Question 1

Which of the following materials acts as the best adsorbent due to its very high surface area and is commonly used to remove impurities, colours and odours from gases and liquids?

Accepted Answer

Introduction / Context: Adsorption is a surface phenomenon in which molecules from a gas or liquid accumulate on the surface of a solid. Some materials have extremely high surface area and are very effective adsorbents, used for purification, decolourisation and deodorisation. This question asks which material acts as the best adsorbent among the options. Understanding the role of activated materials helps students connect colloid and surface chemistry with practical purification methods. Given Data / Assumptions: We are comparing various carbon based materials and one non carbon material for their adsorbing ability. Adsorption efficiency depends strongly on surface area and pore structure. The best adsorbent among the choices should have a very high internal surface area. Context is general laboratory and industrial purification. Concept / Approach: Activated charcoal is produced by heating carbon rich materials such as coconut shells or wood in the absence of air and then activating them to create a network of fine pores. This activation greatly increases the internal surface area, often to hundreds or even thousands of square metres per gram. As a result, activated charcoal can adsorb large amounts of gases, dyes and other impurities. Ordinary charcoal, carbon black and coconut shell charcoal have lower accessible surface area unless specifically activated. Silica gel is also a good adsorbent for moisture, but in many school level treatments, activated charcoal is highlighted as the best general purpose adsorbent. Step-by-Step Solution: Step 1: Recall that adsorption depends on surface area, so materials with very high internal surface area are more effective adsorbents. Step 2: Recognise that activated charcoal is specially processed to create a highly porous structure and enormous surface area. Step 3: Understand that ordinary charcoal and coconut shell charcoal, before activation, have lower surface area and therefore lower adsorption capacity. Step 4: Carbon black consists of fine carbon particles but is mainly used as a reinforcing agent in rubber and as a pigment. Step 5: Silica gel is a good moisture adsorbent, but the question at school level usually treats activated charcoal as the best general adsorbent. Therefore, activated charcoal is the correct choice. Verification / Alternative check: Lab manuals often recommend activated charcoal for decolourising sugar solutions and for removing organic impurities from solutions because of its powerful adsorbing action. Gas masks and water filters frequently use activated carbon granules to trap toxic vapours and odours. Technical data sheets for activated charcoal report extremely high specific surface areas, much greater than those of untreated charcoal or simple carbon black. Although silica gel also has a high surface area and is used in desiccant packets, exam questions on adsorption typically emphasise activated charcoal as the most effective adsorbent overall. Why Other Options Are Wrong: Charcoal: Has some adsorbing capacity but is less effective than properly activated charcoal due to lower porosity and surface area. Coconut shell charcoal: A good raw material for making activated charcoal, but before activation its adsorption capacity is lower. Carbon black: Consists of small carbon particles used mainly as pigment and reinforcing filler; not highlighted as the strongest adsorbent in school chemistry. Silica gel: An important adsorbent for moisture control, but in the context of general adsorption questions, activated charcoal is commonly regarded as superior for a broad range of substances. Common Pitfalls: Students may see several carbon based options and feel unsure which is best. Some may choose silica gel because it is familiar from desiccant packets in packaging. To avoid confusion, remember that activated charcoal is specially processed to maximise adsorption and is widely used in gas masks, water purifiers and decolourisation. Ordinary charcoal and coconut shell charcoal become much better adsorbents only after activation. Associating the word activated with improved performance can help in recalling the correct answer. Final Answer: The material that acts as the best adsorbent among the options is Activated charcoal.

Question 2

In an ecosystem, which of the following groups is responsible for the maximum fixation of solar energy through photosynthesis and thus forms the main producers at the base of food chains?

Accepted Answer

Introduction / Context: Energy flow in ecosystems starts with the capture of solar energy and its conversion into chemical energy. This process is called photosynthesis and is carried out by certain organisms that act as producers. This question asks which group is responsible for the maximum fixation of solar energy in most ecosystems. Understanding this is essential in ecology and environmental science because it explains why plant life is critical for supporting higher trophic levels. Given Data / Assumptions: Solar energy must be captured and converted into chemical energy by photosynthetic organisms. These organisms form the producer level in the food chain. We are considering typical terrestrial and aquatic ecosystems. The options include plants, bacteria, protozoa, fungi, and zooplankton. Concept / Approach: Green plants contain chlorophyll and carry out photosynthesis, converting carbon dioxide and water into glucose and oxygen using sunlight. They are the dominant producers on land and in many freshwater systems, and they fix the majority of solar energy entering the biosphere in these environments. While some bacteria and algae also perform photosynthesis, the bulk of primary productivity in most familiar ecosystems is due to green plants. Bacteria, protozoa, fungi, and zooplankton are often decomposers or consumers rather than primary producers at large scale. Step-by-Step Solution: Step 1: Identify the organisms that perform photosynthesis. Green plants are the most obvious examples, as they contain chlorophyll in their chloroplasts. Step 2: Recognise that photosynthesis is the process through which solar energy is fixed into chemical energy in organic molecules. Step 3: Note that in terrestrial ecosystems, forests, grasslands and crops, green plants account for the vast majority of primary productivity. Step 4: Acknowledge that some bacteria and algae can also fix solar energy, but on the whole, green plants dominate the total global fixation in many ecosystems considered at school level. Step 5: Conclude that green plants are the group responsible for the maximum fixation of solar energy and form the main producer level in food chains. Verification / Alternative check: Ecology textbooks describe food chains starting with producers, usually green plants, followed by herbivores and carnivores. Diagrams of energy pyramids show the broad base occupied by green plants, indicating the large amount of energy they fix. While cyanobacteria and phytoplankton also contribute significantly in oceans and lakes, they are often grouped conceptually with plants as photosynthetic producers. For the purpose of the question, green plants are the most appropriate answer, as they are the principal solar energy fixers in many ecosystems discussed in school curriculum. Why Other Options Are Wrong: Bacteria: Some bacteria are photosynthetic, but many are decomposers or heterotrophs, and overall they do not account for the maximum fixation of solar energy in the usual examples given at this level. Protozoa: Usually heterotrophic and act as consumers, feeding on other organisms, not as primary producers. Fungi: Heterotrophic organisms that obtain food by decomposing organic matter, they do not photosynthesise. Zooplankton: These are small animal like organisms in aquatic systems that feed on phytoplankton or other organisms and are not primary producers. Common Pitfalls: Students may become confused by specialised cases, such as photosynthetic bacteria or algae in oceans, and forget the central role of green plants in typical land based ecosystems. Another error is to think that any microscopic organism must be a producer, when in fact many are consumers or decomposers. A simple way to remember the correct concept is to associate chlorophyll containing organisms, especially green plants, with solar energy fixation and producer status at the base of food chains. Final Answer: The group responsible for the maximum fixation of solar energy in ecosystems is Green plants.

Question 3

In the paper industry, which of the following is commonly used as a bleaching agent for pulp to improve whiteness while reducing the use of elemental chlorine?

Accepted Answer

Introduction / Context: Bleaching of wood pulp is an important step in the paper industry to produce bright, white paper. Historically, elemental chlorine was used for bleaching, but environmental concerns have led to the adoption of alternative bleaching agents. This question asks which substance is commonly used to bleach pulp in the paper industry, highlighting the role of oxidising agents in industrial chemistry and environmental protection. Given Data / Assumptions: The context is pulp bleaching in paper manufacturing. The aim is to remove colour causing lignin residues while preserving cellulose. Modern processes aim to reduce harmful chlorinated by products. The options include acids, enzymes, oxidising agents and other chemicals. Concept / Approach: Hydrogen peroxide and other peroxide compounds are widely used as bleaching agents in the paper industry. Peroxides act as powerful oxidising agents that can break down coloured lignin structures in pulp without producing as many harmful chlorinated organic compounds as elemental chlorine. They are part of elementary chlorine free or total chlorine free bleaching sequences. Mild sulphuric acid is not a bleaching agent, glucose isomerase is an enzyme used in the food industry, and iodine with water is not used for pulp bleaching. Therefore, peroxide is the correct choice among the options. Step-by-Step Solution: Step 1: Identify the goal of pulp bleaching, which is to remove residual lignin and colour bodies from wood pulp to make it whiter. Step 2: Recall that bleaching agents must be strong oxidants that can break down these coloured compounds. Step 3: Recognise that hydrogen peroxide (H2O2) and related peroxides are now widely used in the paper industry as bleaching agents. Step 4: Understand that the use of peroxides helps reduce the formation of chlorinated organic pollutants compared to traditional chlorine based bleaching. Step 5: Compare other options. Mild sulphuric acid is used for pH control, glucose isomerase is used for converting glucose to fructose in food processing, and iodine with water has antiseptic properties but is not used in pulp bleaching. Therefore, peroxide is the correct answer. Verification / Alternative check: Industrial chemistry and environmental engineering texts describe several bleaching sequences for pulp, such as peroxide bleaching, ozone bleaching and chlorine dioxide bleaching. Hydrogen peroxide bleaching is often highlighted as an environmentally friendly method since it breaks down into water and oxygen after use. Process flow diagrams for modern paper mills frequently include peroxide stages. Sulphuric acid appears mainly in pH adjustment and not as a primary bleaching agent. Enzymes like xylanases may be used to aid bleaching, but glucose isomerase is not one of them. These points confirm that peroxide compounds are commonly used bleaching agents. Why Other Options Are Wrong: Mild sulphuric acid: Used mainly to adjust pH or in some chemical treatments but not a primary bleaching agent for pulp. Glucose isomerase: An enzyme used to convert glucose into fructose in the manufacture of high fructose corn syrup, not used in pulp bleaching. Iodine and water: Used as an antiseptic solution for medical purposes, not in bleaching paper pulp. Sodium chloride solution: Common salt solution does not bleach pulp; chlorine based bleaching would require chlorine or chlorine dioxide, not simply sodium chloride. Common Pitfalls: Students may focus on chlorine related chemicals because of older references to chlorine bleaching in paper manufacture and confuse simple salt solutions with the actual bleaching agents. Another mistake is to assume that any strong acid like sulphuric acid must be effective for bleaching, which is not accurate. Remember that modern environmentally conscious paper mills often advertise elemental chlorine free bleaching and rely on oxidising agents like hydrogen peroxide to achieve high brightness with less pollution. Final Answer: In the paper industry, pulp is commonly bleached using Peroxide based bleaching agents such as hydrogen peroxide.

Question 4

In a municipal water treatment plant, the use of chloramines in the treatment process mainly ensures effective __________ of the treated water supplied to consumers.

Accepted Answer

Introduction / Context: Chlorination is one of the most widely used methods for disinfecting drinking water in municipal water treatment plants. Apart from free chlorine, water utilities sometimes use combined chlorine in the form of chloramines. The question is asking what the main purpose of adding chloramines isn't taste, hardness, or weed control, but a specific treatment objective related to safety of the supply. Understanding this helps learners connect environmental chemistry with public health and engineering practice. Given Data / Assumptions: - The setting is a municipal water treatment plant that supplies treated water to consumers through a distribution system. - Chloramines are being added intentionally as part of the water treatment process. - Only one of the listed outcomes is the primary reason for using chloramines in such systems. - We assume standard drinking water treatment practice, where microbiological safety is the top priority. Concept / Approach: Chloramines are formed when chlorine reacts with ammonia in controlled conditions, producing monochloramine as the main species. Free chlorine is a strong and fast acting disinfectant, but it can dissipate relatively quickly in long distribution networks and may form higher levels of some disinfection by products. Chloramines, by contrast, are weaker immediate disinfectants but are more stable over long distances in pipes. Therefore they are often used to maintain a disinfectant residual, which continues to control pathogenic microorganisms as water travels from the plant to distant consumers. The key concept is that chloramines are used primarily for disinfection purposes, not for hardness removal or weed control. Step-by-Step Solution: Step 1: Recall that the main goals of a drinking water treatment plant are removal of suspended solids, removal of dissolved contaminants, and disinfection to kill harmful microorganisms. Step 2: Recognise that chloramines are chemical compounds formed from chlorine and ammonia and are classified as disinfectants used in water treatment. Step 3: Evaluate option related to hardness removal. Permanent hardness is removed by processes such as lime softening, ion exchange, or reverse osmosis, not by chloramines. Step 4: Evaluate the taste and odour option. While chlorination can influence taste and odour, chloramines are chosen more for residual disinfection than for flavour improvement. Step 5: Evaluate the weed control option. Weed control in reservoirs is generally achieved by mechanical removal, shading, or herbicides, not by chloramines added in a treatment plant. Step 6: Conclude that the main reason for using chloramines is to ensure continuing disinfection within the distribution system, so the correct choice is long lasting disinfection of the water supply. Verification / Alternative check: A good way to verify the answer is to think about how utilities describe chloramination in public information brochures. They often explain that chloramines are used to maintain a disinfectant residual, reduce formation of some chlorination by products, and still protect consumers from bacteria and viruses. They do not claim that chloramines remove calcium or magnesium ions that cause permanent hardness, and they are not promoted as agents to kill weeds in open reservoirs. Therefore checking real world descriptions supports the view that disinfection is the main purpose of chloramine use. This cross check confirms that the chosen option is consistent with practical water treatment operations and regulatory guidelines for safe drinking water. Why Other Options Are Wrong: Taste and odour control: While any disinfectant can affect taste and smell, chloramines are not primarily added to improve flavour, so this is not the main objective. Weed control in reservoirs: Weed or algal control is generally managed with different strategies, not by chloramines applied at the plant, so this option is incorrect. Removal of permanent hardness: Permanent hardness is not removed by chlorination or chloramination but by dedicated softening processes, so this statement is wrong for chloramines. Common Pitfalls: A common misunderstanding is to assume that any chemical added during water treatment performs many functions simultaneously, such as softening, disinfection, and taste correction. Students may also confuse chloramines with other water treatment chemicals like alum or lime. Another mistake is to focus on taste complaints related to chloraminated water and therefore assume that taste and odour control is the design objective, when in reality it is an unintended side effect. Remember to distinguish between primary treatment goals and secondary or incidental effects when evaluating statements about treatment chemicals. Final Answer: The use of chloramines in a municipal water treatment plant mainly ensures long lasting disinfection of the water supply as the treated water travels through the distribution system to consumers.

Question 5

When liquid water is converted into solid ice cubes in an ordinary freezer, how does the entropy of the water itself change during this freezing process?

Accepted Answer

Introduction / Context: Entropy is a central concept in thermodynamics and physical chemistry. It is often described as a measure of disorder, randomness, or the number of microscopic arrangements available to a system. This question asks what happens to the entropy of water when it freezes into ice cubes. Understanding this change is important for interpreting phase changes, spontaneity of processes, and the behaviour of systems at different temperatures. Given Data / Assumptions: - The substance is pure water that starts in the liquid state. - The water is cooled in a freezer until it becomes solid ice cubes. - Pressure is assumed to be approximately constant at around one atmosphere. - We are asked specifically about the entropy of the water system, not the entropy of the entire universe. Concept / Approach: In the liquid state, water molecules have considerable freedom to move, rotate, and vibrate, and they can arrange themselves in many different microscopic configurations. In the solid state, ice has a rigid crystalline structure in which each water molecule occupies a more ordered position with restricted motion. Entropy is lower in more ordered states and higher in more disordered states. When water freezes, energy is released to the surroundings and the system becomes more ordered. Therefore the entropy of the water decreases during freezing, even though the total entropy of the system plus surroundings can still increase, which makes the process compatible with the second law of thermodynamics. Step-by-Step Solution: Step 1: Recall that entropy, usually denoted by S, is larger for states with more microscopic disorder and smaller for states with more order. Step 2: Compare molecular motion in liquid water and solid ice. Liquid water has molecules that move freely past one another, whereas solid ice has molecules fixed in a regular lattice. Step 3: Recognise that freezing is a phase transition from a more disordered state to a more ordered state, because the molecules line up in a crystalline arrangement. Step 4: Use the rule that a change from liquid to solid at constant pressure and temperature leads to a decrease in entropy of the substance undergoing the change. Step 5: Among the given options, the statement that the entropy of the water decreases as it freezes matches the thermodynamic description of a liquid to solid transition. Verification / Alternative check: Another way to think about this is to consider the direction of heat flow. When water freezes, it releases latent heat to the surroundings. The water loses energy and becomes more ordered, which corresponds to a negative entropy change for the water. The surroundings gain energy and experience a positive entropy change. For a spontaneous freezing process at a given temperature, the positive entropy gained by the surroundings must outweigh the negative entropy change of the water. This perspective again shows that the entropy of the water itself decreases, confirming that the correct option is the one that states that entropy decreases during freezing. Why Other Options Are Wrong: No change in entropy: A phase change from liquid to solid involves a clear change in molecular order, so entropy cannot stay constant. Entropy increases: This would correspond to a transition to a more disordered state, which is not true when a liquid becomes a crystal. May increase or decrease depending on process: For a given substance at specified temperature and pressure, the direction of entropy change for liquid to solid is fixed and does not depend on the particular freezer used. Common Pitfalls: Many learners confuse the entropy of the system with the entropy of the universe. They know that the total entropy must increase for a spontaneous process and may wrongly assume that every part of the system must show increased entropy. In reality, entropy can decrease in one part, such as the water, while increasing more in the surroundings. Another common mistake is to equate lower temperature directly with lower entropy without considering phase, or to rely too heavily on the idea that freezing is a spontaneous process in a freezer and therefore must involve an entropy increase for the water. Keeping the system and surroundings clearly separated avoids these errors. Final Answer: When liquid water is converted into ice cubes, the entropy of the water system decreases as it freezes because the molecules become more ordered in the solid state.

Question 6

Which one of the following rocks is not classified as an igneous rock in geology?

Accepted Answer

Introduction / Context: Rocks on Earth are broadly grouped into three major classes: igneous, sedimentary, and metamorphic. Competitive examinations often test basic examples of each category. This question asks you to identify which listed rock is not igneous. Recognising common igneous rocks such as granite, basalt, and gabbro, and contrasting them with sedimentary rocks like dolomite, is an important part of general geology and geography knowledge. Given Data / Assumptions: - Four named rocks are provided as options: granite, basalt, dolomite, and gabbro. - Only one of these rocks is not formed from cooling and solidification of molten magma or lava. - Standard geological classification is assumed, without considering rare or unusual rock varieties. Concept / Approach: Igneous rocks form from the cooling and crystallisation of molten material. When this cooling takes place beneath the surface, intrusive igneous rocks such as granite and gabbro develop. When it occurs at the surface from flowing lava, extrusive igneous rocks such as basalt form. Sedimentary rocks, in contrast, are formed by deposition and lithification of sediments, chemical precipitation, or biological processes. Dolomite is a sedimentary rock rich in the mineral dolomite, often formed by alteration of limestone or direct chemical precipitation. Knowing typical examples of each class allows us to distinguish the odd one out in such questions. Step-by-Step Solution: Step 1: Recall examples of intrusive igneous rocks. Granite and gabbro are both classic intrusive igneous rocks that crystallise slowly below the Earth surface. Step 2: Recall examples of extrusive igneous rocks. Basalt is a common extrusive igneous rock formed from lava flows at or near the surface. Step 3: Identify dolomite. Dolomite as a rock, often called dolostone, is primarily a sedimentary rock composed of the mineral dolomite, usually formed by alteration of limestone or chemical precipitation in marine environments. Step 4: Conclude that granite, basalt, and gabbro are all igneous, whereas dolomite belongs to the sedimentary class, so dolomite is the rock that is not igneous. Verification / Alternative check: Textbooks on physical geology and introductory Earth science consistently list granite, basalt, and gabbro among major igneous rocks. They describe granite as coarse grained and rich in quartz and feldspars, basalt as fine grained and mafic, and gabbro as a coarse grained mafic intrusive equivalent of basalt. In contrast, dolomite is grouped with sedimentary carbonate rocks and is often discussed together with limestone in chapters on sedimentary environments. Checking any standard classification table confirms that dolomite is not an igneous rock, which verifies the conclusion drawn from conceptual reasoning. Why Other Options Are Wrong: Granite: Clearly identified as an intrusive igneous rock, so it is not the correct choice for a non igneous example. Basalt: A typical extrusive igneous rock found in oceanic crust and lava plateaus, so it is not the odd one out. Gabbro: Another intrusive igneous rock, coarse grained and mafic, and therefore still igneous and incorrect for this question. Common Pitfalls: Learners sometimes confuse rock names with mineral names or with commercial terms used in the building industry. Dolomite as a mineral and a rock can be mistaken for a type of igneous rock because it is less widely discussed than granite or basalt in school textbooks. Another pitfall is to assume that any rock used as a building stone could be igneous. To avoid these mistakes it is useful to memorise at least a few standard examples of each rock class: granite, basalt, gabbro as igneous; sandstone, shale, limestone, dolomite as sedimentary; and marble, slate, gneiss as metamorphic. Final Answer: The rock that is not classified as igneous is Dolomite, because dolomite is a sedimentary carbonate rock rather than an igneous rock formed from solidified magma or lava.

Question 7

Which gas commonly used in medicine and anaesthesia is popularly known as "laughing gas"?

Accepted Answer

Introduction / Context: Several nitrogen oxides exist in the atmosphere and in industrial and medical applications. One of them, nitrous oxide, is widely known by the common name laughing gas because of its characteristic effect on mood and perception when inhaled in controlled conditions. This question asks you to connect that popular term with the correct chemical species from among several similar sounding nitrogen compounds. Such questions test both chemical nomenclature and general awareness of medical and industrial uses of gases. Given Data / Assumptions: - The question mentions a gas called laughing gas, known for its characteristic effects on humans. - Four nitrogen containing gases are listed as options: nitrous oxide, nitrogen, nitric oxide, and nitrogen peroxide. - Only one of these is popularly called laughing gas in textbooks and common usage. - Standard chemical naming conventions are assumed for the given compounds. Concept / Approach: Nitrous oxide has the chemical formula N2O and is a colourless gas with a slightly sweet odour and taste. When inhaled in small controlled amounts, it can produce a feeling of euphoria and laughter, which gave rise to the name laughing gas. It is used as an anaesthetic and analgesic in dentistry and minor medical procedures, and also appears in some industrial applications. The other listed gases, such as nitrogen (N2), nitric oxide (NO), and nitrogen peroxide (commonly nitrogen dioxide NO2 or dinitrogen tetroxide N2O4), do not have this effect and are not known by this nickname. Therefore we identify nitrous oxide as laughing gas. Step-by-Step Solution: Step 1: Recall that laughing gas is a common name used in both chemistry and medicine for a particular nitrogen oxide. Step 2: Remember that this gas has the formula N2O and the systematic name nitrous oxide. Step 3: Compare with nitrogen, which is diatomic N2 and is a major component of air but is chemically inert under normal conditions and does not cause laughter. Step 4: Compare with nitric oxide, NO, which plays roles in biological signalling and environmental chemistry but is not used as an anaesthetic under the name laughing gas. Step 5: Compare with nitrogen peroxide, a less precise name usually related to nitrogen dioxide or its dimer, which are toxic pollutants and certainly not laughing gas. Step 6: Conclude that nitrous oxide is the only option that matches the common name laughing gas. Verification / Alternative check: A quick check in any basic chemistry or general science textbook lists nitrous oxide as laughing gas and notes its use in dental anaesthesia. Descriptions highlight its mild anaesthetic and analgesic properties along with its ability to induce temporary euphoria and laughter. More advanced references also describe its environmental role as a greenhouse gas. None of these references ever call nitrogen, nitric oxide, or nitrogen dioxide by the name laughing gas. This literature cross check confirms that nitrous oxide is the correct and widely accepted answer. Why Other Options Are Wrong: Nitrogen: This is a chemically inert diatomic gas that forms the bulk of the atmosphere and does not have anaesthetic or laughing effects. Nitric oxide: Although biologically important, it is not used as laughing gas and can be harmful at high concentrations. Nitrogen peroxide: This term usually refers to nitrogen dioxide related species which are toxic air pollutants, not anaesthetic gases used for medical purposes. Common Pitfalls: Candidates sometimes confuse nitrous oxide with nitric oxide because their names sound similar. Remember that nitrous oxide has two nitrogen atoms and one oxygen atom, while nitric oxide has one nitrogen and one oxygen. Another error is to assume that any nitrogen based gas used medically might be laughing gas, without recalling the specific name and formula. A good strategy is to memorise a small set of important gas nicknames, such as laughing gas for nitrous oxide, tear gas for certain irritant gases, and marsh gas for methane, and link them with their correct formulas and uses. Final Answer: The gas popularly known as laughing gas is Nitrous oxide, which has the chemical formula N2O and is used as a mild anaesthetic and analgesic.

Question 8

Most liquids that are able to conduct electricity are actually solutions of acids, bases and which of the following?

Accepted Answer

Introduction / Context: In basic electricity and chemistry, students learn that pure water is a very poor conductor of electricity, yet many everyday liquids, such as salt solutions and acid solutions, conduct electricity quite well. This behaviour is related to the presence of ions in solution. The question asks which additional category of substances, along with acids and bases, typically forms solutions that conduct electricity. Identifying this category requires linking electrochemistry concepts with common examples from daily life. Given Data / Assumptions: - The question focuses on liquids that conduct electricity, usually through ionic conduction in solution. - It states that such liquids are commonly solutions of acids and bases, and asks for a third type of solute that also forms conducting solutions. - Four options are provided: salts, copper, aluminium, and iron. - We assume standard school level definitions of acids, bases, and salts and consider aqueous solutions at room conditions. Concept / Approach: Electrical conductivity in liquids often arises from the mobility of ions. Acids produce hydrogen ions in water, bases produce hydroxide ions, and salts dissociate into positive and negative ions. All these ions can move under an applied potential difference, carrying charge and creating an electric current. Metals such as copper, aluminium, and iron conduct electricity in the solid state by electron movement, not by forming solutions in the way referred to here. Therefore, the missing group alongside acids and bases must be salts, which form ionic solutions known as electrolytes. Step-by-Step Solution: Step 1: Recall that in aqueous solution, acids ionise to generate hydrogen ions and bases ionise to generate hydroxide ions. Step 2: Recognise that salts are ionic compounds composed of positive and negative ions, such as sodium chloride, potassium nitrate, and calcium chloride. Step 3: When salts dissolve in water, they dissociate into their constituent ions, which can move freely and therefore conduct electric current. Step 4: Consider the alternatives: copper, aluminium, and iron are metals, which conduct electricity in solid form but are not typically referred to as conducting liquids when dissolved. Step 5: Conclude that the group that correctly completes the statement is salts, making solutions of acids, bases, and salts the main liquid conductors of electricity. Verification / Alternative check: Practical laboratory experiments reinforce this conclusion. When a bulb is connected in series with electrodes dipped into pure distilled water, the bulb barely glows, indicating poor conductivity. However, if common salt, hydrochloric acid, or sodium hydroxide is added, the bulb glows brightly because these substances produce ions in solution. Science textbooks describe such liquids as electrolytes and specifically list acids, bases, and salts as sources of conducting solutions. Metals such as copper, aluminium, and iron are discussed separately as conductors in solid state, not as solutes responsible for electrolytic conduction. This experimental evidence supports salts as the correct answer. Why Other Options Are Wrong: Copper: A good conductor in solid form but not a typical solute mentioned in the context of conducting solutions of acids, bases, and salts. Aluminium: Again a metallic conductor, but not the intended category of dissolved substances giving rise to conducting liquids. Iron: A solid conductor that may corrode in solutions, but the question is about solutes forming electrolytes, which is not best described by iron as a category. Common Pitfalls: Students sometimes confuse electrical conduction in solids, where electrons move through metallic lattices, with conduction in liquids, where ions are responsible for current. Because copper and aluminium are famous conductors in household wiring, some learners may be tempted to pick these metals. To avoid this error, always note whether the question is about solid conductors or conducting solutions. When the context clearly refers to liquids in which substances are dissolved, think of acids, bases, and salts as the classical electrolyte categories. Final Answer: Most liquids that conduct electricity are solutions of acids, bases, and Salts, because salts also dissociate into ions that carry electric current in aqueous solution.

Question 9

Milk of magnesia, which is used as a mild antacid and laxative, contains which basic compound as its active ingredient?

Accepted Answer

Introduction / Context: Many common medicines and household products contain well known chemical compounds. Milk of magnesia is a familiar example used as an antacid to neutralise excess stomach acid and as a mild laxative. General science and chemistry examinations often test whether candidates can identify the key active ingredient in such preparations. This question asks which base is present in milk of magnesia, so you must link the trade name with the underlying chemical compound. Given Data / Assumptions: - The product mentioned is milk of magnesia, commonly available in pharmacies. - The question specifies that the relevant component is a base, that is, an alkaline substance. - Four possible bases are listed: magnesium hydroxide, calcium hydroxide, sodium hydroxide, and ammonium hydroxide. - Standard formulations of milk of magnesia as described in textbooks and pharmacopeias are assumed. Concept / Approach: Milk of magnesia is essentially a suspension of magnesium hydroxide in water. Magnesium hydroxide is only slightly soluble in water, so it forms a milky white suspension, which is why the product looks like milk. When ingested in small doses, magnesium hydroxide neutralises excess hydrochloric acid in the stomach, providing relief from acidity. It also draws water into the intestines, acting as a mild laxative. Other bases such as sodium hydroxide and calcium hydroxide are far too caustic for direct medicinal use in this way, and ammonium hydroxide is not used for this purpose either. Therefore the only base among the options that correctly matches milk of magnesia is magnesium hydroxide. Step-by-Step Solution: Step 1: Recall from basic chemistry and general knowledge that milk of magnesia is a pharmaceutical preparation of magnesium hydroxide. Step 2: Understand that the term milk refers to the appearance of the suspension and not to the presence of dairy products. Step 3: Consider sodium hydroxide and calcium hydroxide, both of which are strong caustic bases. They can cause severe burns and are not used as gentle antacids in this form. Step 4: Consider ammonium hydroxide, which is mainly used as a cleaning agent and in industry, not as a standard over the counter antacid and laxative. Step 5: Conclude that magnesium hydroxide is the only base that fits both the name milk of magnesia and the medicinal uses described. Verification / Alternative check: Checking a standard school chemistry book or a reliable pharmacology reference confirms that milk of magnesia is defined as a suspension of magnesium hydroxide in water. Composition labels on commercial bottles also list magnesium hydroxide as the active ingredient. These references emphasise its antacid and laxative properties, matching the description students remember from science lessons. No reputable source connects milk of magnesia to sodium hydroxide, calcium hydroxide, or ammonium hydroxide. This documentary evidence supports the conclusion that magnesium hydroxide is the correct answer. Why Other Options Are Wrong: Calcium hydroxide: Known as slaked lime, used in construction and agriculture, and too caustic for direct use in this common form as an antacid suspension. Sodium hydroxide: A very strong base called caustic soda, used in industry and drain cleaners, not in gentle over the counter antacid products. Ammonium hydroxide: Primarily used in cleaning agents and industrial applications, and not the active base in milk of magnesia. Common Pitfalls: Students sometimes confuse different hydroxides because many share similar naming patterns. Calcium hydroxide, for example, is also white and can form suspensions in water, which may lead to confusion if one relies only on appearance. Another mistake is to assume that any base listed in the options could be used medicinally, forgetting that strong caustic bases are unsafe for ingestion at typical cleaning concentrations. To avoid these pitfalls, it helps to memorise a few key medicinal compounds, such as magnesium hydroxide in milk of magnesia and sodium bicarbonate as baking soda, along with their specific uses. Final Answer: Milk of magnesia contains Magnesium hydroxide as its active basic ingredient, which acts as a mild antacid and laxative.

Question 10

The uranium deposits identified in the Ladakh region of India are best described as belonging to which type of natural resource category?

Accepted Answer

Introduction / Context: Geography and environmental science classify natural resources in different ways, such as actual and potential resources, renewable and non renewable resources, and biotic and abiotic resources. Uranium deposits discovered in a remote region like Ladakh provide an excellent example to test understanding of these categories. This question asks you to identify which resource type best describes uranium found there, considering that much of it may not yet be fully explored or exploited. Given Data / Assumptions: - Uranium has been found in the Ladakh region of India according to surveys and exploration reports. - Uranium is a mineral resource used mainly as fuel in nuclear power plants and for other specialised applications. - The question refers to the category of resource based on stage of development and use, not on whether it is living or non living. - It is reasonable to assume that large parts of these deposits are still under exploration and not fully utilised. Concept / Approach: Actual resources are those whose quantity is known and which are currently being exploited using existing technology. Potential resources are those whose existence in a particular region is known, but which have not yet been fully surveyed, quantified, or brought into large scale use. Uranium in Ladakh fits the potential resource category because although its presence is recognised, extraction and full commercial exploitation are still limited compared to established mining areas. The term unnatural resources is not a standard classification in geography, and biotic resources refer to living components like plants and animals, not to minerals such as uranium. Step-by-Step Solution: Step 1: Identify uranium as an abiotic mineral resource that is non renewable on human time scales. Step 2: Recall the definition of actual resources: they are surveyed, their quantity and quality are known, and they are currently used intensively. Step 3: Recall the definition of potential resources: they are known to exist in a particular area but are not yet fully developed or exploited due to lack of technology, investment, or demand. Step 4: Consider uranium in Ladakh, which has been reported by surveys but is not yet a major centre of uranium mining comparable to fully developed uranium fields. Step 5: Recognise that unnatural resources is not a valid textbook category and that biotic resources are living, while uranium is non living. Step 6: Conclude that uranium in Ladakh is best described as a potential resource. Verification / Alternative check: Classroom examples and geography textbooks often mention potential resources like mineral deposits in remote areas, offshore oil fields not yet tapped, or hydroelectric potential in rivers that has not been fully exploited. Uranium in Ladakh matches this pattern because it represents a resource that may be used more extensively in the future as technology, infrastructure, and economic conditions develop. In contrast, actual resources are those already in large scale commercial use, such as coal from well established coalfields. This comparison supports the classification of uranium in Ladakh as a potential resource rather than an actual resource at the current stage. Why Other Options Are Wrong: Actual resources: These are already fully surveyed and widely used, which is not the typical description of the relatively new uranium finds in Ladakh. Unnatural resources: This term does not correspond to any standard category in resource classification and is not used in school level geography. Biotic resources: These are living resources such as forests, fish, and livestock, whereas uranium is an abiotic mineral resource. Common Pitfalls: A frequent mistake is to equate any identified mineral with actual resources, forgetting the requirement that actual resources are already extensively exploited. Another pitfall is confusion between biotic and abiotic resources, leading some learners to overlook that uranium is non living and therefore cannot be biotic. The distractor term unnatural resources may also mislead candidates who are not familiar with proper classifications. To avoid such errors, always return to precise textbook definitions and verify whether a resource is currently exploited or simply known to exist. Final Answer: Uranium discovered in the Ladakh region is best classified as a Potential resource, because its presence is known but it has not yet been fully developed and exploited on a large commercial scale.

Question 11

In space crafts, which of the following substances is commonly used to prevent the fuel from freezing at extremely low temperatures?

Accepted Answer

Introduction / Context: Spacecraft operate in extremely cold environments, where many liquids, including fuels and coolants, are at risk of freezing. Engineers therefore use special additives called antifreezes to lower the freezing point of these liquids. This question asks which of the listed substances is used for preventing fuel from freezing in space crafts. Understanding the basic role of glycols as antifreeze agents reinforces fundamental ideas about colligative properties and practical applications of chemistry in space technology. Given Data / Assumptions: - The setting is a space craft, where fuel can be exposed to very low temperatures. - The design requirement is to prevent the fuel from freezing, so an antifreeze or freezing point depressant is needed. - The options given are benzene, glycol, acetylene, and ester, all of which are organic compounds. - We assume standard knowledge of which of these substances is widely used as an antifreeze in engineering systems. Concept / Approach: Antifreeze agents commonly used in engines and industrial cooling systems are glycols, such as ethylene glycol or propylene glycol. These compounds mix with water or other liquids and significantly lower the freezing point due to colligative effects, preventing the mixture from solidifying at low temperatures. Benzene is a volatile aromatic hydrocarbon, acetylene is a flammable gas used as a fuel in welding, and ester is a general term for a class of compounds with many uses, but none of these are the standard antifreeze agents used in such systems. Therefore, glycol is the best answer among the options. Step-by-Step Solution: Step 1: Recall that antifreeze formulations used in automotive radiators and many cooling systems are based on ethylene glycol or propylene glycol. Step 2: Recognise that the same basic chemistry, lowering of freezing point by mixing glycols with fuels or coolants, can be used in aerospace applications to protect against freezing. Step 3: Examine benzene, which is a liquid hydrocarbon but not a standard antifreeze additive and possesses serious health hazards. Step 4: Examine acetylene, which is a gas used for high temperature welding and is not suitable as a liquid antifreeze agent. Step 5: Examine the generic term ester, which describes a wide group of compounds without specifically indicating antifreeze properties. Step 6: Conclude that glycol is the substance typically used to prevent freezing, making it the correct answer. Verification / Alternative check: Reference materials on automotive and aerospace engineering refer repeatedly to glycols as key components in antifreeze and coolant mixtures. Ethylene glycol based fluids are standard in many terrestrial engines, while propylene glycol is often chosen when lower toxicity is desired. Both types act by lowering the freezing point and raising the boiling point of the mixture. No similar widespread use is reported for benzene, acetylene, or generic esters as primary antifreeze additives in fuel systems. This technical background supports glycol as the correct option in an exam context. Why Other Options Are Wrong: Benzene: An aromatic solvent and fuel component, but not a common antifreeze additive due to toxicity and volatility. Acetylene: A gaseous hydrocarbon used mainly for welding and cutting, not for freezing point depression in fuels. Ester: A broad class of compounds with many uses, including fragrances and plastics, but not a standard antifreeze material mentioned in basic science texts for this purpose. Common Pitfalls: Students may be tempted to choose benzene or another hydrocarbon simply because these are associated with fuels and organic chemistry. Another mistake is to interpret the word ester as a specific technological material rather than a broad functional group. To avoid these errors, it helps to associate the word glycol directly with antifreeze, as seen in automotive coolant labels and science demonstrations that show freezing point depression using ethylene glycol or propylene glycol. Final Answer: In space crafts, substances such as Glycol are used as antifreeze agents to prevent fuel from freezing at very low temperatures.

Question 12

Natural gas used as a domestic and industrial fuel consists mainly of which one of the following components?

Accepted Answer

Introduction / Context: Natural gas is an important fossil fuel used widely for cooking, heating, power generation, and as a feedstock in the chemical industry. It is a mixture of light hydrocarbons, but one component dominates both in proportion and in significance. General science and geography questions often test whether learners can identify this principal constituent. This question asks specifically which substance is the major component of natural gas used as a fuel. Given Data / Assumptions: - The fuel in question is natural gas, taken from underground deposits and used in homes and industries. - A mixture of different hydrocarbons may be present, but one component forms the bulk of the gas. - The options include methane, acetone, chlorine, and hexane. - Standard composition of pipeline quality natural gas is assumed, where methane is predominant. Concept / Approach: Natural gas is composed primarily of methane with the chemical formula CH4. Depending on the field, it may also contain small amounts of ethane, propane, butane, carbon dioxide, nitrogen, and other trace gases. However, methane typically accounts for 70 percent to more than 90 percent of the volume. Acetone is an organic solvent, not a main component of natural gas. Chlorine is a reactive halogen element, and hexane is a heavier hydrocarbon liquid at room temperature. Therefore, among the options, methane is the only one that correctly represents the major component of natural gas. Step-by-Step Solution: Step 1: Recall from basic science that natural gas is described as a mixture containing mostly methane, along with small amounts of higher hydrocarbons. Step 2: Recognise that methane, CH4, is a light hydrocarbon that is gaseous at normal temperatures and pressures and burns with a clean flame. Step 3: Consider acetone, which is a liquid organic solvent used in nail polish remover and laboratories, not a natural gas component. Step 4: Consider chlorine, which is a yellow green poisonous gas used for disinfection and in industry, not a combustible fuel in natural gas. Step 5: Consider hexane, a heavier hydrocarbon that is liquid at room temperature and is found in petrol fractions, not as the main part of natural gas. Step 6: Conclude that methane is the correct answer as the principal component of natural gas. Verification / Alternative check: Energy statistics and chemistry textbooks describe natural gas as consisting mainly of methane. They often define natural gas almost synonymously with methane when explaining combustion and pollution. In diagrams of fractional composition, methane occupies the largest bar, and ethane, propane, and butane are shown only as smaller fractions. Industrial specifications for liquefied natural gas also emphasise high methane content. None of these references describe acetone, chlorine, or hexane as the main constituent. This evidence confirms that methane is the correct choice. Why Other Options Are Wrong: Acetone: A volatile solvent used in industries and cosmetics, but not a natural component or main part of natural gas reserves. Chlorine: A reactive gas used for water treatment and bleach production, not a hydrocarbon fuel component. Hexane: A liquid hydrocarbon found in petrol and solvents, not the primary constituent of natural gas which is dominated by gaseous methane. Common Pitfalls: Learners may overthink the question and look for a more complex hydrocarbon, forgetting that the simplest hydrocarbon, methane, is actually the main natural gas component. Others may confuse natural gas with liquefied petroleum gas, which is richer in propane and butane. Keeping a clear mental distinction between natural gas, LPG, and petrol helps avoid such confusion. Always remember that in most textbook contexts, natural gas is associated with methane as its leading component. Final Answer: The major component of natural gas is Methane, a light hydrocarbon with the formula CH4 that forms the bulk of the fuel.

Question 13

Carnotite, a yellowish mineral found in certain sedimentary rocks, is an important ore of which metal?

Accepted Answer

Introduction / Context: Many metallic elements occur in nature not as pure metals but as ores, which are minerals that contain a high enough concentration of the metal to be economically extracted. Recognising the names of important ores is a common requirement in general chemistry and geography. Carnotite is one such ore, noted for its yellow colour and association with radioactive elements. This question asks which metal is obtained from carnotite, so you must connect the ore name with the correct element. Given Data / Assumptions: - The mineral in question is carnotite, described in textbooks as a uranium bearing mineral. - Four metallic elements are listed: uranium, beryllium, chromium, and copper. - The question focuses on the principal metal of economic interest extracted from carnotite. - Standard mineralogy data as taught at school and introductory college level is assumed. Concept / Approach: Carnotite is a potassium uranium vanadate mineral, often written with a complex formula that includes uranium and vanadium. It occurs in sandstones and other sedimentary rocks and is a significant ore for uranium production. Although vanadium is also present, carnotite is primarily exploited for its uranium content, especially for use in nuclear fuel and related industries. Beryllium, chromium, and copper all have their own characteristic ores, such as beryl for beryllium, chromite for chromium, and chalcopyrite for copper, not carnotite. Therefore the metal associated with carnotite in exam questions is uranium. Step-by-Step Solution: Step 1: Recall that carnotite is commonly mentioned in textbooks as a uranium ore, often in the context of radioactive minerals. Step 2: Note that the economic importance of carnotite lies in its uranium content, which can be extracted for nuclear fuel. Step 3: Compare with typical ores of other options. Beryllium is obtained from beryl, chromium from chromite, and copper from ores such as chalcopyrite, malachite, and cuprite. Step 4: Confirm that carnotite is not listed as a main ore for beryllium, chromium, or copper in standard tables. Step 5: Conclude that uranium is the metal for which carnotite is an ore. Verification / Alternative check: Mineralogy tables and general science books list carnotite under uranium ores, alongside pitchblende and uraninite. These references describe carnotite as a potassium uranium vanadate with notable radioactivity. They associate it strongly with uranium mining regions and mention vanadium only as a secondary element. No reputable source describes carnotite as a primary ore of beryllium, chromium, or copper. This consistent classification in reference materials verifies that uranium is the correct choice. Why Other Options Are Wrong: Beryllium: Generally obtained from minerals like beryl and not from carnotite. Chromium: Extracted mainly from chromite, a chromium iron oxide mineral, not from carnotite. Copper: Mined from ores such as chalcopyrite, malachite, and cuprite, not from carnotite which is associated with uranium and vanadium. Common Pitfalls: A common error is to confuse different ore names because many sound somewhat similar or are associated with mining regions in general. Another mistake is to assume that any ore with a complex formula might be related to a widely used metal like copper without checking specific textbook tables. To avoid such problems, it is helpful to memorise short lists of important ores: for example, bauxite for aluminium, hematite and magnetite for iron, and carnotite and pitchblende for uranium. This creates strong mental associations that make exam questions easier to answer. Final Answer: Carnotite is an important ore of Uranium, which is extracted from this mineral for use in nuclear energy and related applications.

Question 14

When exposed to sunlight, which one of the following common laboratory liquids slowly gets converted to the highly poisonous gas phosgene?

Accepted Answer

Introduction / Context: Chemical safety is an important aspect of laboratory practice. Some seemingly harmless liquids can slowly decompose into hazardous substances under certain conditions, such as exposure to light or air. This question asks which common organic liquid turns into phosgene, a highly toxic gas, when exposed to sunlight. Knowing this helps students understand why some chemicals are stored in dark bottles and handled with care. Given Data / Assumptions: - The transformation in question occurs when a liquid is exposed to sunlight over time. - The product formed is phosgene, a poisonous gas historically used as a chemical warfare agent. - The options include chloroform, acetone, benzene, and propylene. - Standard textbook information about stability and photochemical reactions of these compounds is assumed. Concept / Approach: Chloroform, with the formula CHCl3, is a chlorinated solvent that can undergo slow oxidation in the presence of oxygen and sunlight to form phosgene, COCl2, along with hydrogen chloride. Because phosgene is extremely toxic, chloroform is often stored in dark bottles to limit light exposure and may contain stabilisers such as ethanol to inhibit this decomposition. In contrast, acetone, benzene, and propylene do not typically produce phosgene under normal laboratory sunlight exposure in the same way. Therefore, chloroform is the substance of concern in this question. Step-by-Step Solution: Step 1: Recall that phosgene, COCl2, is a toxic gas that can form from the oxidation of certain chlorinated hydrocarbons. Step 2: Identify chloroform, CHCl3, as a chlorinated solvent often mentioned in safety notes regarding its slow conversion to phosgene in the presence of light and air. Step 3: Note that chloroform bottles are commonly amber coloured or kept in dark places, and sometimes contain small amounts of ethanol as a stabiliser. Step 4: Consider acetone, benzene, and propylene, none of which are routinely described as precursors for phosgene in simple sunlight exposure conditions in school level chemistry. Step 5: Conclude that chloroform is the correct answer since it is known to give phosgene upon prolonged exposure to light and oxygen. Verification / Alternative check: Safety data sheets and standard organic chemistry textbooks specifically warn that chloroform can decompose to phosgene on exposure to air and light. They usually recommend storage in tightly closed, light resistant containers and may advise the presence of ethanol to react with any phosgene formed. No similar widely publicised warning exists for acetone, benzene, or propylene producing phosgene under ordinary sunlight. This consistent treatment in safety literature confirms chloroform as the substance named in the question. Why Other Options Are Wrong: Acetone: A common solvent that can be flammable but is not noted for forming phosgene under simple sunlight exposure in air. Benzene: An aromatic hydrocarbon and known carcinogen, but its photochemical reactions do not typically involve formation of phosgene in the way described for chloroform. Propylene: An unsaturated hydrocarbon gas used in industry, not a chlorinated liquid that would readily yield phosgene by simple oxidation in sunlight. Common Pitfalls: Students may confuse different chlorinated solvents or may not remember which specific compound is linked to phosgene formation. Another mistake is to focus only on toxicity and choose benzene because it is a known carcinogen, forgetting that the question is about conversion to phosgene. To avoid such confusion, associate chloroform with the safety advice about phosgene formation and storage in dark bottles, and keep benzene separate in memory as a carcinogenic aromatic hydrocarbon without that particular photochemical hazard. Final Answer: When exposed to sunlight, Chloroform can slowly decompose in the presence of oxygen to form the poisonous gas phosgene, which is why chloroform is stored in dark, well closed bottles with stabilisers.

Question 15

Molybdenite, which appears as a soft, lead grey mineral, is an important ore of which metal?

Accepted Answer

Introduction / Context: Ores and minerals form a key part of both chemistry and geography syllabi. Each important metal is associated with a few characteristic ores that students are expected to recognise. Molybdenite is one such mineral whose name hints at its relationship with the metal molybdenum. This question checks whether you can correctly link molybdenite with the right metal from a short list that includes several other common metals. Given Data / Assumptions: - The mineral named in the question is molybdenite. - Four metals are provided as options: molybdenum, nickel, silver, and tin. - The question asks which metal has molybdenite as an ore or important mineral source. - Standard high school or introductory college level mineralogy information is assumed. Concept / Approach: Molybdenite is a naturally occurring molybdenum disulphide mineral, often written as MoS2. It resembles graphite in appearance and feel but contains molybdenum instead of carbon. Molybdenum metal is extracted primarily from molybdenite and from some copper ores that contain molybdenum as a by product. Nickel, silver, and tin each have their own characteristic ore minerals, such as pentlandite for nickel, argentite for silver, and cassiterite for tin. They are not mainly obtained from molybdenite. Therefore, the correct association is between molybdenite and molybdenum. Step-by-Step Solution: Step 1: Recall that molybdenite has the formula MoS2 and is structurally similar to graphite, but with molybdenum atoms present. Step 2: Recognise that Mo stands for molybdenum in the periodic table, so the mineral clearly contains molybdenum. Step 3: Understand that molybdenum is extracted from molybdenite by roasting and subsequent processing. Step 4: Consider nickel, silver, and tin. Their key ores are pentlandite and other sulphides for nickel, argentite and related minerals for silver, and cassiterite for tin. Step 5: Conclude that molybdenum is the metal for which molybdenite is a principal ore. Verification / Alternative check: If you consult a table of important ores in a chemistry textbook, molybdenite is almost always listed under molybdenum ores. The entry typically describes it as molybdenum disulphide and explains its use in producing molybdenum metal and lubricants. The same tables do not list molybdenite as an ore of nickel, silver, or tin. Instead, they associate those metals with different minerals. This cross check with standard references verifies that molybdenum is the correct answer. Why Other Options Are Wrong: Nickel: Commonly obtained from pentlandite, nickeliferous laterites, and other ores, not from molybdenite. Silver: Extracted mainly from argentite and other silver bearing minerals, not from molybdenite which contains molybdenum and sulphur. Tin: Produced from cassiterite, a tin oxide ore, rather than from molybdenite. Common Pitfalls: Because molybdenite looks somewhat like graphite, some learners may recall its physical appearance but not its composition, leading to confusion about the associated metal. Another pitfall is to ignore the hint provided by the name itself, which shares the root molyb with molybdenum. Remembering that many ore names contain parts of the element name, such as bauxite for aluminium and hematite for iron in some languages, can make these associations easier to learn and recall during exams. Final Answer: Molybdenite is an important ore of Molybdenum, from which molybdenum metal is extracted for use in alloys and industrial applications.

Question 16

An oxygen molecule present in air consists of how many atoms bonded together in its most stable form?

Accepted Answer

Introduction / Context: In basic chemistry, students learn that elements can exist as individual atoms, as molecules containing a fixed number of atoms, or as extended networks. The form in which an element exists under standard conditions affects its chemical and physical properties. This question asks about the molecular form of oxygen in the air we breathe, which is fundamental to understanding combustion, respiration, and many chemical reactions. Given Data / Assumptions: - The element is oxygen in its most stable form present in the Earth atmosphere at normal conditions. - We are dealing with the ordinary oxygen used in breathing and combustion, not special allotropes like ozone. - The options describe molecules with one, two, three, or many atoms: monoatomic, diatomic, triatomic, and polyatomic forms. - Standard school level chemical notation is assumed, with O2 representing molecular oxygen. Concept / Approach: Atmospheric oxygen exists primarily as diatomic molecules, written as O2. Each O2 molecule contains two oxygen atoms chemically bonded together. This diatomic form is more stable than monoatomic oxygen under normal terrestrial conditions. Another allotrope, ozone, has the formula O3 and is triatomic, but it makes up only a tiny fraction of the atmosphere and is concentrated in the ozone layer. Therefore, when a general question refers to an oxygen molecule in air, it almost always refers to O2, making oxygen diatomic. Step-by-Step Solution: Step 1: Recall the chemical symbol for molecular oxygen in air, which is O2. Step 2: Interpret O2 as a molecule made up of two oxygen atoms bonded together. Step 3: Recognise that monoatomic oxygen, O, is a highly reactive species that is not the dominant form in the atmosphere. Step 4: Recognise that ozone, O3, is triatomic oxygen and exists in much smaller amounts compared to diatomic oxygen. Step 5: Compare with the answer choices and select the option that describes diatomic oxygen as two atoms bonded together. Verification / Alternative check: Chemistry textbooks and environmental science resources consistently state that approximately twenty one percent of the atmosphere by volume is oxygen, and that this oxygen is present chiefly as O2 molecules. Gas law problems also treat oxygen as diatomic when calculating molar masses, using 32 grams per mole, which corresponds to two oxygen atoms in each molecule. The small fraction of atmospheric ozone, while important for absorbing ultraviolet radiation, is explicitly identified as O3 and not as the standard form. These references confirm that oxygen in air is diatomic, not monoatomic or triatomic in bulk. Why Other Options Are Wrong: One atom (monoatomic): This would be correct for noble gases such as helium or neon, but not for the molecular form of oxygen in air. Three atoms (triatomic): This describes ozone, O3, which is an allotrope of oxygen found in much smaller quantities, not the main atmospheric form. A large number of atoms (polyatomic): This description fits network solids such as diamond or silica, not the simple molecules of oxygen gas. Common Pitfalls: Some learners confuse the terms oxygen and ozone because both relate to the same element and are involved in atmospheric processes. They may mistakenly think that ordinary oxygen is triatomic. Another pitfall is to focus too much on the idea that oxygen supports combustion and incorrectly assume that more atoms in a molecule always mean more reactivity. To avoid these errors, remember the standard chemical formulas: O2 for the oxygen we breathe and O3 for ozone in the stratosphere. Final Answer: An oxygen molecule in air is Diatomic, consisting of Two atoms (diatomic) bonded together in the stable form O2.

Question 17

Which one of the following gases is both a greenhouse gas and a substance that can deplete the ozone layer in the stratosphere?

Accepted Answer

Introduction / Context: Global environmental issues often involve gases that have more than one harmful effect. Some gases act as greenhouse gases that trap heat in the atmosphere, while others or the same ones can contribute to ozone layer depletion. This question asks which chemical among the options is both a greenhouse gas and capable of depleting the ozone layer in the stratosphere. Recognising chlorofluorocarbons is crucial for understanding ozone depletion and climate change. Given Data / Assumptions: - Four chemical formulas are provided: CCl2F2, NH3, CH2N2, and BBr3. - The gas we seek contributes significantly to the greenhouse effect and also breaks down ozone molecules in the stratosphere. - Standard environmental science knowledge about greenhouse gases and ozone depleting substances is assumed. Concept / Approach: CCl2F2 is a type of chlorofluorocarbon, often designated as CFC 12. Chlorofluorocarbons are stable in the lower atmosphere but break down under ultraviolet radiation in the stratosphere, releasing chlorine atoms. These chlorine atoms catalyse the destruction of ozone, thinning the ozone layer. CFCs also act as strong greenhouse gases because they absorb infrared radiation and persist for long periods. In contrast, ammonia (NH3) is not a major ozone depleting gas, CH2N2 (diazomethane) is a reactive laboratory reagent not typically discussed as a greenhouse gas, and BBr3 (boron tribromide) is a specialised chemical used in industry, not a known greenhouse or ozone depleting gas at global scale. Step-by-Step Solution: Step 1: Identify CCl2F2 as a chlorofluorocarbon, a class of compounds once widely used in refrigeration and aerosol propellants. Step 2: Remember that CFCs are important greenhouse gases and also destroy ozone when they release chlorine atoms under ultraviolet light in the stratosphere. Step 3: Consider NH3, which is ammonia used in fertilisers and refrigeration but is not a primary ozone depleting substance. Step 4: Consider CH2N2, a highly reactive laboratory gas, not recognised as a significant greenhouse gas or ozone depleting substance in atmospheric science. Step 5: Consider BBr3, a specialist reagent, again not listed among major greenhouse gases or ozone depleting chemicals affecting global climate. Step 6: Conclude that CCl2F2 is the only gas among the options that is both a greenhouse gas and contributes to ozone layer depletion. Verification / Alternative check: Environmental reports and textbooks on atmospheric chemistry discuss chlorofluorocarbons in detail, especially CFC 11 and CFC 12. They explain that these compounds are potent greenhouse gases with long atmospheric lifetimes and that they are responsible for significant ozone depletion, which led to international agreements such as the Montreal Protocol to phase them out. Such references do not treat ammonia, diazomethane, or boron tribromide as major global ozone depleting greenhouse gases. This consistent documentation confirms CCl2F2 as the correct answer. Why Other Options Are Wrong: NH3: While ammonia can have environmental impacts, it is not classified as a principal ozone depleting greenhouse gas in global climate discussions. CH2N2: Diazomethane is a laboratory reagent that is toxic and explosive, but it is not a significant atmospheric pollutant in the way CFCs are. BBr3: Boron tribromide is used in chemical synthesis and does not have the widespread atmospheric presence needed to be a major greenhouse or ozone depleting gas. Common Pitfalls: Students may focus on the presence of nitrogen or bromine and guess that any compound containing these elements could affect the atmosphere. Another mistake is to overlook the famous role of CFCs in ozone depletion and instead choose a less familiar formula. A good strategy is to remember that many ozone depleting substances are chlorofluorocarbons and halons, and that formulas with both chlorine and fluorine like CCl2F2 are classic examples of such gases. Final Answer: The gas that acts as a greenhouse gas and can deplete the ozone layer is CCl2F2, a chlorofluorocarbon commonly known as CFC 12.

Question 18

According to Dalton's atomic theory, the smallest indivisible particle of matter that takes part in a chemical reaction is called what?

Accepted Answer

Introduction / Context: Dalton proposed one of the earliest modern atomic theories to explain the laws of chemical combination, such as the law of conservation of mass and the law of constant proportions. His theory introduced the idea that matter is made up of very small, indivisible particles. This question asks what name Dalton gave to those smallest particles that participate in chemical reactions. Understanding this term is foundational for further study of atomic structure and chemistry. Given Data / Assumptions: - The theory under consideration is Dalton atomic theory, developed in the early nineteenth century. - Dalton described matter as composed of tiny indivisible particles which cannot be created or destroyed in chemical reactions. - The question seeks the term used for these smallest particles in his model. - Options provided are atom, molecule, compound, and element. Concept / Approach: Dalton proposed that all matter consists of indivisible particles called atoms. According to his theory, atoms of a given element are identical in mass and properties, and chemical reactions involve rearrangement of these atoms. Molecules, compounds, and elements are related concepts, but the smallest indivisible particle in Dalton view is specifically the atom. Molecules are combinations of atoms, compounds are substances containing atoms of different elements, and an element is a pure substance made of only one type of atom. Therefore, the correct answer is atom. Step-by-Step Solution: Step 1: Recall the key postulates of Dalton atomic theory, especially that matter is made of very small particles that cannot be subdivided further in chemical processes. Step 2: Remember that Dalton named these small particles atoms, borrowing from earlier philosophical ideas but giving them a scientific basis. Step 3: Distinguish the term atom from molecule, which refers to two or more atoms bonded together, such as O2 or H2O. Step 4: Recognise that compound refers to a substance composed of atoms of different elements chemically combined in fixed proportions. Step 5: Recognise that element is a pure substance consisting of only one kind of atom, but the smallest particle itself is still called the atom. Step 6: Conclude that atom is the term used by Dalton for the smallest indivisible particle of matter. Verification / Alternative check: Textbooks in chemistry summarise Dalton theory by listing atoms as the fundamental particles that cannot be created, divided, or destroyed in chemical reactions. They also state that atoms combine in simple whole number ratios to form compounds. Historical discussions emphasise that Dalton revived and refined the ancient idea of indivisible atoms and provided quantitative evidence. Molecules and compounds are discussed later as structures formed from atoms. This hierarchy, consistently presented in educational resources, confirms that atom is the correct answer to the question. Why Other Options Are Wrong: Molecule: This is made up of one or more atoms, and is therefore not the smallest indivisible particle in Dalton sense. Compound: A compound consists of molecules or formula units containing atoms of different elements, so it is not the smallest particle either. Element: An element is a substance made of only one kind of atom, but the smallest unit that retains the chemical identity of the element is the atom itself, not the abstract concept of the element. Common Pitfalls: Students sometimes confuse atom and molecule, especially when learning about diatomic gases such as hydrogen and oxygen. They may think that the molecule is the smallest particle because it is the smallest unit of a substance that can exist independently. However, Dalton specifically focused on indivisible units in chemical reactions, which he called atoms. Modern science has discovered subatomic particles, but Dalton model remains a useful starting point. Keeping the definitions of atom, molecule, compound, and element clear helps avoid this confusion. Final Answer: According to Dalton atomic theory, the smallest indivisible particle of matter that takes part in a chemical reaction is called an Atom.

Question 19

Which one of the following elements has an atomic number greater than that of Calcium (atomic number 20)?

Accepted Answer

Introduction / Context: The periodic table arranges elements in order of increasing atomic number, which is the number of protons in the nucleus of an atom. Questions that compare atomic numbers help students become familiar with this ordering. Calcium has atomic number 20, and the question asks which listed element has an atomic number greater than that of calcium. Recognising approximate positions of common elements in the periodic table is an important skill for chemistry exams. Given Data / Assumptions: - Calcium has atomic number 20, which is taken as a reference point. - Four elements are provided: scandium, argon, chlorine, and sulphur. - The task is to identify which of these has an atomic number higher than 20. - Standard periodic table atomic numbers are assumed: sulphur 16, chlorine 17, argon 18, scandium 21. Concept / Approach: Elements are ordered in the periodic table by atomic number from left to right and top to bottom. Sulphur (S) has atomic number 16, chlorine (Cl) has 17, argon (Ar) has 18, calcium (Ca) has 20, and scandium (Sc) has 21. Among the options, only scandium lies after calcium in the periodic table and therefore has a greater atomic number. The others have smaller atomic numbers and are located in periods before or at the same level as calcium, but to the left. Step-by-Step Solution: Step 1: Recall the atomic number of sulphur, which is 16. Step 2: Recall the atomic number of chlorine, which is 17. Step 3: Recall the atomic number of argon, which is 18. Step 4: Recall the atomic number of calcium, given as 20. Step 5: Recall that scandium, which follows calcium in the periodic table, has atomic number 21. Step 6: Compare each atomic number with 20 and conclude that only scandium has an atomic number greater than calcium. Verification / Alternative check: Looking at a periodic table confirms the sequence of elements by atomic number: argon (18), potassium (19), calcium (20), and scandium (21). This shows that scandium directly follows calcium as the next element. Sulphur and chlorine appear earlier in the same period as argon, with atomic numbers 16 and 17. Therefore, they are clearly less than 20. This visual check on the periodic table aligns with the numerical comparison and verifies that scandium is the correct answer. Why Other Options Are Wrong: Argon: Atomic number 18, which is less than 20 and hence not greater than calcium. Chlorine: Atomic number 17, again less than 20 and located earlier in the same period. Sulphur: Atomic number 16, also less than 20 and clearly before chlorine and argon in the periodic table. Common Pitfalls: A frequent mistake is to confuse atomic mass with atomic number, especially because some heavier elements with lower atomic numbers may have greater relative atomic masses. Another error is to misremember the order of elements around calcium, leading to wrong conclusion that argon lies after calcium. To avoid such mistakes, students should memorise at least the first twenty or thirty elements in sequence and repeatedly practice reading the periodic table so that patterns become familiar. Final Answer: Among the given options, the element whose atomic number is greater than that of calcium is Scandium, which has atomic number 21.

Question 20

Which one of the following statements about acids in aqueous solution is incorrect?

Accepted Answer

Introduction / Context: Acids are a fundamental class of compounds in chemistry, and their properties are studied early in school curricula. Questions that list several statements about acids and ask which one is incorrect help reinforce a clear understanding of these properties. This question focuses on acids in aqueous solution and includes statements about ion formation, hydrogen content, litmus behaviour, and electrical conductivity. Identifying the incorrect statement requires knowledge of how acids behave when dissolved in water. Given Data / Assumptions: - The discussion is about acids in aqueous solution, that is, acids dissolved in water. - Four statements describe typical properties of acids: ionisation, composition, effect on litmus, and electrical conductivity. - Only one statement is incorrect and must be identified. - Standard Arrhenius theory of acids and basic properties of acid solutions are assumed. Concept / Approach: According to the Arrhenius concept, acids are substances that produce hydrogen ions in aqueous solution. Many common acids indeed contain hydrogen in their formulas, such as hydrochloric acid and sulphuric acid. Acids turn blue litmus paper red, which is a standard laboratory test. As for conductivity, aqueous solutions of acids contain ions and therefore conduct electricity well, especially when the acid is strong. Thus, any statement claiming that acids are bad conductors of electricity in aqueous solution contradicts this understanding and must be the incorrect one. The other statements correctly describe known properties of acids in water. Step-by-Step Solution: Step 1: Evaluate the statement that acids give hydrogen ions in aqueous solution. This matches the Arrhenius definition and is correct. Step 2: Evaluate the statement that most acids contain hydrogen. Common acids such as hydrochloric, nitric, acetic, and sulphuric acids all contain hydrogen, so this statement is broadly correct at basic level. Step 3: Evaluate the statement that acids turn blue litmus red. This is a standard indicator test and is accurate. Step 4: Evaluate the statement that acids are bad conductors of electricity in aqueous solution. In fact, acid solutions are usually good conductors because they contain mobile ions. Step 5: Conclude that the statement about poor electrical conductivity is incorrect and should be selected as the answer. Verification / Alternative check: Simple experiments in school laboratories use an electric circuit with a bulb or LED to test conductivity of solutions. Distilled water shows almost no conduction, while dilute hydrochloric acid or sulphuric acid makes the bulb glow brightly, demonstrating good conductivity. Textbooks describe acid solutions as electrolytes that allow electric current to pass. They also emphasise that blue litmus turns red in acidic solutions and that acids contain hydrogen atoms that ionise in water. These multiple sources of evidence confirm that only the statement about acids being bad conductors in aqueous solution is wrong. Why Other Options Are Wrong (as answers): They give H+ ions in aqueous solution: This is correct and is part of the definition of acids in the Arrhenius model. Most acids contain hydrogen in their molecules: This is correct for common acids such as hydrochloric, nitric, acetic, and sulphuric acids at the school level. They turn blue litmus paper red: This is also correct, reflecting a standard property of acids in indicator tests. Common Pitfalls: Learners sometimes assume that only strong acids conduct electricity well and that weak acids do not conduct at all, leading them to incorrectly accept the statement that acids are bad conductors. In reality, even weak acid solutions conduct electricity, though not as strongly as solutions of strong acids. Another pitfall is to over analyse unusual acids at higher levels and question the statement that most acids contain hydrogen. In the context of general school science, that statement is intended to be correct. Always pay attention to the basic level at which the question is framed. Final Answer: The incorrect statement is They are bad conductors of electricity in aqueous solution, because acid solutions actually conduct electricity well due to the presence of ions.

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