Question 1

Which type of glass is commonly used for making chemical laboratory apparatus such as beakers, flasks, and test tubes because of its resistance to heat and chemical attack?

Accepted Answer

Introduction / Context: Laboratory glassware such as beakers, flasks, test tubes, and measuring cylinders must withstand heating, cooling, and contact with a variety of chemicals without breaking or reacting. Not all types of glass are suitable for this purpose. Understanding which glass is used in chemical apparatus helps explain why some kitchen glassware cannot be safely used over a flame, while laboratory glassware can. This question asks you to identify the type of glass commonly used for chemical apparatus. Given Data / Assumptions: The options are potash glass, hard glass, Jena glass, and soda glass. Chemical apparatus must resist heat shock and chemical corrosion. We assume standard terminology where “hard glass” refers to borosilicate glass used in laboratories. Soda glass is the ordinary glass used in window panes and bottles. Concept / Approach: Hard glass is a type of borosilicate glass containing silica and boron compounds. It has a low coefficient of thermal expansion, which means it can withstand rapid temperature changes without cracking, and it is more resistant to chemical attack than ordinary soda-lime glass. This makes it ideal for laboratory apparatus that may be heated over a flame or come into contact with acids and alkalis. Potash glass and Jena glass are specialised glass types, while soda glass is soft glass used in everyday items like bottles and window panes, which cannot tolerate high thermal shock. Therefore, for typical school and college laboratories, hard glass (borosilicate glass) is used to make beakers and flasks. Step-by-Step Solution: Step 1: Consider the requirements of laboratory glassware. It must endure heating, cooling, and contact with chemicals without cracking or reacting. Step 2: Recall the common laboratory glass type. Borosilicate glass, often branded as Pyrex or similar names, is used for laboratory items and is referred to in textbooks as hard glass. Step 3: Evaluate soda glass. Soda-lime glass is cheaper and used for bottles and window panes but is not suitable for direct heating. Step 4: Recognise that potash glass and Jena glass are not the standard answer in basic courses for common chemical apparatus. Step 5: Select “Hard glass” as the correct type for beakers and flasks. Verification / Alternative check: Descriptions of laboratory equipment often mention that beakers, flasks, and test tubes are made from borosilicate or hard glass to resist thermal shock and chemical attack. Safety instructions warn against using ordinary soda glass on burners because it may crack or shatter. Potash glass (potassium glass) and Jena glass are mentioned less frequently at school level, whereas “hard glass” is the typical textbook term. This pattern confirms that hard glass is the type generally associated with chemical apparatus. Why Other Options Are Wrong: Option A (Potash glass): Contains potassium oxide; it has some specialised uses but is not the standard term given for general laboratory glassware in school texts. Option C (Jena glass): A type of heat-resistant glass developed in Jena, Germany, but not the common exam answer for typical beakers and flasks. Option D (Soda glass): Ordinary soft glass used for bottles and windows; it cannot withstand strong heating and rapid temperature changes like hard glass can. Common Pitfalls: Students sometimes choose soda glass because it is the most familiar type or because they see it used in everyday containers. Another source of confusion is the presence of several technical glass names; without recalling which is linked to labware, they may guess randomly. To avoid this, remember that hard glass (borosilicate glass) is used in laboratories, while soda-lime glass is used in everyday items and is not flame-proof. Final Answer: Chemical apparatus like beakers and flasks are commonly made from Hard glass (borosilicate glass).

Question 2

Copper has atomic number 29. The atomic number of which of the following elements is greater than that of copper, meaning it appears later in the periodic table?

Accepted Answer

Introduction / Context: The periodic table arranges elements in order of increasing atomic number, which is the number of protons in the nucleus. Being able to compare atomic numbers of common elements helps you quickly determine their relative positions and understand periodic trends. This question asks you to identify which element among the options has an atomic number greater than that of copper, which is 29, and therefore lies to the right or below copper in the table. Given Data / Assumptions: The atomic number of copper (Cu) is given as 29. The options are iron (Fe), chromium (Cr), zinc (Zn), and manganese (Mn). We assume standard periodic table values for atomic numbers. We compare each element's atomic number with 29. Concept / Approach: Atomic number is unique for each element and increases as you move from left to right across a period and down groups in the periodic table. For the 3d transition series, chromium has atomic number 24, manganese 25, iron 26, cobalt 27, nickel 28, copper 29, and zinc 30. From this sequence, it is clear that zinc has atomic number 30, which is greater than 29. The other elements listed—chromium, manganese, and iron—all have atomic numbers less than 29, so they appear before copper in the periodic sequence. Step-by-Step Solution: Step 1: Recall atomic numbers for the given elements. Chromium (Cr): 24 Manganese (Mn): 25 Iron (Fe): 26 Zinc (Zn): 30 Step 2: Compare each atomic number with that of copper (29). 24, 25, and 26 are all less than 29, so Cr, Mn, and Fe have atomic numbers below that of copper. 30 is greater than 29, so Zn has an atomic number greater than copper. Step 3: Conclude that zinc is the only element in the list with atomic number greater than 29. Verification / Alternative check: If you look at the 4th period transition metal row of the periodic table, you will see the sequence: Sc (21), Ti (22), V (23), Cr (24), Mn (25), Fe (26), Co (27), Ni (28), Cu (29), Zn (30). This ordering by atomic number confirms that copper is followed immediately by zinc, while iron, chromium, and manganese appear earlier. Atomic number is fundamental and does not change, so any reliable periodic table will confirm these values and the conclusion that zinc has the higher atomic number. Why Other Options Are Wrong: Option A (Iron): Atomic number 26, which is less than 29; iron appears before copper in the periodic table. Option B (Chromium): Atomic number 24; earlier in the same period than copper. Option D (Manganese): Atomic number 25; also earlier than copper. Common Pitfalls: Students sometimes confuse relative positions because they memorise elements alphabetically or by common usage rather than by atomic number. Another mistake is to misremember the exact atomic numbers of transition metals, which are closely spaced. A helpful strategy is to memorise short sequences, such as the 3d series from Sc to Zn, or to remember a few key reference points like Fe (26), Cu (29), and Zn (30) so you can quickly compare nearby elements. This makes questions about “greater than” or “less than” atomic numbers much easier to answer. Final Answer: The element whose atomic number is greater than that of copper is Zinc.

Question 3

Which of the following gases is commonly used as a fuel to generate a high-temperature flame for producing light and for welding or cutting metals in oxy-fuel welding?

Accepted Answer

Introduction / Context: Oxy-fuel welding and cutting processes rely on burning a fuel gas in oxygen to produce a very high-temperature flame capable of melting or cutting metals. Different fuel gases can be used, but one of the most common in traditional gas welding is acetylene. Understanding which compounds are gases used as fuels and which are liquids or solids used for other purposes helps in both chemistry and basic engineering questions. This question asks you to identify the gas used to generate light and to weld metals. Given Data / Assumptions: The options are ethylene, acetylene, glycol, and oxalic acid. We are looking for a fuel gas suitable for generating a high-temperature flame. The flame should be suitable for both illumination (historically) and metal welding or cutting. We assume standard industrial and textbook descriptions of oxy-acetylene welding. Concept / Approach: Acetylene (C2H2) is a hydrocarbon gas that burns in oxygen to form a very hot flame, reaching temperatures high enough for welding and cutting steel. The oxy-acetylene flame can exceed 3000 degrees Celsius and has been widely used in metal fabrication. Ethylene (C2H4) is also a gas and burns with a flame, but it is not the standard gas for welding in basic exam questions. Glycol and oxalic acid are not gases; glycol is a liquid (for example, ethylene glycol used in antifreeze), and oxalic acid is a solid organic acid used in cleaning and bleaching. Therefore, acetylene is the fuel gas traditionally associated with high-temperature welding flames and “carbide lamps” used historically for light. Step-by-Step Solution: Step 1: Identify which substances are gases at room temperature. Ethylene and acetylene are gases; glycol is a liquid and oxalic acid is a solid. Step 2: Recall common welding gas names. The common gas used in traditional gas welding is acetylene in combination with oxygen (oxy-acetylene flame). Step 3: Consider historical use for light. Acetylene was also used in lamps (carbide lamps) for bright illumination in mines and bicycles. Step 4: Match this with the options and select “Acetylene”. Verification / Alternative check: Engineering and welding handbooks describe oxy-acetylene welding as one of the earliest and most widely used gas welding methods. The fuel gas is generated from acetylene cylinders or from reaction of calcium carbide with water. The high flame temperature and controllable flame characteristics make it suitable for welding and cutting. Ethylene is more commonly discussed in polymer chemistry as a monomer for polyethylene rather than as a primary welding gas. Glycol and oxalic acid are not fuel gases at all. This clear association between acetylene and welding confirms it as the correct answer. Why Other Options Are Wrong: Option A (Ethylene): Although it is a combustible gas, ethylene is not the classic welding gas referred to in basic exam questions; its main use is as a petrochemical feedstock. Option C (Glycol): Glycol is a liquid (for example, ethylene glycol) used as antifreeze and in plastics production, not as a welding gas. Option D (Oxalic acid): Oxalic acid is a solid organic acid used in cleaning and bleaching; it is not a fuel gas and cannot produce a welding flame. Common Pitfalls: Students may confuse ethylene and acetylene because their names sound similar and both are small hydrocarbons. Another mistake is not recognising that glycol and oxalic acid are not gases at room temperature. To avoid confusion, remember the phrase “oxy-acetylene welding”, which directly links acetylene with welding, and note that acetylene was also historically used in lamps to generate bright light. Final Answer: The gas used to generate light and to weld metals is Acetylene.

Question 4

Which poisonous industrial gas leaked during the Bhopal Gas Tragedy in December 1984 in India, causing widespread deaths and long term health damage?

Accepted Answer

Introduction / Context: The Bhopal Gas Tragedy of December 1984 is one of the worst industrial disasters in history. It occurred in Bhopal, India, and involved the accidental release of a highly toxic industrial gas from a pesticide plant. This question tests knowledge of environmental chemistry and industrial safety by asking which specific gas leaked during this tragedy. Remembering the name of the gas helps link chemical hazards to real world consequences. Given Data / Assumptions: The disaster occurred in December 1984 in Bhopal, Madhya Pradesh, India. The leak happened at a pesticide manufacturing plant. The gas was used as an intermediate chemical in the production of certain carbamate pesticides. The gas was extremely toxic, volatile, and dangerous when inhaled. Concept / Approach: The pesticide plant involved in the Bhopal disaster stored large quantities of methyl isocyanate, often abbreviated as MIC. Methyl isocyanate is a reactive and highly poisonous chemical used to manufacture carbamate pesticides. Due to poor maintenance and safety failures, water entered a storage tank containing MIC, leading to a runaway reaction and the release of a massive cloud of this gas over the nearby densely populated areas. By recalling this well documented incident, we can identify methyl isocyanate as the gas that leaked during the Bhopal Gas Tragedy. Step-by-Step Solution: Step 1: Recall the industrial setting of the Bhopal Gas Tragedy. It was a pesticide factory that used methyl isocyanate as a key raw material. Step 2: Remember that the disaster involved a sudden and uncontrolled release of a toxic gas stored in large underground tanks. Step 3: Identify methyl isocyanate (MIC) as the specific gas that was stored in those tanks and that escaped into the atmosphere. Step 4: Confirm that other named compounds such as methyl phosphate or methyl isochlorate are not historically associated with this particular tragedy. Step 5: Conclude that methyl isocyanate is the correct answer. Verification / Alternative check: Accounts of the Bhopal disaster consistently mention methyl isocyanate as the gas involved. MIC is known for its low boiling point, high volatility, and extreme toxicity. In Bhopal, thousands of people died within hours due to exposure, and many more suffered long term health effects such as respiratory problems, eye damage, and birth defects. Official reports, textbooks on environmental chemistry, and case studies on industrial safety all confirm that methyl isocyanate was the chemical released, not other similarly named compounds. Why Other Options Are Wrong: Methyl isochlorate: A different compound name that is not historically linked with the Bhopal incident. Methyl phosphate: This name suggests a phosphorus containing compound and does not match the known gas involved. Methyl isopropate: This is not the standard name for the chemical used in the tragedy and is not the gas that leaked. Methyl carbamate: Related to carbamate pesticides but not the specific toxic gas that escaped in Bhopal. Common Pitfalls: Students sometimes confuse similar sounding chemical names, especially when they share the methyl prefix. Another mistake is to focus only on the trade names of pesticides rather than the intermediate chemicals used in their production. To avoid confusion, remember that the Bhopal Gas Tragedy is specifically associated with methyl isocyanate, often abbreviated as MIC. Recognising the importance of proper storage and handling of such hazardous chemicals is also a key learning point from this event. Final Answer: The poisonous gas that leaked in the Bhopal Gas Tragedy was Methyl isocyanate.

Question 5

The oceanic crust of the Earth mainly consists of silica and which other major component, often summarised together as sima in basic geology?

Accepted Answer

Introduction / Context: Earth science and basic chemistry often overlap when we discuss the composition of the Earths crust. The crust is divided into continental and oceanic types, each with characteristic chemical makeup. This question asks about the main components of oceanic crust, focusing on the common shorthand terms used in school level geology, such as sial and sima. Understanding these terms helps students connect elemental chemistry with large scale geological structures. Given Data / Assumptions: The crust of the Earth is divided into continental crust and oceanic crust. Continental crust is often described as rich in silica and aluminium (sial). Oceanic crust is often described as rich in silica and another major element (sima). The question asks specifically for the second major component of oceanic crust along with silica. Concept / Approach: In school level geology, the terms sial and sima are used to summarise the composition of different parts of the crust. Sial refers to silica (SiO2) and aluminium rich rocks that dominate continental crust. Sima refers to silica and magnesium rich rocks that dominate oceanic crust. The oceanic crust is mainly composed of basaltic rocks, which contain significant amounts of magnesium and iron silicates. However, in the traditional shorthand, the emphasis is on silica and magnesium, hence the term sima (silica plus magnesium). Therefore the correct answer is magnesium. Step-by-Step Solution: Step 1: Recall that continental crust is described as sial, meaning it is rich in silica and aluminium. Step 2: Remember that oceanic crust is described as sima, meaning it is rich in silica and magnesium. Step 3: Note that the question mentions silica as one component and asks for the other major component of oceanic crust. Step 4: From the term sima, identify magnesium as the second key component. Step 5: Conclude that oceanic crust mainly consists of silica and magnesium. Verification / Alternative check: Basalt, which is the dominant rock type forming the oceanic crust, contains minerals such as pyroxenes and olivine that are rich in magnesium and iron silicates. While iron is also important, the traditional classification emphasises magnesium when defining sima. Textbooks typically state that the oceanic crust is mainly made of silica and magnesium rich rocks, confirming that magnesium is the intended answer. Thus, the combination of silica and magnesium correctly summarises the average chemistry of the oceanic crust at basic study level. Why Other Options Are Wrong: Iron: Although oceanic crust does contain iron bearing minerals, the standard sima term is derived from silica and magnesium, not silica and iron. Manganese: Present only in minor amounts and not used as a defining component of crustal composition. Sulphur: Mostly associated with volcanic gases and sulphide minerals, not a primary component of crustal rocks. Calcium: Found in certain minerals such as plagioclase feldspar but not the main element used in the sima classification. Common Pitfalls: Students sometimes confuse the roles of magnesium and iron in basaltic rocks and may think that iron should be the second main component. Another common error is mixing up sial and sima, forgetting which one is associated with continental crust and which with oceanic crust. A simple memory aid is that sial has al for aluminium and sima has ma for magnesium. Keeping this relationship clear helps in answering similar questions correctly. Final Answer: The oceanic crust mainly consists of silica and magnesium, commonly summarised together as sima.

Question 6

When solid magnesium oxide (MgO) reacts with water (H2O), which hydroxide compound is formed as the main product of this combination reaction?

Accepted Answer

Introduction / Context: This question is about a simple inorganic reaction between a basic oxide and water. Magnesium oxide is a metal oxide that shows basic character. When it reacts with water, it forms a metal hydroxide. Understanding this type of reaction helps students recognise general patterns in the chemistry of metal oxides and hydroxides, and it reinforces the idea that basic oxides often produce alkaline solutions when dissolved in water. Given Data / Assumptions: Reactants: magnesium oxide (MgO) and water (H2O). The reaction is a combination type reaction between an oxide and water. We are looking for the correct empirical formula of the magnesium hydroxide product. Magnesium has valency 2 in its common compounds. Concept / Approach: Magnesium oxide is a basic oxide of a divalent metal. When a metal oxide like MgO dissolves or reacts with water, it forms the corresponding metal hydroxide. For magnesium, the common hydroxide is magnesium hydroxide with the formula Mg(OH)2. The balanced chemical equation is MgO + H2O gives Mg(OH)2. In this compound, magnesium has a charge of +2 and each hydroxide ion OH has a charge of -1, so two hydroxide ions are required to balance the charge on one magnesium ion. Step-by-Step Solution: Step 1: Identify the type of reaction. It is a reaction of a basic oxide with water, usually producing a hydroxide. Step 2: Recall the valency of magnesium. Magnesium commonly forms Mg2+ ions in its compounds. Step 3: Consider the hydroxide ion OH, which carries a charge of -1 in ionic compounds. Step 4: To balance the +2 charge on Mg2+, two OH- ions are needed. This gives the formula Mg(OH)2 for the neutral hydroxide. Step 5: Write the overall reaction: MgO + H2O gives Mg(OH)2, which is the correct product. Verification / Alternative check: Another way to verify the formula is to check charge balance and atom balance. In Mg(OH)2, there is one magnesium atom, two oxygen atoms, and two hydrogen atoms. Combining MgO and H2O gives exactly that composition: one Mg, two O, and two H atoms. The ions Mg2+ and 2 OH- combine to give a neutral compound. Magnesium hydroxide is also known in everyday life as the active ingredient in some antacid medicines, and its formula is widely given as Mg(OH)2 in textbooks and product information. This independent confirmation agrees with the reasoning above. Why Other Options Are Wrong: Mg(OH): This would imply a +1 charge on magnesium, which is incorrect for its usual chemistry and does not form a stable neutral compound. Mg2(OH): This formula is not charge balanced and does not match the stoichiometry of the reaction with water. Mg(O2H)2: This is not a standard way to write the hydroxide and misrepresents the arrangement of the atoms. Mg2(OH)2: This would suggest two magnesium ions and two hydroxide groups, which would not be neutrally balanced without additional counter ions. Common Pitfalls: Students sometimes forget to match the valency of the metal with the number of hydroxide groups and may try to guess the formula by trial. Another mistake is to misplace brackets when writing hydroxide formulas, leading to confusion about how many OH groups are present. A good habit is to first write the charges of the ions, cross multiply them to get the correct ratio, and then check the total number of each atom and the overall neutrality of the compound. Final Answer: When magnesium oxide reacts with water, the main product formed is Mg(OH)2, magnesium hydroxide.

Question 7

In common food chemistry, which organic acid is mainly responsible for the sour taste of grapes and is naturally present in significant amounts in grape juice?

Accepted Answer

Introduction / Context: Many fruits have a characteristic sour taste due to the presence of specific organic acids. Knowing which acid is associated with which fruit is a common question in general science and food chemistry. This question focuses on grapes and asks which acid is mainly responsible for their sour taste. Understanding this link helps students relate chemistry to everyday foods and beverages. Given Data / Assumptions: We are dealing with the natural acid content of grapes. Several common organic acids are listed, each associated with different foods. The question asks for the main acid in grapes, not in other fruits or fermented products. No fermentation is assumed, so wine making processes are not the focus, only fresh grapes. Concept / Approach: Grapes are known to contain tartaric acid as one of their principal organic acids, along with smaller amounts of malic acid. Tartaric acid contributes significantly to the sour and tart flavour of grapes and grape juice. The sediment sometimes observed in wine barrels and bottles, called wine crust or argol, is largely potassium hydrogen tartrate, which is a salt of tartaric acid. Other acids such as lactic, acetic, and formic acids are associated with milk fermentation, vinegar, and insect stings respectively, not primarily with fresh grapes. Step-by-Step Solution: Step 1: Recall that tartaric acid is commonly cited as the characteristic acid of grapes and wine. Step 2: Note that lactic acid is mainly produced by bacteria in milk and is associated with curd and sour dairy products. Step 3: Recognise that acetic acid is the main acid in vinegar, obtained by oxidation of ethanol. Step 4: Formic acid is associated with ant stings and bee stings rather than with fruits like grapes. Step 5: Therefore, identify tartaric acid as the primary acid responsible for the sour taste of grapes. Verification / Alternative check: Food chemistry references list tartaric acid as a major acid in grapes at concentrations that significantly influence the taste. Malic acid is also present, but tartaric acid is emphasised in school exams and in winemaking literature. The presence of potassium hydrogen tartrate crystals in grape products further confirms the important role of tartaric acid. On the other hand, lactic, acetic, and formic acids are emphasised in different contexts and are not described as the main acids in grapes. This agreement among multiple sources supports the choice of tartaric acid as the correct answer. Why Other Options Are Wrong: Lactic acid: Main acid in fermented milk products such as curd and yoghurt, not in fresh grapes. Formic acid: Found in stings of ants and some insects, not a major component of grape juice. Acetic acid: Responsible for the sourness of vinegar, produced from ethanol, not the principal acid in grapes. Oxalic acid: Present in some leafy vegetables such as spinach, not commonly cited as the characteristic acid of grapes. Common Pitfalls: Students may confuse tartaric acid with acetic or lactic acid because all of them are organic acids with similar sounding names. Another common mistake is to memorise only one or two fruit acid associations and then guess for others. A helpful study technique is to create a small table: citric acid with citrus fruits like lemon and orange, tartaric acid with grapes, lactic acid with milk products, and acetic acid with vinegar. Reviewing this table regularly makes it easier to answer such questions accurately. Final Answer: The acid mainly responsible for the sour taste of grapes is Tartaric acid.

Question 8

Limestone is an important mineral raw material. In basic industrial chemistry, limestone is primarily used as a key raw material by which of the following industries?

Accepted Answer

Introduction / Context: Limestone is a sedimentary rock rich in calcium carbonate and is widely used as a raw material in many industries. Recognising which industry depends most heavily on limestone is a common question in general science and economic geology. This question asks which industry primarily uses limestone as a main raw material, highlighting the link between natural resources and industrial products such as building materials. Given Data / Assumptions: Limestone is mainly composed of calcium carbonate (CaCO3). It is quarried in large quantities in many parts of the world. Several industries may use limestone in smaller amounts, but one industry uses it as a major feedstock. We consider typical industrial processes taught at school level. Concept / Approach: The cement industry, especially Portland cement manufacture, depends heavily on limestone as its chief raw material. In a cement plant, limestone is mixed with clay or shale and heated in a rotary kiln to form clinker, which is then ground to make cement. The calcium component of cement mainly comes from the calcium carbonate in limestone. While limestone may have minor uses in other industries, such as as a flux in metallurgy or as a soil conditioner, its principal industrial role at basic level is in cement production. Step-by-Step Solution: Step 1: Identify the main composition of limestone as calcium carbonate, CaCO3. Step 2: Recall that in the manufacture of cement, particularly Portland cement, limestone is heated with clay in a kiln to form clinker. Step 3: Understand that this clinker, rich in calcium silicates, is then ground to produce cement, making limestone a critical raw material. Step 4: Consider other industries. Aluminium production mainly depends on bauxite, not limestone. Fertiliser industries depend on raw materials like natural gas, sulphur, phosphates, and ammonium compounds. Petrochemicals are derived from crude oil and natural gas. Step 5: Conclude that the cement industry is the primary industry that uses limestone as a main raw material. Verification / Alternative check: Industrial chemistry and geography textbooks typically list limestone deposits alongside cement factories, emphasising that cement plants are often located near limestone quarries to minimise transport costs. Process flow diagrams of cement manufacturing begin with crushing and grinding limestone. Although limestone can be used in other sectors, such as in the production of lime (CaO), glass, and as a filler in some products, these uses either support the cement industry or are not as dominant in basic curricula. This reinforces that cement is the intended answer. Why Other Options Are Wrong: Aluminium: The main ore is bauxite (hydrated aluminium oxide), not limestone. Fertiliser: Uses raw materials like ammonia, phosphates, potash, natural gas, and sulphur, not primarily limestone. Petrochemicals: Derived from petroleum and natural gas fractions, not from limestone. Paper: Uses wood pulp and other chemicals such as alum and bleaching agents; limestone may have minor uses but is not the primary raw material. Common Pitfalls: Students may think of limestone as widely used in many industries and may over generalise its importance. While limestone does have diverse applications, exams often focus on its key role in cement manufacture. Another confusion arises between lime (calcium oxide) and limestone (calcium carbonate). Lime is produced by heating limestone, and it has its own uses, but the starting mineral is still most strongly associated with the cement industry. Final Answer: Limestone is primarily used as a key raw material in the cement industry.

Question 9

For isotopes of a given chemical element, which statement about their chemical properties is most appropriate, considering that they have the same atomic number but different mass numbers?

Accepted Answer

Introduction / Context: Isotopes are atoms of the same element that have the same atomic number but different mass numbers due to different numbers of neutrons. This concept is important in both physics and chemistry. The key question for chemists is whether isotopes of an element behave the same way in chemical reactions. This question asks which statement best describes the chemical properties of isotopes of a given element. Given Data / Assumptions: Isotopes have the same atomic number, so they have the same number of protons. They differ only in the number of neutrons and therefore in mass number. Chemical properties depend mainly on electronic configuration and valence electrons. Nuclear properties such as stability may differ, but electronic arrangements remain identical for isotopes of the same element. Concept / Approach: Chemical properties of an element are determined primarily by the arrangement of electrons around the nucleus, especially the number of valence electrons. Isotopes of an element have the same number of protons and electrons, so their electron configurations are identical. Since the electrons are responsible for chemical bonding, isotopes show almost identical chemical behaviour. Differences in mass can lead to small kinetic isotope effects, but at school level and for most reactions, isotopes are considered to have the same chemical properties. Step-by-Step Solution: Step 1: Recall the definition of isotopes: same atomic number (same number of protons), different mass number (different neutrons). Step 2: Note that the number of electrons in a neutral atom equals the number of protons, so isotopes share the same electronic configuration. Step 3: Understand that chemical reactions involve making and breaking bonds by electrons, especially valence electrons. Step 4: Since isotopes have identical electron configurations, they interact with other atoms in essentially the same way during chemical reactions. Step 5: Conclude that their chemical properties must be the same to a very good approximation, which matches the correct option. Verification / Alternative check: Consider examples such as hydrogen isotopes: protium, deuterium and tritium. All have one proton and one electron, so they form similar compounds like H2, D2, and T2 and can form water, heavy water, and tritiated water. Their chemical reactions are very similar, though reaction rates may differ slightly due to mass differences. Similarly, isotopes of chlorine, such as chlorine 35 and chlorine 37, have the same valence electrons and show identical chemistry, forming the same compounds like sodium chloride. This consistent behaviour in many examples verifies that isotopes have the same chemical properties. Why Other Options Are Wrong: Their chemical properties must be different: This contradicts the fact that chemical properties depend on electrons, which are the same for isotopes of one element. Their chemical properties need not be the same and may vary widely: Variations are minor and kinetic in nature, not fundamental differences in chemical identity. Their chemical properties need not be different in any case: This wording is confusing and does not clearly state the equality of properties. Their chemical properties are always unpredictable: This is incorrect; isotopic chemistry is well understood and predictable. Common Pitfalls: Students sometimes focus on the word different in the definition of isotopes and assume that different mass numbers must mean different chemistry. Others may confuse nuclear properties, such as radioactivity, with chemical behaviour. It is important to keep in mind that while isotopes can have different nuclear stability and applications in nuclear medicine or dating, their chemical bonding patterns remain the same because they have the same electronic structure. Final Answer: For isotopes of a given element, their chemical properties must be the same because they have identical electronic configurations.

Question 10

The characteristic sour taste of lemon juice is mainly due to the presence of which organic acid in the fruit?

Accepted Answer

Introduction / Context: Chemistry of everyday life often focuses on the acids present in common fruits and food items. Lemons have a sharp sour taste that most people recognise. This question asks which organic acid is mainly responsible for that sourness. Understanding such associations helps connect textbook chemistry with real experiences in the kitchen and diet. Given Data / Assumptions: The fruit under consideration is lemon. Lemon juice is clearly acidic in nature. Several well known organic acids are listed as options. We are looking for the principal acid naturally present in lemon and other citrus fruits. Concept / Approach: The dominant organic acid in lemons and most citrus fruits, including oranges, limes and grapefruits, is citric acid. Citric acid is a weak organic acid that contributes to the sour taste and preservative properties of citrus juice. It is widely used as a food additive, often labelled with the code E330. Although other acids like acetic, oxalic and formic acid are important in different contexts, they are not the main acids in lemon juice. Step-by-Step Solution: Step 1: Recall that citric acid is named after citrus fruits and is widely associated with their sour taste. Step 2: Remember that lemon juice is a classic example used in textbooks to demonstrate acidity and the presence of citric acid. Step 3: Consider acetic acid, which is the main acid in vinegar, not in lemons. Step 4: Oxalic acid is associated more with leafy vegetables such as spinach and rhubarb than with citrus fruits. Step 5: Formic acid is found in some insect stings and is not a principal component of lemon juice. Therefore, citric acid is the correct choice. Verification / Alternative check: Food chemistry references list citric acid as making up a significant fraction of the acids in lemon juice, often alongside small amounts of other acids. It is also produced industrially and added to many soft drinks and candies to give a sour taste similar to citrus fruits. Laboratory tests that titrate lemon juice with base often assume citric acid as the main acid for calculations. These consistent references confirm that citric acid is the primary acid responsible for the sour taste of lemons. Why Other Options Are Wrong: Acetic acid: Responsible for the sour taste of vinegar, produced by oxidation of ethanol, not characteristic of lemon. Oxalic acid: Found in vegetables like spinach and may contribute to kidney stones, but not the main acid in citrus fruits. Formic acid: Present in ant stings and some insect venoms, not a major acid in lemon juice. Tartaric acid: Characteristic of grapes and tamarind rather than lemons. Common Pitfalls: Students sometimes mix up different fruit acid associations, such as citric acid in lemons and tartaric acid in grapes. Another error is to think that any sour taste must always be due to one common acid. In reality many different organic acids exist, each associated with different foods. Creating a simple table matching citric acid with citrus fruits, tartaric acid with grapes, lactic acid with curd, and acetic acid with vinegar can help reduce such confusion. Final Answer: The sour taste of lemon is mainly due to the presence of Citric acid.

Question 11

Ethylene glycol is an important organic compound. It is used as one of the raw materials in the manufacture of which synthetic fibre known for its use in textiles and plastic bottles?

Accepted Answer

Introduction / Context: Ethylene glycol is a simple dihydric alcohol that has several industrial uses, including as an antifreeze and as a raw material in polymer production. One of its most important roles in polymer chemistry is as a starting material for a widely used synthetic fibre. This question asks which synthetic fibre is manufactured using glycol, linking organic chemistry with materials science and everyday synthetic fabrics. Given Data / Assumptions: The compound in question is glycol, commonly referring to ethylene glycol (HO CH2 CH2 OH). We are looking for a synthetic fibre that uses glycol as a key raw material. The options include several widely known fibres and polymers. The question focuses on school level names rather than detailed polymer structures. Concept / Approach: Terylene is the trade name for polyethylene terephthalate, often abbreviated as PET. Terylene is produced by the condensation polymerisation of ethylene glycol with terephthalic acid or its dimethyl ester. During this reaction, ester linkages form and long chains of PET are produced. Nylon is made from diamines and dicarboxylic acids, artificial silk commonly refers to rayon produced from cellulose, and rubber and polyvinyl chloride have different raw materials. Therefore, the fibre directly associated with glycol as a raw material is terylene. Step-by-Step Solution: Step 1: Identify glycol as ethylene glycol, a dihydric alcohol with two hydroxyl groups. Step 2: Recall that terylene or PET is made by condensing ethylene glycol with terephthalic acid or its dimethyl ester. Step 3: Understand that this condensation reaction forms repeating ester units, creating the polyester fibre terylene. Step 4: Consider nylon, which is produced by combining diamines with dicarboxylic acids such as hexamethylene diamine with adipic acid, not glycol. Step 5: Recognise that artificial silk or rayon is derived from cellulose, and rubber and PVC come from monomers such as isoprene and vinyl chloride respectively. Conclude that terylene is the correct answer. Verification / Alternative check: Polymer chemistry references describe PET as a polyester made from ethylene glycol and terephthalic acid. Terylene fibres are used in clothing, plastic bottles, films, and many other products. Diagrams of the polymerisation process always show glycol as one of the monomers. In contrast, nylon is described as a polyamide and does not list glycol as a key raw material. Rayon processes focus on cellulose, and rubber chemistry focuses on isoprene based monomers. This strong association between glycol and terylene confirms the choice. Why Other Options Are Wrong: Nylon: A polyamide made from diamines and dicarboxylic acids such as hexamethylene diamine and adipic acid, not from glycol. Artificial silk: Usually refers to rayon made from regenerated cellulose, not from glycol. Rubber: Natural rubber is made from isoprene units; synthetic rubbers come from related unsaturated monomers, not from glycol. Polyvinyl chloride: A polymer of vinyl chloride monomer and not produced from ethylene glycol. Common Pitfalls: Students sometimes confuse different synthetic fibres, especially nylon and terylene, because both are used in textiles and appear together in exam questions. Another common mistake is to assume that all synthetic fibres use similar raw materials. The best way to avoid this is to remember a few key pairings: nylon with diamine plus dicarboxylic acid, terylene with glycol plus terephthalic acid, and rayon with cellulose. Associating each fibre with its monomers or starting materials helps in answering such questions accurately. Final Answer: Ethylene glycol is used as a raw material in the manufacture of the synthetic fibre Terylene.

Question 12

Among the following greenhouse gases, which one has the greatest heat trapping ability per molecule and is therefore especially powerful in contributing to global warming?

Accepted Answer

Introduction / Context: Climate change and global warming are closely linked to the presence of greenhouse gases in the atmosphere. Different gases trap heat with different effectiveness. This question asks which greenhouse gas among the options has the greatest heat trapping ability per molecule, often described in terms of global warming potential. Understanding this helps students appreciate why some gases, even at low concentrations, can have a large impact on climate. Given Data / Assumptions: We are comparing greenhouse gases by their heat trapping ability per molecule, not by total quantity in the atmosphere. The options include common greenhouse gases and related compounds. Carbon dioxide is the most abundant anthropogenic greenhouse gas but not the strongest per molecule. Chlorofluorocarbons are present in much smaller amounts but have very high global warming potential. Concept / Approach: Global warming potential is a measure of how much heat a greenhouse gas traps in the atmosphere compared to carbon dioxide over a specific time period. By this measure, many chlorofluorocarbons (CFCs) have extremely high global warming potential, often thousands of times greater than that of carbon dioxide. Methane and nitrous oxide also have higher global warming potential than carbon dioxide, but typically less than many CFCs. Therefore, when asked which gas has the greatest heat trapping ability among the listed options, chlorofluorocarbon is the correct choice. Step-by-Step Solution: Step 1: Recall that carbon dioxide is used as the reference gas with a global warming potential of 1. Step 2: Recognise that methane has a global warming potential many times that of carbon dioxide, and nitrous oxide even higher. Step 3: Understand that many chlorofluorocarbons have global warming potential values in the thousands compared to carbon dioxide. Step 4: Note that although CFCs are less abundant, their strong absorption of infrared radiation and long atmospheric lifetimes make them extremely powerful greenhouse gases per molecule. Step 5: Conclude that among the given options, chlorofluorocarbon has the greatest heat trapping ability. Verification / Alternative check: Environmental science texts often provide tables of global warming potential values for different gases. While the exact numbers vary from one CFC to another, many values are in the range of thousands of times that of carbon dioxide. Methane is commonly in the tens, and nitrous oxide in the hundreds, but CFCs usually exceed these significantly. Carbon dioxide, although very important due to its quantity, has a much lower global warming potential per molecule. This comparison confirms that chlorofluorocarbon is the strongest heat trapping gas among the listed choices. Why Other Options Are Wrong: Nitrous oxide: Stronger than carbon dioxide and methane in warming potential, but generally weaker per molecule than many CFCs. Methane: Has higher heat trapping ability than carbon dioxide but less than nitrogen oxides and most CFCs. Carbon dioxide: The most discussed greenhouse gas due to its abundance, but not the strongest per molecule. Ozone in the lower atmosphere: Acts as a greenhouse gas and pollutant but still does not usually exceed the per molecule heat trapping ability of CFCs. Common Pitfalls: Students often confuse the total impact of a gas, which depends on both its global warming potential and its concentration, with the heat trapping ability per molecule. Because carbon dioxide is discussed so much, some learners assume it must be the strongest greenhouse gas. Others may focus only on methane. To avoid this, it is useful to remember that CFCs, although present in much smaller amounts, have very high global warming potential and therefore are extremely powerful greenhouse gases molecule for molecule. Final Answer: The greenhouse gas with the greatest heat trapping ability per molecule among the options is Chlorofluorocarbon.

Question 13

Iron and manganese sometimes occur as dissolved pollutants in water. Which of the following treatment methods alone cannot effectively remove these dissolved iron and manganese ions from water?

Accepted Answer

Introduction / Context: Water treatment processes are designed to remove different types of impurities, including dissolved metals such as iron and manganese. These metals can cause staining, bad taste, and operational problems in distribution systems. This question asks which water treatment method, when used alone, is not effective for removing dissolved iron and manganese ions. Understanding this helps students differentiate between processes used for disinfection and those used for metal removal. Given Data / Assumptions: The pollutants of concern are dissolved iron and manganese ions in water. Several treatment methods are listed, including physical, chemical, and ion exchange processes. The question asks which method alone cannot remove these metals effectively. We assume typical concentration ranges seen in groundwater or raw water sources. Concept / Approach: Dissolved iron and manganese in water are usually present as ferrous (Fe2+) and manganous (Mn2+) ions. To remove them, they are often oxidised to ferric (Fe3+) and higher oxidation states, which form insoluble hydroxides or oxides that can be removed by settling and filtration. Processes such as aeration, chemical oxidation, lime soda treatment, manganese zeolite filtration, and ion exchange can all be used for this purpose. Chlorination, while excellent for disinfection and oxidation of some substances, is not designed on its own to precipitate and remove iron and manganese unless combined with filtration processes. As a simple stand alone disinfection step, chlorination does not remove dissolved metal ions from the water. Step-by-Step Solution: Step 1: Identify the goal: removal of dissolved iron and manganese ions from water. Step 2: Recall that effective removal usually involves oxidising these ions to form insoluble precipitates, followed by settling and filtration. Step 3: Oxidation followed by settling and filtration is clearly designed to transform dissolved ions into filterable solids and is effective for iron and manganese removal. Step 4: Ion exchange processes can replace iron and manganese ions with harmless ions such as sodium, and thus can remove them. Step 5: Lime soda and manganese zeolite processes are specifically known for removing hardness and for oxidation and filtration of iron and manganese. Step 6: Chlorination by itself mainly serves as a disinfectant, killing bacteria and pathogens. Without additional steps like filtration or precipitation, chlorination alone does not remove dissolved iron and manganese ions from the water. Verification / Alternative check: Standard water treatment schemes often show a sequence of processes: aeration or chemical oxidation, sedimentation, filtration, and then disinfection by chlorination. Iron and manganese removal steps are placed before chlorination. Technical references emphasise that chlorination is primarily for disinfection and, although chlorine is an oxidant, its main use is not to precipitate metals unless specifically engineered with other steps. Moreover, many iron and manganese removal units operate without chlorine, using aeration and filtration instead. This confirms that chlorination alone is not considered an iron and manganese removal method. Why Other Options Are Wrong: Oxidation followed by settling and filtration: This is a standard approach for iron and manganese removal, as it converts dissolved ions into insoluble forms that can be removed. Ion exchange process: Can selectively remove dissolved metal ions by exchanging them with other ions on a resin. Lime soda process or manganese zeolite process: Both are known for treating hard water and for helping remove iron and manganese by precipitation and filtration. Aeration followed by filtration: Aeration can oxidise iron and manganese, and filtration then removes the precipitated solids. Common Pitfalls: Students sometimes think of chlorination as a universal solution for all water quality problems because it is widely used in treatment plants. This leads to the mistaken belief that chlorination removes any pollutant, including metals. It is important to remember that chlorination is primarily for disinfection and sometimes taste and odour control. Physical removal of metals usually requires oxidation combined with settling, filtration, or ion exchange. Keeping this distinction clear helps to choose the correct answer. Final Answer: Dissolved iron and manganese present as pollutants in water cannot be effectively removed by Chlorination alone.

Question 14

When iron objects rust in moist air, how does their weight change overall as a result of the rusting process?

Accepted Answer

Introduction / Context: Rusting of iron is a common example used to illustrate corrosion and basic redox chemistry. When iron objects are exposed to moist air, they gradually form rust, which is mainly hydrated iron oxide. This question focuses on how the weight of an iron object changes as rusting occurs. Understanding this helps students connect mass changes with chemical reactions involving oxygen from the atmosphere. Given Data / Assumptions: Rusting occurs when iron is exposed to oxygen and moisture over time. Rust is a compound of iron and oxygen, often with water molecules. No large flakes of rust are assumed to have broken off and fallen away during the period considered. The question is about the net change in weight of the iron object itself plus the rust that forms on it. Concept / Approach: Rusting is an oxidation process in which iron metal reacts with oxygen from the air and water to form hydrated iron oxide. During this reaction, oxygen atoms are added to the iron. Since oxygen has mass, the total mass of the iron plus the oxygen that has combined with it is greater than the original mass of the iron alone. Therefore, as long as rust remains attached to the object, the weight of the rusted object becomes greater than it was before rusting. Step-by-Step Solution: Step 1: Recall the simplified rusting reaction: iron plus oxygen plus water gives hydrated iron oxide (rust). Step 2: Understand that in this reaction, iron atoms from the object combine with oxygen atoms from the air. Step 3: Since oxygen has mass, adding oxygen to iron produces a compound whose total mass is the sum of the masses of iron and oxygen. Step 4: As long as the rust remains on the object and does not flake off, the object now consists of iron plus additional oxygen in the form of rust. Step 5: Therefore, the overall weight of the rusted object is greater than the weight of the original clean iron object. Verification / Alternative check: Consider a simpler oxidation example such as burning magnesium ribbon in air. The mass of the white magnesium oxide formed is greater than the original mass of the metal ribbon because magnesium has combined with oxygen. In rusting, the process is slower but conceptually similar. Conservation of mass tells us that the total mass of iron plus oxygen equals the mass of rusted iron. Unless material falls off, more atoms are present in the solid on the object after rusting, so its weight increases. Experimental demonstrations with carefully weighed iron samples also show a gain in mass after controlled oxidation. Why Other Options Are Wrong: Decreases: This might seem plausible if rust flakes off, but the question focuses on the basic chemical process, where oxygen addition increases mass. Remains the same: This ignores the fact that oxygen atoms are being added to the metal. First increases and then decreases: This could happen if rust forms and then flakes off, but such behaviour is more detailed than what is tested in a school level theoretical question. First decreases and then increases: There is no common chemical reason for an initial mass loss before oxidation adds mass in the basic rusting process. Common Pitfalls: Students sometimes assume that corrosion always means loss of material, so they think the weight must decrease. While it is true that heavily rusted structures can lose strength when rust flakes off, the initial chemical reaction adds oxygen to the iron and causes a mass gain. Another error is to forget that gases like oxygen have mass, even though they are not visible. Keeping in mind that oxidation involves adding atoms from the environment helps avoid such misunderstandings. Final Answer: When iron rusts in moist air, its weight overall increases because oxygen combines with the iron to form rust.

Question 15

Many metals, such as aluminium and zinc, react with aqueous sodium hydroxide under suitable conditions. In such reactions, which gas is evolved as a product along with soluble complex salts?

Accepted Answer

Introduction / Context: Certain metals show amphoteric behaviour, meaning they can react with both acids and bases. Metals such as aluminium and zinc react with strong bases like sodium hydroxide to produce soluble complexes and a gas. This question asks which gas is evolved during such reactions. Understanding these reactions reinforces knowledge about metal activity, hydrogen evolution, and the behaviour of amphoteric metals. Given Data / Assumptions: We are considering the reaction of metals like aluminium or zinc with aqueous sodium hydroxide solution. The reaction occurs in the presence of water and often with heating. A gas is evolved as one of the products. Soluble complex salts such as sodium aluminate or sodium zincate are also formed. Concept / Approach: When active metals react with water or aqueous alkalies, they can displace hydrogen from water molecules, producing hydrogen gas. Aluminium reacts with sodium hydroxide and water to form sodium aluminate and hydrogen gas. Similarly, zinc forms sodium zincate and hydrogen gas. The balanced equations show hydrogen gas as a clear product. Oxygen gas is not liberated in these reactions, and sodium metal or chlorine gas are not produced because the reaction conditions and reactants do not support such changes. Step-by-Step Solution: Step 1: Consider aluminium reacting with sodium hydroxide and water. A typical equation is: 2Al + 2NaOH + 6H2O gives 2Na[Al(OH)4] + 3H2. Step 2: Observe that hydrogen gas (H2) is clearly shown as a product of this reaction. Step 3: Similarly, zinc reacts with sodium hydroxide according to: Zn + 2NaOH + 2H2O gives Na2[Zn(OH)4] + H2. Step 4: In both these reactions, hydrogen gas is released as a product while complex aluminate or zincate ions form in solution. Step 5: There is no formation of oxygen gas, sodium vapour, or chlorine gas in these standard reactions, so hydrogen gas must be the correct answer. Verification / Alternative check: Practical laboratory experiments demonstrate that when aluminium foil or zinc granules are added to warm sodium hydroxide solution, bubbles of gas are evolved. This gas can be tested by bringing a burning splint near the mouth of the test tube; a characteristic pop sound confirms the presence of hydrogen gas. Textbooks describing the behaviour of amphoteric metals with alkalies consistently show hydrogen gas as the gaseous product. These observations support the conclusion that hydrogen, not oxygen or other gases, is produced. Why Other Options Are Wrong: Oxygen gas: Oxygen is not liberated in these reactions; instead, the metal is oxidised and hydrogen is reduced from water. Sodium vapour: Sodium is present as sodium ions in solution and does not vaporise as metal under these conditions. Water vapour only: Although heating may produce some water vapour, the key chemical gaseous product is hydrogen. Chlorine gas: Chlorine is not part of the reactants and is not produced in reactions involving sodium hydroxide and metals like aluminium or zinc. Common Pitfalls: Students sometimes confuse these reactions with electrolysis or other processes where oxygen might be produced. Another confusion is to think that because sodium hydroxide is involved, sodium metal might appear as a product, which does not happen in aqueous solution. The key is to remember that active metals reacting with water or alkalies can displace hydrogen, leading to hydrogen gas evolution. Recognising the typical pop test for hydrogen helps in remembering the correct gas. Final Answer: When metals such as aluminium or zinc react with aqueous sodium hydroxide, they evolve Hydrogen gas as a product.

Question 16

When magnesium metal (Mg) burns in oxygen (O2), which oxide compound is formed as the principal product of this combustion reaction?

Accepted Answer

Introduction / Context: The burning of magnesium ribbon in air is a classic school experiment that demonstrates combustion and oxidation of metals. When magnesium burns, it combines with oxygen to form a white powdery oxide. This question asks for the correct formula of that oxide, reinforcing understanding of valency rules, formula writing, and basic redox chemistry. Given Data / Assumptions: Reactants: magnesium metal (Mg) and oxygen gas (O2). The reaction is a combustion or oxidation reaction. We are asked for the correct empirical formula of the oxide product. Magnesium is a Group 2 metal with valency 2. Concept / Approach: Magnesium forms a simple ionic oxide in which magnesium has an oxidation state of +2 and oxygen has an oxidation state of -2. To balance charges, one Mg2+ ion combines with one O2- ion, giving the formula MgO. The balanced chemical equation for the reaction is 2Mg + O2 gives 2MgO. Other formulas such as Mg2O or MgO4 are not consistent with the usual valencies of magnesium and oxygen in this context. Step-by-Step Solution: Step 1: Recall that magnesium is a divalent metal and commonly forms Mg2+ ions. Step 2: Oxygen in oxides typically forms O2- ions with a charge of -2. Step 3: To form a neutral compound, one Mg2+ ion will pair with one O2- ion. Step 4: This charge balance leads to the empirical formula MgO for magnesium oxide. Step 5: The balanced reaction is 2Mg + O2 gives 2MgO, confirming MgO as the product. Verification / Alternative check: In the laboratory, burning magnesium produces a bright white flame and leaves behind a white powder identified as magnesium oxide. Chemical analysis and textbooks consistently give the formula of this oxide as MgO. If we tried other formulas, such as Mg2O, the charges would not balance properly for the simple oxide. MgO also fits with trends among Group 2 metals, which typically form MO type oxides, such as CaO and BaO. This pattern supports MgO as the correct formula for the product. Why Other Options Are Wrong: Mg2O: Implies a ratio that would not maintain simple charge neutrality with Mg2+ and O2- ions. MgO4: Suggests an unrealistic oxidation state for magnesium in a simple oxide and does not correspond to known magnesium compounds formed by simple combustion. O2Mg: This is just another way of writing MgO2 if misinterpreted and is not the standard formula for the common magnesium oxide product. Mg2O3: Does not correspond to the stable oxide formed during burning of magnesium in air. Common Pitfalls: Students sometimes misinterpret the stoichiometric coefficients in the balanced equation as subscripts in the formula. For example, seeing 2Mg + O2 gives 2MgO might lead some to think the compound is Mg2O, which is not correct. To avoid this, always derive the formula from charge balance and valency, not from equation coefficients. Another error is to overcomplicate the formula by adding extra oxygen atoms that are not required for charge neutrality. Final Answer: When magnesium burns in oxygen, the main product formed is MgO, magnesium oxide.

Question 17

In basic acid base chemistry, what is the reaction between an acid and a base called, in which a salt and water are usually formed?

Accepted Answer

Introduction / Context: Acids and bases are central to many chemical reactions. When an acid and a base react together, they typically produce a salt and water. This fundamental process has a specific name that students encounter early in chemistry. The question asks for that name, reinforcing understanding of one of the most common reaction types in inorganic chemistry. Given Data / Assumptions: Reactants: an acid and a base. Typical products: a salt and water. The reaction involves the combination of H+ ions from the acid and OH- ions from the base. We are dealing with standard aqueous acid base reactions. Concept / Approach: A reaction in which an acid reacts with a base to form salt and water is called a neutralisation reaction. In such reactions, the hydrogen ions from the acid and hydroxide ions from the base combine to form water, removing the acidic and basic characteristics. The remaining ions form the corresponding salt. This is distinct from processes like desalination, crystallisation, sublimation, or oxidation, which involve different physical or chemical changes. Step-by-Step Solution: Step 1: Recall the general form of the reaction: acid plus base gives salt plus water. Step 2: Consider a simple example: hydrochloric acid plus sodium hydroxide gives sodium chloride plus water. Step 3: Observe that H+ from the acid and OH- from the base combine to form H2O, effectively cancelling each other. Step 4: Recognise that because the acidic and basic properties are neutralised, this type of reaction is called neutralisation. Step 5: Match this description with the options and select neutralisation as the correct name. Verification / Alternative check: Textbooks on chemistry introduce neutralisation as the core concept when discussing reactions between acids and bases. The classic experiment of adding an acid to a base while monitoring pH demonstrates that the solution moves towards a neutral pH as the two react. Many real world processes, such as treating acid spills with lime or antacid tablets neutralising excess stomach acid, are described using the term neutralisation. These consistent usages confirm that the correct name for the reaction is neutralisation. Why Other Options Are Wrong: Desalination: Refers to the process of removing salt from sea water to obtain fresh water, not to acid base reactions. Crystallisation: A physical process where dissolved substances form solid crystals, usually on cooling or evaporation. Sublimation: The direct conversion of a solid to a gas without passing through the liquid state, not related to acids and bases. Oxidation: Involves loss of electrons or gain of oxygen and is part of redox chemistry, not specifically the reaction of acids with bases. Common Pitfalls: Students may sometimes confuse neutralisation with more general terms like reaction or combination. Another error is to mix it up with desalination because both words involve removal of something from water. A helpful tip is to remember that neutralisation brings the pH towards 7, which is neutral, whereas desalination changes salt content but may not change pH significantly. Associating neutralisation with antacid tablets and common laboratory titrations can help lock in the correct concept. Final Answer: The reaction between an acid and a base that forms salt and water is called Neutralisation.

Question 18

Gelidium and Gracilaria are red algae used in industry. Which of the following commercial products obtained from these algae is widely used to grow microbes in laboratories and in preparations of ice creams and jellies?

Accepted Answer

Introduction / Context: Many marine algae are important sources of industrial products used in food, microbiology and pharmaceuticals. Red algae such as Gelidium and Gracilaria are especially significant in this regard. This question asks which commercial product derived from these algae is used to grow microbes and in preparations of ice creams and jellies. Understanding this link between biology and industry is valuable in both biology and chemistry education. Given Data / Assumptions: Gelidium and Gracilaria are species of red algae. They are processed to obtain a polysaccharide product. The product is used as a solidifying agent in microbiological media. The same product is also used in food as a gelling or stabilising agent. Concept / Approach: From red algae such as Gelidium and Gracilaria, a jelly like substance called agar is extracted. Agar is a mixture of polysaccharides that dissolve in hot water and form a firm gel on cooling. In microbiology laboratories, agar is used as a solidifying agent in culture media to grow bacteria and fungi. In the food industry, agar is used in desserts, jellies, and as a stabiliser in ice cream. Other options like Chlorella and Spirulina are microalgae used as protein rich food supplements, not as gelling agents, and gymnosperms are seed plants, not algae. Step-by-Step Solution: Step 1: Recall that Gelidium and Gracilaria are red algae from which agar is extracted. Step 2: Understand that agar forms a clear, firm gel and is used as a solid base for growing microbes in petri dishes and test tubes. Step 3: Recognise that agar is also used as a gelling agent in food products such as jellies and in stabilising ice creams. Step 4: Consider the other options. Chlorella and Spirulina are green and blue green microalgae used as health supplements due to their high protein content. Step 5: Gymnosperms are higher plants like pines and cycads, not algae. Therefore agar is the only option that matches the description in the question. Verification / Alternative check: Biology and biotechnology textbooks state that agar is obtained mainly from certain red algae such as Gelidium and Gracilaria. In every microbiology laboratory, agar plates and agar slants are standard tools for culturing microbes. Nutrient agar, MacConkey agar, and many other media rely on agar as the solidifying agent. Food science references also mention agar as a vegetarian substitute for gelatin in desserts and as a stabiliser in products like ice cream. These cross disciplinary references all point to agar as the correct commercial product derived from these red algae. Why Other Options Are Wrong: Chlorella: A unicellular green alga used as a protein rich food supplement, not as a gelling agent. Spirulina: A filamentous cyanobacterium (blue green alga) also used as a health supplement, not used for agar production. Gymnosperms: A group of seed plants, including conifers, unrelated to marine red algae and not a commercial product. Alginates: These are extracted mainly from brown algae such as Laminaria and are used as thickening agents, not specifically as the product named in the question. Common Pitfalls: Students may confuse different algal products such as agar and alginates or may mix up the specific algae from which each is obtained. Another mistake is to focus only on food uses and ignore microbiological uses. A helpful approach is to remember that agar comes from red algae and is used to grow microbes, whereas alginates come from brown algae and are used to thicken products like toothpaste and ice cream. Chlorella and Spirulina are more associated with health foods and tablets. Final Answer: The commercial product obtained from Gelidium and Gracilaria and used to grow microbes and prepare jellies and ice creams is Agar.

Question 19

Which of the following is an example of a thermosetting plastic, that is, a polymer which once moulded and hardened cannot be softened by reheating?

Accepted Answer

Introduction / Context: Plastics are broadly divided into thermoplastics and thermosetting plastics. Thermoplastics soften on heating and can be remoulded, while thermosetting plastics undergo a permanent hardening process and cannot be softened again by heat. This question asks for an example of a thermosetting plastic. Knowing the difference between these two classes is important in materials science and everyday applications. Given Data / Assumptions: We are comparing common plastics: Bakelite, PVC, polyethylene and others. Thermosetting plastics harden irreversibly when first moulded. Thermoplastics can be repeatedly softened and reshaped by heating. The correct answer must be a thermosetting plastic. Concept / Approach: Bakelite is a classic example of a thermosetting plastic. It is a phenol formaldehyde resin that forms an extensively cross linked network when cured. Once this network is formed, Bakelite does not soften on reheating and keeps its shape even at relatively high temperatures. In contrast, PVC (polyvinyl chloride), polyethylene, and polystyrene are thermoplastics that soften when heated and can be reshaped multiple times. Therefore, among the options given, Bakelite is the thermosetting plastic. Step-by-Step Solution: Step 1: Recall that thermosetting plastics have heavily cross linked polymer chains that lock the structure in place when cured. Step 2: Identify Bakelite as a phenol formaldehyde resin that forms such a cross linked network and is widely used in electrical switches and handles due to its heat resistance. Step 3: Consider PVC, polyethylene and polystyrene. These are linear or slightly branched polymers that soften on heating and are classed as thermoplastics. Step 4: Since the question asks for a thermosetting plastic, Bakelite is the correct choice. Step 5: Option stating that no option is correct can be rejected because Bakelite clearly fits the thermosetting category. Verification / Alternative check: Materials science references list Bakelite as one of the earliest synthetic plastics and emphasise that it is thermosetting. Historical descriptions explain how Bakelite revolutionised electrical insulation because it did not soften when devices became warm. In contrast, PVC is commonly used for pipes and cable insulation that can be moulded by heat, polyethylene for bags and bottles, and polystyrene for foam packaging, all of which are reprocessable by heating. These examples confirm the classification of Bakelite as thermosetting and of the others as thermoplastics. Why Other Options Are Wrong: PVC: Polyvinyl chloride is a thermoplastic and softens on heating, allowing it to be extruded into pipes and other shapes. Polyethylene: A typical thermoplastic used for carry bags and bottles that can be melted and reshaped. No option is correct.: Incorrect because Bakelite is indeed a thermosetting plastic. Polystyrene: Another thermoplastic used in foam products and disposable cups that softens when heated. Common Pitfalls: Students sometimes assume that all plastics behave similarly when heated, not realising that thermosetting plastics are fundamentally different from thermoplastics. Another error is to confuse Bakelite with more modern plastics such as PVC. One way to remember the difference is to associate Bakelite with old style electrical switches and handles that remain hard and brittle, indicating a thermosetting nature, while PVC and polyethylene are flexibile and can be softened or welded with heat. Final Answer: An example of a thermosetting plastic is Bakelite.

Question 20

Removal of fine carbon particles, such as soot and smoke, from industrial exhaust air in electrostatic precipitators involves which scientific principle of charged particle movement?

Accepted Answer

Introduction / Context: Industrial chimneys and power plants often emit flue gases containing fine carbon particles, ash and dust. To reduce air pollution, devices such as electrostatic precipitators are used to remove these particles before the gases are released into the atmosphere. This question asks which principle is involved in the removal of carbon particles from air in such devices. Understanding this principle links physical chemistry and environmental engineering. Given Data / Assumptions: We are dealing with fine solid particles like carbon or soot suspended in a gas. The removal method is not simply mechanical filtering. Electrostatic precipitators are common in large industrial installations. Charged particles move under the influence of an electric field. Concept / Approach: Electrostatic precipitators work on the principle of electrophoresis, which is the movement of charged particles in a fluid (which may be a gas or a liquid) under the influence of an electric field. In such devices, flue gases pass between highly charged electrodes. Particles become electrically charged and are then attracted to plates of opposite charge. They stick to these plates and can be removed periodically. This is different from simple filtration or sedimentation, which rely on mechanical sieving or gravity rather than electric forces. Step-by-Step Solution: Step 1: Recognise that the question is about removal of fine carbon particles from air, not about settling of heavy particles in water. Step 2: Recall that electrostatic precipitators use high voltage electrodes to charge particles in the flue gases. Step 3: Under the influence of an electric field, charged particles move towards plates with opposite charge and deposit on them. Step 4: This directed movement of charged particles due to an electric field is called electrophoresis. Step 5: Mechanical filtration, sedimentation, and simple precipitation do not involve this movement in an electric field, so electrophoresis must be the correct principle. Verification / Alternative check: Descriptions of electrostatic precipitators in environmental engineering texts explicitly mention that particles in the gas stream are electrically charged by corona discharge and then move to collecting plates because of the electric field. This is essentially the same principle used in electrophoresis of colloidal particles in liquids, adapted to a gas phase system. Diagrams show particles migrating along electric field lines until they strike the plates. While filtration and sedimentation are also used for particulate removal in some systems, the question emphasises the mechanism used in devices designed specifically for air pollution control in chimneys, which is electrophoresis. Why Other Options Are Wrong: Filtration: Involves passing air or water through a porous medium to trap particles mechanically, not through movement in an electric field. Sedimentation: Relies on gravity for particles to settle out of a fluid and is more common in water treatment than in gas cleaning for very fine particles. Precipitation: A general term often used for formation of a solid from solution but does not specify the electric field driven motion of particles. Diffusion: Refers to the random spreading of particles from high concentration to low concentration, not a directed movement towards charged plates. Common Pitfalls: Students sometimes choose filtration because it is the most familiar method for removing particles from air or water, such as in masks or water filters. However, industrial smoke from large chimneys contains extremely fine particles that are difficult to capture by simple filters at high flow rates. Electrostatic precipitators use electric forces instead, and the term electrophoresis specifically captures the idea of charged particles moving under an electric field. Remembering that electrostatic devices rely on electrical forces can help avoid confusion with purely mechanical processes. Final Answer: The removal of carbon particles from air in electrostatic precipitators is based on the principle of Electrophoresis, the movement of charged particles in an electric field.

Chemistry Questions