Question 1

Which gas is commonly used to fill luminous advertising tubes (often called neon or tube lights) to produce a bright coloured glow?

Accepted Answer

Introduction / Context: This general knowledge chemistry question asks about the gas used in glowing advertising tubes, often referred to as neon lights or tube lights. When a suitable gas is placed under low pressure in a sealed glass tube and an electric potential is applied, the gas can emit characteristic coloured light. Different gases give different colours, but the classic bright reddish orange glow is produced by neon gas, which is why such signs are commonly called neon signs. Given Data / Assumptions: - The context is luminous tubes used for signage and lighting. - Options include oxygen, hydrogen, neon, and nitric oxide. - We assume a typical low pressure gas discharge tube commonly used in advertising displays. Concept / Approach: Noble gases like neon, argon, and others emit characteristic colours when excited by electricity at low pressure. Neon, in particular, produces a bright orange red glow that is strongly associated with advertising signage. Oxygen and hydrogen are reactive diatomic gases that, under normal conditions, are not used alone to make stable luminous sign tubes. Nitric oxide is a reactive gas that is not appropriate for standard lighting applications. Therefore, among the options provided, neon is the gas most commonly used for neon sign tubes and is the correct answer. Step-by-Step Solution: Step 1: Recall that neon signs are named after the gas neon, which is used in many classic advertising tubes. Step 2: Neon is a noble gas, chemically inert under most conditions, and safe to use in sealed glass tubes. Step 3: When a high voltage is applied across the electrodes of a tube containing low pressure neon, electrons in neon atoms are excited and then fall back to lower energy levels, emitting light. Step 4: The light emitted by excited neon atoms has a characteristic bright orange red colour, making neon ideal for visible signage. Step 5: Oxygen and hydrogen do not produce such characteristic decorative glows in standard low pressure tubes and are not typically used for coloured advertising lights. Step 6: Nitric oxide is a reactive gas and is not suitable for everyday lighting tubes. Verification / Alternative check: Practical observations of street signs and advertising boards show long glowing tubes that are commonly called neon signs. Reference material on lighting technology states that neon gas is used to produce the classic orange red emission, while other noble gases or phosphor coatings are used for different colours. Oxygen and hydrogen are mainly associated with combustion or industrial processes, not decorative discharge lamps. This confirms that neon is the standard gas used in such tubes. Why Other Options Are Wrong: - Oxygen: While essential for combustion and respiration, oxygen is not used on its own as a gas for colourful advertising tubes and would present safety concerns. Why Other Options Are Wrong (continued): - Hydrogen: Hydrogen is flammable and explosive in mixtures with air, making it unsuitable for common lighting tubes. - Nitric oxide: This is a reactive nitrogen oxide gas with uses in medicine and industry but not in standard tube lighting for advertisements. Common Pitfalls: Sometimes learners confuse different types of tubes, such as fluorescent lamps, mercury vapour lamps, and neon signs. While fluorescent lamps use mercury vapour and phosphors, the classic narrow glass advertising tubes that glow orange red are usually filled with neon. Remember that the word neon in neon light refers directly to the gas neon, which helps avoid confusion with other lighting technologies. Final Answer: Neon is the gas commonly used in neon or tube lights for bright advertising displays.

Question 2

In industrial and biochemical processes, the conversion of sugar into alcohol by the action of microorganisms is called what?

Accepted Answer

Introduction / Context: This question is about a key biochemical and industrial process: the conversion of sugars into alcohol and carbon dioxide. This process is widely used in brewing, wine making, baking, and certain industrial applications. The correct term for this sugar to alcohol conversion carried out by yeast or other microorganisms is fermentation. Recognising this term helps link everyday products like bread, beer, and wine to the underlying biochemistry. Given Data / Assumptions: - The process described is the conversion of sugar to alcohol. - The options include moulding, infection, fermentation, and alcohol fixation. - We assume common yeast based fermentation as in brewing and baking. Concept / Approach: Fermentation is a metabolic process in which microorganisms such as yeast convert sugars into alcohol and carbon dioxide in the absence of oxygen, or under low oxygen conditions. In alcoholic fermentation, glucose and other simple sugars are transformed primarily into ethanol and carbon dioxide. Moulding refers to the growth of mould fungi on surfaces, infection refers to harmful invasion by pathogens, and alcohol fixation is not a standard term for sugar conversion. Therefore, the correct technical term for sugar being converted into alcohol by microorganisms is fermentation. Step-by-Step Solution: Step 1: Identify the key transformation described: sugars such as glucose are being converted into alcohol. Step 2: Recall that yeast, such as Saccharomyces species, carry out alcoholic fermentation, in which they use sugar as a substrate to produce ethanol and carbon dioxide. Step 3: The process is widely used in producing beverages such as beer and wine and in baking, where carbon dioxide gas helps dough rise. Step 4: The scientific name for this metabolic pathway is fermentation. Specifically, the type is alcoholic fermentation. Step 5: Other options like moulding and infection describe different biological phenomena and not the sugar to alcohol conversion. Step 6: Alcohol fixation is not a recognised standard term in this context, so it cannot be correct. Verification / Alternative check: Biology and chemistry textbooks describe fermentation as an anaerobic breakdown of carbohydrates, where yeast convert glucose to ethanol and CO2. The general equation for alcoholic fermentation can be written as glucose gives ethanol plus carbon dioxide plus energy. In everyday language, when grapes are fermented, their sugars are turned into wine. The same term is used for brewing beer from malted grains. No other term in the options is used in these contexts, confirming fermentation as the correct choice. Why Other Options Are Wrong: - Moulding: This refers to the growth of mould fungi on food or surfaces, often causing spoilage, but it does not necessarily describe sugar to alcohol conversion. Why Other Options Are Wrong (continued): - Infection: This refers to invasion and multiplication of harmful microorganisms in a host organism, which is a medical concept rather than a sugar conversion process. - Alcohol fixation: This is not a standard term in biochemistry for any common process and is not used for the conversion of sugar to alcohol. Common Pitfalls: Learners may confuse different microbial processes, such as mould growth or infection, with fermentation. Some might think any microbial activity on food counts as fermentation, but fermentation specifically describes metabolic pathways that break down carbohydrates, often without oxygen, producing alcohol or organic acids. Associating fermentation with familiar products such as bread, beer, wine, yogurt, and pickles can help distinguish it from unrelated biological terms. Final Answer: Fermentation is the name of the process in which sugar is converted into alcohol by microorganisms.

Question 3

In the modern periodic table, gold (Au) is located in which horizontal period?

Accepted Answer

Introduction / Context: This question checks familiarity with the layout of the modern periodic table, specifically the position of the element gold. Periods are the horizontal rows in the periodic table, numbered from 1 at the top to 7 at the bottom for known elements. Gold, with symbol Au and atomic number 79, is a well known precious metal, and knowing its period helps with understanding its electron configuration and chemical relatives. Given Data / Assumptions: - The element in question is gold, symbol Au, atomic number 79. - The options for its period number are 11, 5, 6, and 7. - We assume the standard long form periodic table with 7 periods. Concept / Approach: Period number corresponds to the highest principal quantum number (n) that contains electrons in the ground state electron configuration of an element. Elements in period 6 have electrons filling the 6s, 4f, and 5d subshells, among others. Gold has an electron configuration that places it in period 6. There is no period 11 in the standard table, and gold is too heavy to be in periods 5 or 4, but not as heavy as the elements in period 7, which include the actinides and superheavy elements. Step-by-Step Solution: Step 1: Recall that gold has atomic number 79 and symbol Au. Step 2: The approximate ground state electron configuration of gold can be written in a simplified way as [Xe] 4f14 5d10 6s1, indicating that its outermost electrons involve the n equals 6 shell. Step 3: The highest principal quantum number present in this configuration is 6, which means that gold belongs to period 6. Step 4: The standard periodic table has only 7 periods. There is no period 11, so option 11 is impossible. Step 5: Period 5 contains elements with electrons filling up to n equals 5, and period 7 contains even heavier elements than gold, including some radioactive and synthetic elements. Gold does not belong to those periods. Step 6: Therefore, among the choices given, the correct period for gold is period 6. Verification / Alternative check: If you examine a standard periodic table, you will find gold (Au) in the sixth horizontal row, positioned among the transition metals, below silver (Ag) and copper (Cu) in group 11. This visual confirmation from any accurate periodic table matches the electron configuration based reasoning and clearly places gold in period 6, not in periods 5 or 7. Why Other Options Are Wrong: - 11: The long form periodic table has only 7 periods, so period 11 does not exist in the standard classification. Why Other Options Are Wrong (continued): - 5: Period 5 contains lighter transition metals such as silver (Ag) and cadmium (Cd). Gold is heavier and is placed in the next period down. - 7: Period 7 includes very heavy and mostly radioactive elements, such as francium, radium, and the actinide series. Gold, with atomic number 79, appears one period above them in period 6. Common Pitfalls: Students may confuse group numbers (vertical columns) with period numbers (horizontal rows). For example, gold is in group 11, but that does not mean it is in period 11. Another confusion arises from not having the periodic table memorised and guessing the period based on atomic number alone. Remember that gold is just below silver and copper in the same group, and copper is in period 4, silver in period 5, and gold in period 6, which forms a neat trend down the group. Final Answer: 6 is the period number in which gold (Au) occurs in the modern periodic table.

Question 4

An alloy primarily made from copper and zinc together is commonly called what?

Accepted Answer

Introduction / Context: This general chemistry question asks about the name of a common alloy made from copper and zinc. Alloys are mixtures of metals that often have improved properties compared with their pure components. Knowing the names and compositions of familiar alloys such as brass and bronze is useful in both chemistry and everyday life, as these materials are widely used in coins, fittings, and decorative items. Given Data / Assumptions: - The alloy in question contains copper and zinc as its principal components. - The options are bronze, gold, silver, and brass. - We assume standard metallurgical definitions where brass is copper plus zinc and bronze is mainly copper plus tin. Concept / Approach: Brass is the name given to alloys of copper and zinc, sometimes with small additions of other elements to modify properties. Bronze refers primarily to alloys of copper and tin, though modern bronzes may contain other metals as well. Gold and silver are pure elements when not alloyed or are used in jewellery alloys with small amounts of other metals, but they are not the standard names for copper zinc mixtures. Therefore, the correct term for the copper zinc alloy is brass. Step-by-Step Solution: Step 1: Recall that brass is commonly described in textbooks as an alloy of copper and zinc. Step 2: Bronze, on the other hand, is typically described as an alloy of copper and tin, sometimes with additional metals. Step 3: Gold and silver are names of pure metals and are not the standard names used for copper zinc alloys. Step 4: Since the question specifically states that the alloy is made from copper and zinc, the well known name for this alloy is brass. Verification / Alternative check: Engineering and materials science references list brass as a family of alloys containing copper and zinc, used in applications such as musical instruments, plumbing fittings, and decorative hardware. Bronze is associated with statues, bells, and tools, and is typically based on copper and tin. No reputable source describes gold or silver as the names of copper zinc alloys. This confirms that brass is the correct name for an alloy of copper and zinc. Why Other Options Are Wrong: - Bronze: This is usually an alloy of copper and tin, not copper and zinc, although some modern bronzes may contain other additions. Why Other Options Are Wrong (continued): - Gold: Gold is a precious metal element with symbol Au and is not the standard name for any copper zinc alloy. - Silver: Silver is another precious metal element with symbol Ag and is not used as the name for a copper zinc alloy. Common Pitfalls: Students frequently confuse brass and bronze because both are copper based alloys and have similar colour. Remembering their main alloying elements helps: brass is copper plus zinc, while bronze is copper plus tin. Another trick is to associate brass with items like door handles and some musical instruments, and bronze with statues and historical tools. These mental images help keep the names and compositions straight. Final Answer: Brass is the alloy primarily made from copper and zinc.

Question 5

Which class of hydrocarbons consists of saturated compounds that contain only single covalent bonds between carbon atoms?

Accepted Answer

Introduction / Context: This organic chemistry question asks about saturated hydrocarbons, which are compounds that contain only single bonds between carbon atoms and have the maximum number of hydrogen atoms possible. Hydrocarbons are divided into three main classes: alkanes, alkenes, and alkynes. Recognising which of these is saturated helps in predicting reactions and properties such as addition reactions and hydrogenation. Given Data / Assumptions: - The classes of hydrocarbons listed are alkenes, alkanes, and alkynes. - Saturated means that all carbon carbon bonds are single bonds and the molecule contains as many hydrogen atoms as possible for its carbon skeleton. - We assume standard definitions used in introductory organic chemistry. Concept / Approach: Alkanes are saturated hydrocarbons with only carbon carbon single bonds (C C) and general formula CnH2n+2 for acyclic molecules. Alkenes contain at least one carbon carbon double bond and are unsaturated. Alkynes contain at least one carbon carbon triple bond and are also unsaturated. Unsaturated hydrocarbons can add hydrogen across their multiple bonds in addition reactions, whereas saturated hydrocarbons cannot increase hydrogen content without breaking the skeleton. Therefore, among alkenes, alkanes, and alkynes, only alkanes are saturated hydrocarbons. Step-by-Step Solution: Step 1: Recall that alkanes have only carbon carbon single bonds and are considered saturated because no more hydrogen atoms can be added without breaking the carbon framework. Step 2: Alkenes contain one or more carbon carbon double bonds (C=C). These double bonds can open up to add hydrogen, showing that alkenes are unsaturated. Step 3: Alkynes contain one or more carbon carbon triple bonds (C≡C). These triple bonds can also be hydrogenated, indicating that alkynes are unsaturated. Step 4: The question specifically asks for the class that represents saturated hydrocarbons, which must therefore be the alkanes. Step 5: Options suggesting that all of the above are saturated are incorrect because alkenes and alkynes are unsaturated. Verification / Alternative check: Textbook tables of hydrocarbon series list alkanes as saturated hydrocarbons with general formula CnH2n+2, alkenes as unsaturated with CnH2n, and alkynes as unsaturated with CnH2n−2 for acyclic forms. Examples such as ethane (C2H6) are alkanes, ethene (C2H4) is an alkene, and ethyne (C2H2) is an alkyne. Only ethane among these has no multiple bonds and is fully saturated. This pattern confirms that only the alkane family is saturated. Why Other Options Are Wrong: - Alkene: Contains at least one double bond and can undergo addition reactions with hydrogen, so it is unsaturated. Why Other Options Are Wrong (continued): - Alkyne: Contains at least one triple bond and is also unsaturated, capable of adding hydrogen across the triple bond. - All of the above: This is incorrect because only alkanes are saturated, while alkenes and alkynes are unsaturated hydrocarbons. Common Pitfalls: Students sometimes confuse the names alkane, alkene, and alkyne due to their similar spelling. A useful memory trick is that the simplest saturated hydrocarbon, methane, ends in ane, matching the family name alkane. Another source of confusion is thinking that a molecule is saturated if it contains any single bond, but the correct criterion is that it must have only single carbon carbon bonds and carry the maximum hydrogen content for its carbon skeleton. Final Answer: Alkane is the class of hydrocarbons that consists of saturated compounds with only single carbon carbon bonds.

Question 6

In the context of the water cycle, infiltration is the process by which rainwater gradually becomes which type of water?

Accepted Answer

Introduction / Context: This environmental science and geography question focuses on infiltration, an important process in the hydrological or water cycle. After rain falls on land, some of the water flows over the surface, some evaporates, and some seeps into the soil. The process by which rainwater soaks into the ground, eventually replenishing underground water reserves, is known as infiltration. Understanding this term helps explain how groundwater supplies are maintained and why soil and vegetation are important for water management. Given Data / Assumptions: - The process is described as the conversion of rainwater into another form of water through soaking into the ground. - Options include river water, ground water, salt water, and ocean water. - We assume a typical landscape where rain falls on soil or rock surfaces. Concept / Approach: Infiltration refers to the movement of water from the surface of the land into the soil and porous rock beneath. Once water infiltrates, it may percolate deeper, eventually reaching aquifers where it becomes groundwater. River water and ocean water are associated with surface flow and large bodies of water. Salt water is usually associated with oceans and saline lakes rather than with the initial step of water soaking into soil. Therefore, the type of water formed when rainwater infiltrates the ground is groundwater. Step-by-Step Solution: Step 1: Consider what happens when rain falls on land. Some of it runs off into streams and rivers, some evaporates, and some soaks into the soil. Step 2: The soaking of water into the soil surface is called infiltration. Step 3: The infiltrated water moves downward through pores and cracks, a process known as percolation, and eventually accumulates in underground layers called aquifers. Step 4: Water stored in aquifers and saturated zones beneath the surface is called groundwater. Step 5: River water and ocean water are associated with surface runoff and large water bodies, not with the initial soaking into soil. Step 6: Therefore, the process of infiltration turns rainwater into groundwater. Verification / Alternative check: Diagrams of the water cycle in textbooks typically label infiltration as the process where precipitation seeps into the ground, leading to groundwater storage. These diagrams show separate arrows for surface runoff that goes to rivers and oceans and for infiltration that leads downwards to underground water. This visual representation confirms that infiltration is connected specifically with groundwater, not directly with river or ocean water. Why Other Options Are Wrong: - River water: River water is mainly fed by surface runoff and contributions from groundwater discharge, but the process of infiltration itself does not directly convert rainwater into river water. Why Other Options Are Wrong (continued): - Salt water: Salt water is characteristic of oceans and some saline lakes, not the immediate product of rainwater soaking into soil. - Ocean water: Oceans are large reservoirs that receive water from rivers and direct precipitation, but infiltration refers specifically to the movement into ground, not into the ocean. Common Pitfalls: Students may confuse infiltration with runoff and think that any movement of water after rainfall might be called infiltration. Another confusion is between infiltration and percolation. Infiltration is the entry of water into the soil surface, while percolation is its downward movement through the soil profile. Remember that only the pathway into the ground leading to underground storage is called infiltration and that the resulting stored water is known as groundwater. Final Answer: Ground water is the type of water that results when rainwater undergoes infiltration.

Question 7

All atoms of the element uranium share which identical fundamental property, regardless of isotope or ionisation state?

Accepted Answer

Introduction / Context: This atomic structure question addresses the difference between atomic number, atomic mass, mass number, and electron count. An element such as uranium can exist in different isotopic and ionic forms, but there is one fundamental property that remains the same for all atoms of that element. Understanding which property is constant is essential for correctly interpreting the periodic table and for distinguishing isotopes from one another. Given Data / Assumptions: - The element under discussion is uranium. - Options include atomic mass, number of electrons, atomic number, and mass number. - We assume standard definitions where atomic number is the number of protons and defines the element. Concept / Approach: Atomic number is defined as the number of protons in the nucleus of an atom. All atoms of the same element have the same atomic number and therefore the same number of protons. Isotopes differ in the number of neutrons, which changes the mass number and atomic mass. Ions differ in the number of electrons. Therefore, atomic number is the unique property that is the same for all atoms of a given element, including uranium. Step-by-Step Solution: Step 1: Recall that atomic number (Z) is the number of protons in the nucleus and determines the identity of the element. Uranium has atomic number 92, so every uranium atom has 92 protons. Step 2: Atomic mass and mass number involve both protons and neutrons. Different isotopes of uranium, such as uranium 235 and uranium 238, have different numbers of neutrons, so their mass numbers and atomic masses differ. Step 3: The number of electrons can vary if the uranium atom gains or loses electrons to form ions, such as U4 plus or U6 plus, so electron count is not constant for all uranium atoms. Step 4: Since only the proton count is fixed for all atoms of uranium, atomic number is the property that remains the same. Verification / Alternative check: Periodic tables list each element with a specific atomic number. Uranium is always listed with atomic number 92. Isotope notation such as 235U or 238U indicates different mass numbers (235 and 238) but always includes the element symbol U, which implies 92 protons. Ion notation may add charges, but the underlying element remains uranium as long as the proton count is 92. This consistent use of atomic number to define identity confirms that all uranium atoms share the same atomic number. Why Other Options Are Wrong: - Atomic mass: This is often a weighted average of the masses of all naturally occurring isotopes, and individual isotopes of uranium have different atomic masses. Why Other Options Are Wrong (continued): - Number of electrons: In neutral uranium atoms the electron count equals the proton count, but ions of uranium can lose or gain electrons, so this number can change. - Mass number: Mass number varies between isotopes because it is the sum of protons and neutrons, so different isotopes of uranium have different mass numbers. Common Pitfalls: Students sometimes confuse atomic mass with atomic number because both are numbers printed near the element symbol. Another common error is to think that the number of electrons is always fixed, forgetting about ions. To avoid these mistakes, remember that atomic number is always an integer that defines the element, while atomic mass is often a decimal and mass number can change with isotopes. The element uranium is defined by having atomic number 92, regardless of its isotope or charge state. Final Answer: Atomic number is the property that is the same for all atoms of uranium.

Question 8

In physical geography and geology, which of the following processes is not commonly classified as a type of chemical weathering of rocks?

Accepted Answer

Introduction / Context: This environmental science question concerns chemical weathering, which is the breakdown of rocks and minerals through chemical reactions with water, gases, and dissolved substances. Common types of chemical weathering include solution, oxidation, and carbonation. The term corrosion is frequently used in chemistry to describe the degradation of metals, especially iron, but it is not one of the standard categories used to describe weathering processes acting on rocks in geomorphology. Given Data / Assumptions: - The processes listed are corrosion, solution, oxidation, and carbonation. - The question asks which is not typically classified as a type of chemical weathering in the context of rocks and landforms. - We assume standard physical geography terminology where solution, oxidation, and carbonation are key weathering processes. Concept / Approach: Chemical weathering of rocks is usually grouped into several main processes: solution (dissolving of soluble minerals), oxidation (reaction with oxygen, especially of metals such as iron), carbonation (reaction with carbonic acid formed from carbon dioxide and water), and sometimes hydrolysis. While the word corrosion can be used broadly to mean chemical degradation, in school geography and geology it is more commonly associated with the chemical decay of metals like iron in engineering, not as a distinct named category of rock weathering. Therefore, corrosion is the option that does not fit the standard list of chemical weathering types for rocks. Step-by-Step Solution: Step 1: Recognise that solution is a type of chemical weathering in which water dissolves soluble minerals, such as halite or limestone, and carries the ions away. Step 2: Oxidation is another well known chemical weathering process where minerals, especially those containing iron, react with oxygen to form oxides, often giving rocks a reddish colour. Step 3: Carbonation involves the reaction of carbonic acid (formed when carbon dioxide dissolves in water) with minerals such as calcite in limestone, leading to dissolution and formation of karst landforms. Step 4: Corrosion is a general term in chemistry for the deterioration of metals, particularly iron, by reactions with the environment. It is not typically used as a specific category in lists of rock weathering processes. Step 5: Since solution, oxidation, and carbonation are all standard types of chemical weathering of rocks, and corrosion is not usually listed alongside them in geology, corrosion is the correct choice. Verification / Alternative check: Physical geography textbooks frequently present tables listing mechanical and chemical weathering processes. Under chemical weathering, they generally include solution, oxidation, carbonation, and sometimes hydrolysis and hydration. The term corrosion appears mainly in engineering contexts or in discussions of metal structures, not in those standard lists. Rock weathering diagrams emphasise dissolution, rusting of iron bearing minerals (oxidation), and carbonic acid reactions, all under specific names, supporting the idea that corrosion is not a main category for rocks. Why Other Options Are Wrong: - Solution: This is a classical type of chemical weathering where water dissolves minerals, especially in limestone and rock salt, altering the rock. Why Other Options Are Wrong (continued): - Oxidation: This is a standard chemical weathering process where minerals react with oxygen, often weakening rocks and changing their colour and structure. - Carbonation: This is also a standard process where carbonic acid in water reacts with minerals such as calcite, contributing to features like caves and sinkholes in limestone regions. Common Pitfalls: Students sometimes use corrosion as a catch all term for any chemical deterioration, including rock weathering, because they hear about metal corrosion. While the underlying chemistry may be similar, the terminology used in physical geography is more specific. To avoid confusion, learners should remember that solution, oxidation, and carbonation are the named processes for chemical weathering of rocks, while corrosion is reserved mainly for metals in engineering contexts. Final Answer: Corrosion is not typically classified as a standard type of chemical weathering of rocks in physical geography.

Question 9

Which one of the following calcium compounds is commonly used as a nitrogenous fertilizer in agriculture (often referred to as calcium cyanamide)?

Accepted Answer

Introduction / Context: This agricultural chemistry question asks about a calcium compound used as a fertilizer, specifically a nitrogenous fertilizer. Farmers often apply nitrogen containing compounds to improve plant growth, and some of these fertilizers are calcium salts. One such compound is calcium cyanamide, sometimes loosely referred to as calcium cyanide in older texts, which releases nitrogen into the soil. Recognising which calcium compound provides nitrogen helps distinguish it from other calcium salts used for different purposes. Given Data / Assumptions: - The options are calcium carbide, calcium carbonate, calcium cyanide (intended to represent calcium cyanamide), and calcium sulphate. - The question focuses on use as a fertilizer, particularly for nitrogen supply. - We assume common agricultural uses: nitrogenous fertilizers versus soil conditioners. Concept / Approach: Calcium cyanamide, often written as CaCN2, is a nitrogenous fertilizer that slowly releases nitrogen into the soil. Although the option text says calcium cyanide, in many exam contexts it represents calcium cyanamide, which is the agriculturally important compound. Calcium carbonate (limestone) and calcium sulphate (gypsum) are mainly used to correct soil acidity or improve soil structure, not as primary nitrogen sources. Calcium carbide is used to generate acetylene gas, not as a fertilizer. Therefore, the calcium compound that functions as a nitrogenous fertilizer is calcium cyanamide, represented in the options as calcium cyanide. Step-by-Step Solution: Step 1: Identify which compounds contain nitrogen. Calcium cyanamide, CaCN2, contains nitrogen, while calcium carbide (CaC2), calcium carbonate (CaCO3), and calcium sulphate (CaSO4) do not contain nitrogen in their formulas. Step 2: Recognise that nitrogen containing compounds are used as nitrogenous fertilizers because they can release nitrogen in forms that plants can absorb. Step 3: Calcium carbonate is commonly used to reduce soil acidity (liming) and is considered a soil conditioner rather than a nitrogen fertilizer. Step 4: Calcium sulphate, or gypsum, is used to improve soil structure and supply calcium and sulphur, but not as a major nitrogen source. Step 5: Calcium carbide is mainly used industrially to produce acetylene gas and is not applied as a fertilizer. Step 6: Calcium cyanamide (represented in the option as calcium cyanide) is specifically manufactured and sold as a nitrogenous fertilizer, so this is the correct choice. Verification / Alternative check: Agricultural references list calcium cyanamide as a nitrogen fertilizer that slowly breaks down in the soil to release ammonia, which plants can use. Its use is distinct from that of lime (calcium carbonate) and gypsum (calcium sulphate), which are recommended primarily for adjusting soil pH and improving soil structure. Safety data sheets and product labels for calcium cyanamide clearly identify it as a fertilizer, while calcium carbide is labelled as a gas generating chemical and is not applied to fields for plant nutrition. This confirms that the nitrogen supplying calcium compound is calcium cyanamide, reflected in the option about calcium cyanide. Why Other Options Are Wrong: - Calcium carbide: Used mainly to generate acetylene gas for welding and other industrial purposes; it is not a standard agricultural fertilizer. Why Other Options Are Wrong (continued): - Calcium carbonate: Used to reduce soil acidity and as a soil conditioner, but it does not provide nitrogen to plants. - Calcium sulphate: Known as gypsum, it supplies calcium and sulphur and helps improve soil physical properties, but it is not primarily a nitrogenous fertilizer. Common Pitfalls: Students may confuse calcium carbonate and calcium sulphate because they are both used in soil treatment and contain calcium. Another source of confusion is the wording calcium cyanide, which in many exam questions is intended to represent calcium cyanamide, the actual fertilizer. To avoid mistakes, focus on whether the compound contains nitrogen when the question specifies a nitrogenous fertilizer, and remember that calcium cyanamide is the relevant fertilizer in this group. Final Answer: Calcium cyanide (calcium cyanamide) is the calcium compound used as a nitrogenous fertilizer in agriculture.

Question 10

Among the common allotropes of phosphorus, which solid form is the most chemically reactive under ordinary conditions?

Accepted Answer

Introduction / Context: Phosphorus is a non metal that exists in several different solid forms called allotropes, such as white, red, black, and violet phosphorus. These allotropes have the same element, but different arrangements of atoms in the solid, which leads to very different physical and chemical properties. General chemistry and competitive exams often ask which allotrope is the most reactive, because this is directly connected to its structure and safety in handling. Given Data / Assumptions: We are comparing the reactivity of different solid forms of phosphorus: white, red, black, violet, and related amorphous forms. All allotropes consist of phosphorus atoms but differ in how the atoms are bonded and packed in the solid. Reactivity here refers mainly to how easily the allotrope reacts with oxygen and other reagents at normal temperatures. Normal laboratory conditions and air exposure are assumed. Concept / Approach: Reactivity of an allotrope depends on factors such as bond strain, bond angles, stability of the structure, and surface area. White phosphorus is made of P4 tetrahedral molecules with significant bond angle strain, and the molecules are held together only by weak intermolecular forces. This makes white phosphorus unstable and very eager to react, especially with oxygen in air, where it can ignite spontaneously. Red, black, and violet phosphorus have more extended polymeric or layered structures that are thermodynamically more stable and therefore less reactive. Step-by-Step Solution: Step 1: Recall that white phosphorus consists of discrete P4 molecules with strained P–P–P bond angles of about 60 degrees. Step 2: Understand that this high strain and weak intermolecular forces make white phosphorus metastable and ready to react, especially in air. Step 3: Note that red phosphorus has a polymeric structure, where P atoms form long chains or networks with less bond angle strain and greater stability. Step 4: Recognise that black and violet phosphorus have layered structures similar to graphite, which are even more stable and less reactive. Step 5: Conclude that among the listed forms, white phosphorus is the most reactive allotrope under ordinary conditions. Verification / Alternative check: In practice, white phosphorus must be stored under water because it can oxidise and even catch fire in air at relatively low temperatures. It glows in the dark (chemiluminescence) due to slow oxidation, which is another sign of high reactivity. Red phosphorus, in contrast, is used safely on matchbox striking surfaces, where it reacts only when triggered by friction and heat. Black phosphorus is even more inert and is studied as a layered material. Textbooks consistently state that white phosphorus is the most reactive allotrope, confirming this choice. Why Other Options Are Wrong: Red phosphorus is less reactive than white phosphorus and does not ignite spontaneously in air. Black phosphorus is the most thermodynamically stable allotrope and is comparatively unreactive. Violet phosphorus is also more stable than white phosphorus and does not show such extreme reactivity. Amorphous forms can be reactive, but standard teaching focuses on white phosphorus as the most reactive allotrope among the named crystalline forms. Common Pitfalls: A common mistake is to assume that the darker allotropes like black phosphorus must be more reactive because they look less familiar, or to think that red phosphorus used in matches is the most reactive. In reality, safety data and storage requirements show that white phosphorus is far more dangerous and reactive. To remember this, associate white phosphorus with the need to store it under water and with its ability to glow and ignite in air, traits that clearly indicate very high reactivity. Final Answer: Among the common allotropes of phosphorus, the most reactive form is white phosphorus.

Question 11

In the modern periodic table, what is the common characteristic of all elements present in the same vertical group (family)?

Accepted Answer

Introduction / Context: The periodic table is organised in such a way that elements with similar properties fall into the same vertical columns, called groups or families. Students are often asked what basic feature all elements in a given group share, because this feature explains why their chemical behaviour is similar. Understanding this concept is central to predicting valency, typical reactions, and the formation of compounds for main group elements. Given Data / Assumptions: Elements are arranged in periods (rows) and groups (columns) in the periodic table. Atomic number equals the number of protons in the nucleus, and also the number of electrons in a neutral atom. Electrons are arranged in shells and subshells around the nucleus. Elements in the same group show similar valency and chemical properties. Concept / Approach: The key to group similarity lies in valence electrons, the electrons in the outermost shell. These electrons participate in bonding and determine the typical valency and reactivity of an element. All elements of a given group have the same number of valence electrons, even though they may differ in total electrons, total protons, atomic mass, and number of shells. For example, all alkali metals in group 1 (Li, Na, K, and so on) have one electron in their outermost shell, and all halogens in group 17 have seven outer electrons. This shared valence electron count produces similar chemical behaviour within a group. Step-by-Step Solution: Step 1: Recall that in the periodic table, groups are vertical columns labelled 1 to 18 in the modern system. Step 2: Remember that electronic configuration of elements in a group follows the same pattern in the outermost shell, even though the inner shells differ. Step 3: Check examples: group 2 elements (Be, Mg, Ca, etc.) each have two electrons in their outermost shell. Step 4: Recognise that this shared valence electron number leads to similar valency and similar types of compounds. Step 5: Conclude that the most important common feature of elements in the same group is the same number of electrons in the outermost shell. Verification / Alternative check: Compare sodium (Na, atomic number 11) and potassium (K, atomic number 19). Their electronic configurations end with 3s1 and 4s1 respectively, both having one valence electron, so they behave similarly as alkali metals. Compare chlorine (Cl, atomic number 17) and bromine (Br, atomic number 35). Their configurations end with 3s2 3p5 and 4s2 4p5, both having seven valence electrons, which explains their similar halogen chemistry. These examples confirm that a common number of outer shell electrons is the key group feature. Why Other Options Are Wrong: The total number of electrons (and protons) increases steadily from top to bottom in a group, so it is not the same for all members. Atomic weight also increases down a group and is not constant. The number of occupied shells increases from top to bottom, so that also differs between elements in the same group. Therefore, those alternatives cannot explain the similarity in chemical properties within a group. Common Pitfalls: Learners sometimes assume that elements in the same group must have the same total number of electrons or similar atomic masses, which is not correct. Another confusion is between periods and groups, leading students to associate the same number of shells with the same group, whereas shell number is actually common to a period. To avoid mistakes, memorise that groups share valence electrons, while periods share the number of occupied shells. Final Answer: Elements in the same group have in common that they possess the same number of electrons in their outermost shell.

Question 12

In basic thermochemistry, what is meant by an exothermic reaction in terms of heat exchange with the surroundings?

Accepted Answer

Introduction / Context: Thermochemistry deals with the heat changes that accompany chemical reactions. One of the first distinctions students learn is between exothermic and endothermic reactions. Understanding this difference helps explain everyday phenomena such as why fuels feel hot when burned or why some chemical packs warm up when activated. This question asks for a clear definition of an exothermic reaction in terms of heat flow between the reacting system and its surroundings. Given Data / Assumptions: The system is the reacting chemicals, and the surroundings include everything else, such as the container and the air. Heat flow can be described as energy released from the system to the surroundings or absorbed from the surroundings into the system. Temperature changes can indicate the direction of heat flow. The question is about the general definition, not about a specific reaction equation. Concept / Approach: In thermochemistry, an exothermic reaction is defined as one in which the enthalpy change of reaction is negative, meaning the system loses heat and the surroundings gain heat. In simple language, heat is released to the surroundings. This often leads to a temperature rise in the surroundings if the heat is not quickly lost. By contrast, an endothermic reaction absorbs heat from the surroundings, leading to a temperature decrease. Therefore, any definition of exothermic behaviour must involve release of heat from the reacting system to the outside environment. Step-by-Step Solution: Step 1: Identify the key word "exothermic". The prefix "exo" means "outside" or "outward". Step 2: Connect this to heat flow: exothermic reactions send heat out of the system to the surroundings. Step 3: Recall examples such as combustion of fuels, neutralisation reactions between strong acids and bases, and respiration in the body, all of which release heat. Step 4: Translate this understanding into a simple statement: the reaction releases heat energy to the surroundings. Step 5: Compare this with the definitions of endothermic reactions, which explicitly absorb heat, and neutral situations where there is negligible heat change. Verification / Alternative check: Consider the burning of methane in oxygen. The reaction produces carbon dioxide, water, and a significant amount of heat. The surroundings get warmer, and the enthalpy change is negative, which is a hallmark of exothermic reactions. Chemical hand warmers are another everyday example: when activated, they undergo an exothermic reaction and release heat, warming the hands. In contrast, cold packs for sports injuries use endothermic reactions that absorb heat and cool the surroundings, showing the opposite behaviour. These examples confirm that exothermic reactions release heat energy outward. Why Other Options Are Wrong: Saying that an exothermic reaction absorbs heat from the surroundings describes an endothermic reaction, not an exothermic one. Claiming that neither heat is released nor absorbed would correspond to a reaction with negligible enthalpy change, not specifically exothermic. Defining exothermic reactions as those that occur only at high temperatures is incorrect, because some exothermic reactions occur even at room temperature. The idea that an exothermic reaction always cools the surroundings to absolute zero is unrealistic and scientifically wrong. Common Pitfalls: A common confusion is mixing up the signs of enthalpy change and the terms exothermic and endothermic. Some learners think "exo" means energy goes into the system instead of out. Others focus only on temperature change without considering the direction of heat flow. To avoid these mistakes, remember the simple phrase: exothermic equals heat exiting the system, while endothermic equals heat entering the system. Final Answer: An exothermic reaction is a reaction in which heat energy is released from the system to the surroundings.

Question 13

What are the main components of Brass Alloy?

Accepted Answer

Copper and Zinc

Question 14

In household and laboratory chemistry, what is the common English name of the compound sodium bicarbonate (NaHCO3)?

Accepted Answer

Introduction / Context: Many inorganic compounds used in homes and industries have simple common names that differ from their chemical names. For examination questions, it is important to connect the chemical formula with the everyday name. Sodium bicarbonate, NaHCO3, is one such compound that appears frequently in cooking, cleaning, and science experiments. This question asks for its common English name, which students often encounter in school chemistry and daily life. Given Data / Assumptions: The compound has the formula NaHCO3 and the chemical name sodium hydrogen carbonate or sodium bicarbonate. It is a mild alkali used in baking, cleaning, and as an antacid. Related products include washing soda (Na2CO3), plaster of Paris (CaSO4·0.5H2O), bleaching powder, and baking powder. The focus is on the standard common name recognised in everyday use. Concept / Approach: Sodium bicarbonate, NaHCO3, is commonly known as baking soda. It is used in baking as a leavening agent because it decomposes to release carbon dioxide gas when heated or when it reacts with acids. Washing soda is sodium carbonate, Na2CO3, which is a stronger alkali used mainly for cleaning and softening water. Plaster of Paris and bleaching powder are different calcium compounds. Baking powder is a mixture that contains baking soda plus an acid salt and a filler. Therefore, baking soda is the correct common name for sodium bicarbonate. Step-by-Step Solution: Step 1: Identify the given compound as NaHCO3, sodium bicarbonate or sodium hydrogen carbonate. Step 2: Recall that this compound is used in cooking to make cakes and breads rise due to gas release. Step 3: Remember the common name baking soda for the white crystalline powder that performs this function. Step 4: Distinguish baking soda from washing soda, which is Na2CO3 and used for laundry and cleaning. Step 5: Confirm that none of the other given common names correspond to NaHCO3. Verification / Alternative check: If you check food ingredient labels for baking soda, they usually list sodium bicarbonate as the only chemical. Baking powder labels typically mention sodium bicarbonate along with acid salts such as cream of tartar and a starch filler, confirming that baking powder is a mixture, not a single compound. Laundry products marketed as washing soda are sodium carbonate, not sodium bicarbonate. These comparisons reinforce that NaHCO3 corresponds specifically to baking soda. Why Other Options Are Wrong: Washing soda refers to sodium carbonate, Na2CO3, which has a different formula and is a stronger base. Plaster of Paris is calcium sulphate hemihydrate, used in casts and moulds, not for baking. Bleaching powder is mainly calcium oxychloride, used as a disinfectant and bleach, not as a mild baking alkali. Baking powder is a mixture that contains sodium bicarbonate but is not the same as pure NaHCO3. Thus, none of these options correctly name sodium bicarbonate. Common Pitfalls: Students often mix up baking soda and baking powder because both are used in cooking and have similar sounding names. Another confusion occurs between washing soda and baking soda, both being sodium carbonates with different degrees of hydration and acidity. To avoid errors, remember that baking soda is NaHCO3, washing soda is Na2CO3, and baking powder is a ready made mixture that includes baking soda plus an acid and filler. Final Answer: The common name of sodium bicarbonate, NaHCO3, is baking soda.

Question 15

In basic inorganic chemistry, which of the following is the correct chemical formula for quicklime, the substance obtained by heating limestone strongly?

Accepted Answer

Introduction / Context: Quicklime is an important industrial chemical used in construction, metallurgy, and environmental applications. It is produced by heating limestone, and it can further react with water to form slaked lime. Understanding the correct formula of quicklime and how it is produced is a standard topic in school level chemistry. This question asks you to choose the correct chemical formula for quicklime from several calcium compounds. Given Data / Assumptions: Limestone is mainly calcium carbonate, CaCO3. When limestone is strongly heated (calcination), it decomposes into quicklime and carbon dioxide. Quicklime is also called burnt lime. Related calcium compounds include calcium hydroxide, calcium chloride, and others used in different processes. Concept / Approach: The thermal decomposition of calcium carbonate on heating is a classic example of a decomposition reaction. The balanced reaction is: CaCO3 (s) → CaO (s) + CO2 (g) The product CaO is called quicklime or burnt lime. When quicklime reacts with water, it forms calcium hydroxide, Ca(OH)2, commonly known as slaked lime. Calcium chloride, CaCl2, and other calcium compounds have different uses and are not formed directly in this decomposition. Therefore, the correct formula for quicklime is CaO. Step-by-Step Solution: Step 1: Identify limestone as calcium carbonate, formula CaCO3. Step 2: Recall that strong heating of CaCO3 causes it to lose carbon dioxide gas. Step 3: Write the reaction CaCO3 → CaO + CO2 and see that the solid residue CaO is quicklime. Step 4: Recognise that Ca(OH)2 is obtained only after adding water to CaO, giving slaked lime. Step 5: Choose CaO as the correct formula from the list of options. Verification / Alternative check: Industrial lime kilns operate by heating limestone to high temperatures, producing quicklime CaO, which is then often converted to calcium hydroxide for use in mortar and plaster. Textbooks consistently define quicklime as calcium oxide and slaked lime as calcium hydroxide. Material safety data sheets for quicklime list the formula as CaO. No standard reference refers to CaCO3, Ca(OH)2, or CaCl2 as quicklime, confirming that CaO is the correct choice. Why Other Options Are Wrong: CaCO3 is limestone itself, the starting material, not the product after strong heating. Ca(OH)2 is slaked lime, produced when CaO reacts with water. CaCl2 is calcium chloride, a salt often used as a drying agent and de icing compound, not related to lime production. CaO2 (if considered) would suggest calcium peroxide, which is not the common substance known as quicklime. Thus, only CaO correctly represents quicklime. Common Pitfalls: Some learners confuse quicklime with slaked lime and may mistakenly choose Ca(OH)2. Others may think that limestone and quicklime share the same formula because both are calcium compounds. To avoid these errors, remember the sequence: limestone (CaCO3) on heating gives quicklime (CaO), and quicklime plus water gives slaked lime (Ca(OH)2). Linking this three step chain in memory helps with many related questions. Final Answer: The correct chemical formula for quicklime is CaO (calcium oxide).

Question 16

Dichlorodifluoromethane (CCl2F2) is a chlorofluorocarbon refrigerant. What is its well known common trade name?

Accepted Answer

Introduction / Context: Many industrial chemicals are sold under trade names that become widely used in science and engineering. Dichlorodifluoromethane, with formula CCl2F2, is a chlorofluorocarbon that was once very common in refrigeration and air conditioning systems. It is better known to students and technicians by its common trade name. Recognising this name helps in understanding discussions about refrigerants and environmental issues such as ozone layer depletion. Given Data / Assumptions: The compound in question is dichlorodifluoromethane, formula CCl2F2. It is a refrigerant gas and belongs to the chlorofluorocarbon (CFC) family. Options include names of minerals and industrial products such as galena, gypsum, borax, and Teflon, as well as a refrigerant trade name. We assume standard usage of these common names in school chemistry. Concept / Approach: The family of chlorofluorocarbon refrigerants was marketed under the trade name Freon. Specific members of this family are numbered, and dichlorodifluoromethane (CCl2F2) is known as Freon-12. Galena is lead sulphide ore, gypsum is calcium sulphate dihydrate, borax is hydrated sodium borate, and Teflon is polytetrafluoroethylene. None of these are refrigerant gases of the CCl2F2 type. Therefore, the correct common name for dichlorodifluoromethane is Freon-12, usually shortened to Freon in general statements. Step-by-Step Solution: Step 1: Identify the chemical as dichlorodifluoromethane, a molecule with one carbon, two chlorine, and two fluorine atoms. Step 2: Recall that this type of molecule belongs to the chlorofluorocarbons, which were widely used as refrigerants and propellants. Step 3: Remember that DuPont and other companies sold these compounds under the trade name Freon, with different numbers for different formulations. Step 4: Note that CCl2F2 corresponds to Freon-12 in this numbering system. Step 5: Select the option that clearly names Freon-12 as the common trade name. Verification / Alternative check: Older textbooks and technical manuals on refrigeration refer to CCl2F2 as R-12 or Freon-12. Environmental discussions on CFCs also mention Freon as a major class of ozone depleting substances, with dichlorodifluoromethane as a typical example. In contrast, mineralogy texts define galena as PbS, gypsum as CaSO4·2H2O, and borax as Na2B4O7·10H2O. Polymer chemistry texts identify Teflon as a solid fluoropolymer, not a gaseous refrigerant. These references confirm that Freon-12 is the correct trade name for CCl2F2. Why Other Options Are Wrong: Galena is an ore of lead, chemically lead sulphide, and has nothing to do with refrigerant gases. Gypsum is a hydrated calcium sulphate used in plaster and cement. Borax is a sodium borate used in detergents and glass manufacture. Teflon is a solid polymer used as a non stick coating and in electrical insulation. None of these substances are dichlorodifluoromethane or share its role as a traditional refrigerant gas. Common Pitfalls: Some learners may simply pick a familiar sounding chemical like Teflon without checking whether it is a gas or solid, or whether it is used as a refrigerant. Others may not recognise the connection between chlorofluorocarbons and the Freon trade name. To avoid such errors, remember that many older refrigerants and aerosol propellants labelled as CFCs were marketed as Freon, and CCl2F2 specifically is Freon-12. Final Answer: The common trade name of dichlorodifluoromethane (CCl2F2) is Freon-12, commonly referred to as Freon.

Question 17

Considering the given elements, which one has the lowest melting point: xenon, iodine, barium, or magnesium?

Accepted Answer

Introduction / Context: Melting point trends in the periodic table are influenced by the type and strength of bonding in the solid state. Metals, nonmetal solids, and noble gases all show very different melting behaviours. This question asks you to compare the melting points of four elements from different groups: xenon, iodine, barium, and magnesium. Recognising which has the lowest melting point helps reinforce the idea that weak intermolecular forces in noble gases lead to very low melting temperatures. Given Data / Assumptions: Xenon is a noble gas in group 18. Iodine is a halogen in group 17, existing as a molecular solid I2 at room conditions. Barium and magnesium are metals in groups 2 (alkaline earth metals). Metals have metallic bonding, molecular solids have van der Waals forces, and noble gases are held together only by very weak dispersion forces in the solid state. Concept / Approach: The melting point of a substance depends on how strongly its particles are held together in the solid. Metals like barium and magnesium have metallic bonding with a sea of delocalised electrons, which is strong and gives relatively high melting points. Iodine exists as I2 molecules in a solid lattice, held by intermolecular forces stronger than those in noble gases. Xenon, as a noble gas, forms a molecular solid held together only by very weak London dispersion forces, so it melts at a very low temperature. Therefore, xenon has the lowest melting point among the four elements listed. Step-by-Step Solution: Step 1: Classify xenon as a noble gas, iodine as a nonmetal molecular solid, and barium and magnesium as metals. Step 2: Recall that metallic bonding in barium and magnesium is strong, so their melting points are relatively high. Step 3: Note that iodine has discrete I2 molecules but still forms a solid at room temperature, indicating moderate intermolecular forces. Step 4: Recognise that xenon is gaseous at room temperature and only condenses at very low temperatures, meaning its solid is held together by the weakest forces. Step 5: Conclude that xenon has the lowest melting point among xenon, iodine, barium, and magnesium. Verification / Alternative check: If approximate data are consulted, xenon melts at about minus 112 degrees Celsius, iodine melts near 114 degrees Celsius above zero, magnesium melts above 600 degrees Celsius, and barium melts above 700 degrees Celsius. These values confirm that xenon has by far the lowest melting point. The order matches the bonding strength: weak van der Waals forces for xenon, stronger molecular forces for iodine, and strongest metallic bonding for magnesium and barium. Why Other Options Are Wrong: Iodine has a significantly higher melting point than xenon because the I2 molecules have more electrons and stronger dispersion forces and are arranged in a molecular crystal. Barium and magnesium are metals with strong metallic bonds, so their melting points are much higher than those of molecular solids and noble gases. The suggestion that all have approximately the same melting point is clearly incorrect because their bonding types and phase at room temperature are quite different. Common Pitfalls: A common mistake is to focus on atomic mass alone and assume that heavier elements must have higher or lower melting points without considering bonding. Another error is to forget that noble gases are weakly bound and tend to liquefy and solidify only at very low temperatures. To avoid these issues, always link melting point primarily to bonding type: metallic, ionic, covalent network, molecular, or noble gas solids, with noble gas solids having among the lowest melting points. Final Answer: Among xenon, iodine, barium, and magnesium, the element with the lowest melting point is xenon.

Question 18

In a typical acid–base neutralisation reaction between an acid and a base, which product is formed along with a salt?

Accepted Answer

Introduction / Context: Acid–base reactions are among the most common reactions studied in introductory chemistry. When an acid reacts with a base, they often neutralise each other. Students are expected to know the general form of such reactions and the typical products formed. This question focuses on the classic neutralisation reaction and asks which substance is produced along with a salt when an acid reacts with a base in aqueous solution. Given Data / Assumptions: We are considering standard neutralisation between an acid (providing H+ ions) and a base (providing OH− ions) in water. The base may be a hydroxide such as NaOH, KOH, Ca(OH)2, and so on. The acid may be hydrochloric acid, sulphuric acid, nitric acid, or other common acids. The solution is aqueous, and no redox gas forming reactions are specifically described. Concept / Approach: The essential ionic reaction in a simple acid–base neutralisation is: H+ (aq) + OH− (aq) → H2O (l) At the same time, the remaining ions from the acid and base (such as Na+ and Cl−) combine to form a salt that remains dissolved or may crystallise. Therefore, the general products are a salt and water. Hydrogen gas, oxygen gas, carbon dioxide, or chlorine gas may be produced in specific types of reactions, but they are not the standard products of a straightforward acid plus base neutralisation in water. Step-by-Step Solution: Step 1: Write a general example, such as HCl (acid) reacting with NaOH (base). Step 2: The molecular equation is HCl + NaOH → NaCl + H2O. Step 3: Identify NaCl as the salt and H2O as the other product. Step 4: Recognise that the net ionic equation highlights formation of water from H+ and OH− ions. Step 5: Conclude that in general, a neutralisation reaction forms a salt and water. Verification / Alternative check: Check other examples: sulphuric acid plus sodium hydroxide produces sodium sulphate and water; nitric acid plus potassium hydroxide produces potassium nitrate and water. In every case, the common product besides the salt is water. When acids react with metals or carbonates, gases like hydrogen or carbon dioxide can be released, but those are not simple acid–base neutralisations involving hydroxide bases. This confirms that water is the standard co product with salt in neutralisation reactions. Why Other Options Are Wrong: Hydrogen gas is produced when acids react with some active metals, not when they neutralise bases. Oxygen gas may appear in some decomposition or redox reactions, not in typical acid–base neutralisation. Carbon dioxide is produced when acids react with carbonates or bicarbonates, such as Na2CO3 or NaHCO3, but this is a different category of reaction. Chlorine gas might be evolved in certain redox reactions with strong oxidising agents but is not a normal neutralisation product. These options therefore do not match the general acid plus base case. Common Pitfalls: Students sometimes mix up gas forming reactions of acids with metals or carbonates and assume that those gases appear in all acid reactions. Another mistake is to focus only on the salt formation and forget that neutralisation also produces water. To avoid confusion, remember that neutralisation means H+ from the acid and OH− from the base combine to form H2O, while the leftover ions form the salt. Final Answer: In a typical acid–base neutralisation reaction, the products are a salt and water.

Question 19

In temperature scales, absolute zero is defined as which of the following physical conditions?

Accepted Answer

Introduction / Context: Absolute zero is a fundamental concept in thermodynamics and temperature measurement. It forms the lower limit of the Kelvin scale and is closely related to the behaviour of gases and the kinetic theory of matter. Many exam questions ask for the correct physical definition or interpretation of absolute zero, because understanding it helps explain why there is a natural lowest possible temperature and how particle motion relates to temperature. Given Data / Assumptions: Temperature is related to the average kinetic energy of the particles in a substance. Kelvin is an absolute temperature scale where 0 K is defined as absolute zero. Real gases deviate from ideal behaviour at very low temperatures, but the definition is based on an idealised model. Absolute zero corresponds approximately to minus 273.15 degrees Celsius. Concept / Approach: According to kinetic theory, the temperature of a substance is proportional to the average kinetic energy of its particles. As a substance is cooled, this energy decreases. Absolute zero is defined as the temperature at which the average kinetic energy of the particles would reach a minimum and classical translational motion would effectively cease. In reality, quantum effects remain, but for basic chemistry and physics, absolute zero is described as the point where molecular motion stops. It is also the point where an ideal gas would extrapolate to zero volume when plotted using Charles law, though real gases liquefy before this point is reached. Step-by-Step Solution: Step 1: Recall that the Kelvin temperature scale starts at absolute zero, which corresponds to 0 K. Step 2: Connect temperature with molecular motion: higher temperature means faster average particle motion, lower temperature means slower motion. Step 3: Understand that as temperature approaches absolute zero, the average kinetic energy of particles approaches a minimum value. Step 4: Recognise that for simple explanations, absolute zero is defined as the point where molecular motion would cease. Step 5: Relate this to gas laws: extrapolating the volume of an ideal gas versus temperature graph to zero volume gives a temperature near minus 273.15 degrees Celsius, which is defined as absolute zero. Verification / Alternative check: In many textbooks, absolute zero is introduced via Charles law, which states that for a fixed amount of gas at constant pressure, volume is proportional to temperature on the Kelvin scale. Plotting volume versus Celsius temperature and extending the line to where volume would be zero gives a value of about minus 273 degrees Celsius, which is defined as 0 K. The explanation given is that at this temperature, particle motion would stop, and no further cooling would be possible. This supports the definition in terms of cessation of molecular motion, rather than boiling of water or helium. Why Other Options Are Wrong: The temperature at which water boils at 298 K is nonsensical, because 298 K is about 25 degrees Celsius, where water does not boil at standard pressure. The boiling point of liquid helium depends on pressure and is not used to define absolute zero. The statement that gas volume becomes zero in practice is not accurate, because real gases liquefy and solidify before reaching absolute zero; zero volume is an ideal extrapolation, not an experimental fact. The temperature at which ice and steam coexist in equilibrium relates to the triple point or phase diagram, not to absolute zero. These options misrepresent the true definition. Common Pitfalls: Some students mix up absolute zero with the freezing point of water, the boiling point of water, or the triple point. Others may think that absolute zero is simply where volume is zero, without understanding that this result comes from extrapolation of an ideal gas law. To avoid confusion, remember that absolute zero is fundamentally about the cessation of thermal motion and the lowest possible value of internal energy on the Kelvin scale, not about a specific substance boiling or freezing. Final Answer: Absolute zero is defined as the temperature at which, in theory, all molecular motion ceases.

Question 20

Bronze is a historically important alloy used in tools and statues. Which two metals are primarily combined to form bronze?

Accepted Answer

Introduction / Context: Bronze is one of the earliest alloys developed by humans and gave its name to the Bronze Age. It has been used for weapons, tools, coins, and works of art due to its hardness, durability, and resistance to corrosion. For general knowledge and basic chemistry, students are often asked to identify which metals make up bronze, and to distinguish it from other alloys such as brass and steel. Given Data / Assumptions: Bronze is an alloy, meaning a mixture of metals. Common candidate metals in such questions include copper, tin, zinc, iron, nickel, and carbon. Brass is another copper alloy, and steel is based on iron and carbon. The question asks specifically which two metals form bronze in its classical composition. Concept / Approach: Classical bronze is mainly an alloy of copper (Cu) and tin (Sn). Small amounts of other elements may be present, but the essential identity of bronze comes from this copper–tin combination. Brass, by contrast, is copper plus zinc, and is used in cartridges, fittings, and musical instruments. Steel is an alloy of iron and carbon, sometimes with additional elements like chromium or nickel. Therefore, to answer this question correctly, we must associate bronze with copper and tin, and not confuse it with brass or other alloys. Step-by-Step Solution: Step 1: List the metal pairs given: copper and tin, copper and zinc, copper and iron, iron and nickel, iron and carbon. Step 2: Recall that bronze is the alloy that historically replaced stone tools and weapons and that it is copper based. Step 3: Remember the standard textbook description: bronze is an alloy of copper and tin. Step 4: Distinguish this from brass, which is copper and zinc, and from steel, which is iron and carbon. Step 5: Select copper and tin as the correct pair for bronze. Verification / Alternative check: Materials science references list bronze as Cu–Sn alloys. Archaeological studies of Bronze Age artefacts show that typical bronze contains a high percentage of copper with a smaller amount of tin. Industry standards refer to various bronzes, such as phosphor bronze, that still centre on copper and tin. In contrast, brass specifications always emphasise copper–zinc compositions. This confirms that bronze is correctly identified as an alloy of copper and tin. Why Other Options Are Wrong: Copper and zinc together form brass, not bronze. Copper and iron are not a standard named alloy in basic textbooks; they can form some specialty alloys but not bronze. Iron and nickel form alloys used in special applications such as Invar, not bronze. Iron and carbon form steel and cast iron rather than bronze. These pairs therefore correspond to different materials and not to the alloy the question is about. Common Pitfalls: Students frequently mix up bronze and brass because both are copper based alloys and their names sound similar. Some may also confuse them with steel because all are used for tools. To avoid mistakes, it helps to memorise a simple association: bronze equals copper plus tin, brass equals copper plus zinc, steel equals iron plus carbon. Once this mapping is clear, such multiple choice questions become straightforward. Final Answer: Bronze is primarily an alloy of copper and tin.

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