Chemical Reactions and Equations

\((aq)\) means that a substance is dissolved in water and is present as an aqueous solution, for example, \(\mathrm{NaCl(aq)}\). \((l)\) means that the substance itself is in the liquid state, for example, \(\mathrm{H_2O(l)}\). Thus, \((aq)\) does not merely mean liquid; it specifically indicates dissolution in water.

Hydrogen has oxidation number 0 in \(\mathrm{H_2}\), which becomes +1 in \(\mathrm{H_2O}\); therefore, hydrogen is oxidised. Copper has oxidation number +2 in \(\mathrm{CuO}\) and 0 in \(\mathrm{Cu}\); therefore, copper(II) oxide is reduced. Option B states the opposite changes.

In option A, zinc is more reactive than copper and displaces copper from \(\mathrm{CuSO_4}\). Zinc is oxidised because its oxidation number changes from \(0\) to \(+2\), whereas copper is reduced because its oxidation number changes from \(+2\) to \(0\). Therefore, this is both a displacement and a redox reaction. Option B is a double-displacement (precipitation) reaction, not a displacement reaction.

The oxidation number of hydrogen in \(H_2\) increases from 0 to +1 in \(H_2O\), so \(H_2\) is oxidised. At the same time, copper in \(CuO\) is reduced from +2 to 0 in \(Cu\). A substance that is itself oxidised while causing reduction of another substance is the reducing agent; therefore, \(H_2\) is the reducing agent. In contrast, \(CuO\) is the oxidising agent.

The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction, provided the complete system is considered. A closed container prevents gases, vapours, or solids from leaving, so measuring the total mass before and after gives a reliable comparison. In an open vessel, escaping gas makes the remaining material appear lighter. Colour and smell are observations, not measurements of total mass; therefore option A is correct.

Oxidation can be identified by an increase in oxidation number or addition of oxygen, while reduction involves a decrease in oxidation number or removal of oxygen. In H₂, hydrogen has oxidation number 0; in H₂O it becomes +1, so hydrogen is oxidised. Copper in CuO is +2 and becomes elemental copper with oxidation number 0, so CuO is reduced. Since both occur together, this is a redox reaction and option A is correct.

According to the law of conservation of mass, mass is neither created nor destroyed during a reaction in a closed container. Since no gas or other substance leaves the container, the total mass of the products equals the total mass of the reactants, which is 20 g. A value such as 10 g would be possible only if some material escaped from the container.

According to the law of conservation of mass, the total mass remains constant during a chemical reaction. In an open container, a gas produced may escape, so its mass will not be included in the measurement. Therefore, the reaction should be performed in a closed container and the total mass should be measured before and after the reaction. Measuring only the remaining solid is incorrect because the gas is also a product of the reaction.

The coefficient 2 before H₂O means that two water molecules are present on the product side. The subscript 2 in each H₂O molecule shows that each molecule contains two hydrogen atoms. Therefore, the total number of hydrogen atoms is coefficient × subscript = 2 × 2 = 4. The value 2 counts hydrogen atoms in only one water molecule, whereas 6 incorrectly mixes coefficients and subscripts. Thus option C is correct.

The product side contains \(2\mathrm{MgO}\). Each formula unit of \(\mathrm{MgO}\) has 1 oxygen atom, and the coefficient 2 means there are 2 such units. Thus, the total number of oxygen atoms is \(2\times1=2\). Do not confuse this with the subscript 2 in \(\mathrm{O}_2\), which shows the number of atoms in one oxygen molecule on the reactant side.

Subscripts specify the fixed number of atoms of each element in a compound. For example, changing the subscript 2 in \(\mathrm{H_2O}\) to form \(\mathrm{H_2O_2}\) changes water into hydrogen peroxide, a different substance. Therefore, only coefficients are changed to balance an equation, because they change the amount of a substance without changing its identity.

The reaction is \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2 + heat}\). Calcium oxide and water combine to form a single product, calcium hydroxide; therefore, it is a combination reaction. Since heat is released, it is exothermic. In contrast, in a decomposition reaction, one compound breaks down into two or more substances.

Calcium carbonate undergoes thermal decomposition: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). The \(\mathrm{CO_2}\) produced reacts with lime water, \(\mathrm{Ca(OH)_2}\), to form an insoluble white precipitate of \(\mathrm{CaCO_3}\), making the solution milky. Hydrogen, nitrogen, and oxygen do not produce this milkiness.

On heating, calcium carbonate undergoes thermal decomposition: \(\mathrm{CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)}\). Carbon dioxide is released as a gas, while calcium oxide remains as the solid product. Calcium hydroxide is formed when calcium oxide reacts with water, not in this decomposition reaction.

On heating, lead nitrate undergoes thermal decomposition: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide, NO₂, is a reddish-brown gas, so its characteristic colour identifies it as the gas observed. Oxygen is colourless, hydrogen is also colourless and is not produced in this decomposition, and carbon dioxide is not a product of lead nitrate decomposition. Therefore option C is correct.

Lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). Lead oxide, \(\mathrm{PbO}\), is the solid residue, and nitrogen dioxide, \(\mathrm{NO_2}\), is the brown gas. Although oxygen is also produced, it is colourless, so option A does not identify the brown gas correctly.

Silver chloride is photosensitive and decomposes when exposed to sunlight according to 2AgCl → 2Ag + Cl₂. Light energy initiates this decomposition, and the metallic silver produced appears grey, causing the visible colour change. The reaction is not driven by sound or electrical energy in this observation, and water or hydrogen is not formed. Thus the correct combination is light energy and silver, given in option A.

Silver bromide is photosensitive. In the presence of light, it can decompose: \(2\mathrm{AgBr}\xrightarrow{\text{light}}2\mathrm{Ag}+\mathrm{Br}_2\). A dark-coloured bottle reduces exposure to light and therefore prevents photolytic decomposition. It does not increase the solubility of silver bromide in water.

The balanced equation for electrolysis of water is \(2H_2O(l) \rightarrow 2H_2(g) + O_2(g)\). At the same temperature and pressure, gas volumes are proportional to their mole ratio. Therefore, \(H_2:O_2 = 2:1\). Hence, for 20 mL of hydrogen, the oxygen volume is \(20/2 = 10\) mL. Choosing 20 mL would be incorrect because equal volumes of the two gases are not produced.

Calcium carbonate undergoes thermal decomposition: CaCO₃ → CaO + CO₂. In the open test tube, carbon dioxide escapes, so the measured solid loses the mass of the gas and appears lighter. For example, 100 g of calcium carbonate can produce 56 g of calcium oxide and 44 g of carbon dioxide; the total remains 100 g when all products are included. Calcium atoms are not destroyed, and CO₂ has mass. Therefore option A correctly explains the observation.