Chemical Reactions and Equations

Silver chloride decomposes in the presence of light to form silver and chlorine: \(2AgCl \xrightarrow{\text{light}} 2Ag + Cl_2\). The breakdown of one compound into simpler substances is a decomposition reaction. Here, light supplies the energy rather than heat, so this is photolytic decomposition. In contrast, heating calcium carbonate causes \(CaCO_3 \rightarrow CaO + CO_2\), making option B an example of thermal decomposition.

According to the law of conservation of mass, the total mass of reactants and products remains the same if no substance enters or leaves the system. In a closed system, any gas produced is retained; therefore, measuring the mass of the entire system, including the container, before and after the reaction provides a reliable test. In an open container, gas may escape and make the measured mass appear lower. A colour change or the mass of only a precipitate does not represent the mass of the whole system.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. Light-green iron(II) sulphate forms, while reddish-brown copper is deposited on the nail: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). In a combination reaction, reactants generally form one product; here, iron(II) sulphate and copper are two products.

The reaction is \(\mathrm{CaCO_3(s) \rightarrow CaO(s) + CO_2(g)}\). In an intact closed container, no matter, including \(\mathrm{CO_2}\), leaves the system, so the total mass of the container and all its contents remains unchanged. Option B could apply to an open container, where escaping \(\mathrm{CO_2}\) can make the measured mass decrease.

The law of conservation of mass applies to the complete system. In an open container, a produced gas may escape, so it is not included when the container and its remaining contents are weighed later. Therefore, the measured mass appears lower, but total mass has not been destroyed. Option B is incorrect because mass is not destroyed in a chemical reaction.

A balanced chemical equation represents the law of conservation of mass. In an ordinary chemical reaction, atoms are neither created nor destroyed; they are only rearranged to form new substances. Therefore, each element must have the same number of atoms on the reactant and product sides. Balancing does not mean that colour or heat must remain unchanged, and it does not prevent gas formation. Hence option A is correct.

Subscripts specify the fixed number of atoms of each element in a compound. For example, changing H₂O to H₂O₂ changes water into hydrogen peroxide, so it changes the substance itself. Only coefficients should be changed when balancing because they change the number of molecules or moles without changing the chemical formula.

The formula H₂O gives the atomic composition of one water molecule: two hydrogen atoms and one oxygen atom. For two water molecules, multiply the hydrogen count by two: 2 molecules × 2 hydrogen atoms per molecule = 4 hydrogen atoms. Therefore option B is correct. Option A counts only one water molecule, while options C and D do not follow the subscript in the formula or the stated number of molecules.

The product is \(2MgO\). Each formula unit of \(MgO\) contains one oxygen atom, and the coefficient 2 represents two formula units. Therefore, the total number of oxygen atoms is \(2 \times 1 = 2\). Although \(O_2\) also has two oxygen atoms, it is on the reactant side; atoms on the product side are counted from \(2MgO\).

A thin layer of magnesium oxide forms on magnesium when it is exposed to air. This layer can prevent direct contact between magnesium and oxygen and hinder burning. Rubbing with sandpaper removes the oxide layer, exposing fresh magnesium so that it burns readily. The purpose is not to make the ribbon thinner.

Magnesium reacts with oxygen in air to form white magnesium oxide. The balanced equation is \(2Mg + O_2 \rightarrow 2MgO\). This is a combination reaction because two reactants form one product. Magnesium chloride would require chlorine, which is not present in this reaction.

Calcium oxide reacts with water as CaO + H₂O → Ca(OH)₂. Two reactants combine to produce a single product, calcium hydroxide, so the reaction is a combination reaction. Because heat is released to the surroundings, it is also exothermic. Therefore, option A correctly identifies both features; the other options describe different reaction types or energy changes.

On thermal decomposition, calcium carbonate forms solid calcium oxide and carbon dioxide gas: \(\mathrm{CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)}\). Therefore, the solid product is calcium oxide. Calcium hydroxide is formed when calcium oxide reacts with water, not by heating calcium carbonate alone.

Calcium carbonate undergoes thermal decomposition on heating to form calcium oxide and carbon dioxide: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). The released \(\mathrm{CO_2}\) reacts with lime water, \(\mathrm{Ca(OH)_2}\), forming insoluble calcium carbonate: \(\mathrm{CO_2 + Ca(OH)_2 \rightarrow CaCO_3 \downarrow + H_2O}\). This precipitate makes the lime water appear milky. Oxygen does not form this precipitate with lime water.

On heating, lead nitrate undergoes thermal decomposition: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide, NO₂, is a reddish-brown gas, so its brown fumes provide the identifying observation. Oxygen is colourless, hydrogen is colourless and combustible, and carbon dioxide is colourless and does not produce brown fumes. Thus option B is correct.

When lead nitrate is heated, it decomposes according to 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Lead oxide, PbO, remains as the solid residue, while nitrogen dioxide and oxygen escape as gases. Lead sulphate, lead chloride, and lead carbonate are not products of this decomposition because their anions are absent from the reactant. Therefore, option A is correct.

The thermal decomposition reaction is:
\[
2\mathrm{Pb(NO_3)_2} \rightarrow 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}
\]
Here,
\(\mathrm{NO_2}\)
is the brown gas, whereas
\(\mathrm{O_2}\)
is colourless. Therefore, the brown colour identifies
\(\mathrm{NO_2}\)
, not oxygen. Oxygen is indeed formed, but it is confirmed by rekindling a glowing splint, not by observing brown fumes.

Silver chloride undergoes photodecomposition in sunlight: \(2\mathrm{AgCl}(s)\xrightarrow{\text{sunlight}}2\mathrm{Ag}(s)+\mathrm{Cl}_2(g)\). The silver metal produced is grey, so silver chloride gradually appears grey. Although chlorine gas is also formed, it is not responsible for the grey colour.

Silver bromide is photosensitive. In light, it undergoes photodecomposition: \(2\mathrm{AgBr}\xrightarrow{\text{light}}2\mathrm{Ag}+\mathrm{Br}_2\). A dark-coloured bottle reduces the light reaching the compound and therefore helps prevent this decomposition. It is not used to increase thermal decomposition or solubility 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 numbers of moles. Therefore, the volume ratio of hydrogen to oxygen is \(2:1\). The ratio \(1:2\) would represent oxygen to hydrogen, not the order asked in the question.