Chemical Reactions and Equations

When magnesium burns, it reacts with oxygen: \(2\mathrm{Mg} + \mathrm{O}_2 \rightarrow 2\mathrm{MgO}\). Two reactants, magnesium and oxygen, combine to form one type of product, magnesium oxide; therefore, it is a combination reaction. Option B describes a decomposition reaction, in which a compound breaks down.

The reaction is \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Since two reactants combine to form one product, it is a combination reaction. The container becoming hot shows that heat is released, so the reaction is also exothermic. In a decomposition reaction, one compound breaks into simpler substances, which does not occur here.

Calcium carbonate undergoes thermal decomposition: \(\mathrm{CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)}\). Therefore, calcium oxide is the solid product and carbon dioxide is the gaseous product. Forming calcium hydroxide and hydrogen would require a source of water or hydrogen, which is not present in this reaction.

In this reaction, one compound, \(AgCl\), breaks down to form silver \(Ag\) and chlorine gas \(Cl_2\), so it is a decomposition reaction. Because sunlight supplies the energy for this breakdown, it is specifically a photochemical decomposition reaction. In thermal decomposition, heat causes the substance to break down; here, light energy is used instead.

This is a thermal decomposition reaction: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide, NO₂, is a reddish-brown gas, so option B is correct. The oxygen released is colourless, nitrogen is also colourless, and nitric oxide is not the brown gas produced in this reaction. The observation of brown fumes is therefore a useful test for NO₂.

On heating, lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). \(\mathrm{NO_2}\) is a brown gas, whereas \(\mathrm{O_2}\) is colourless. Therefore, oxygen is the correct answer; carbon dioxide is not a product of this reaction.

On heating, lead nitrate breaks down because of heat into lead oxide, nitrogen dioxide, and oxygen: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). Therefore, it is thermal decomposition. Photolytic decomposition requires light, whereas electrolytic decomposition requires an electric current.

In the presence of light, silver chloride undergoes photochemical decomposition: \(2\mathrm{AgCl}(s) \xrightarrow{\text{light}} 2\mathrm{Ag}(s)+\mathrm{Cl}_2(g)\). The silver metal formed is grey, so the white silver chloride appears grey. This is not due to the formation of silver oxide; light causes silver ions to be reduced to silver metal.

Silver chloride is photosensitive. On exposure to light, it undergoes photochemical decomposition to form metallic silver and chlorine: \(2\mathrm{AgCl}\xrightarrow{\text{light}}2\mathrm{Ag}+\mathrm{Cl_2}\). Therefore, it is kept in the dark. Option D is different because light, not heat, is the main cause of decomposition here.

The balanced equation is \(2\mathrm{H_2O}\rightarrow 2\mathrm{H_2}+\mathrm{O_2}\). At the same temperature and pressure, gas volumes are proportional to their moles, so \(\mathrm{H_2}:\mathrm{O_2}=2:1\). Therefore, for 18 mL of hydrogen, the oxygen volume is \(18\div2=9\) mL. The 18 mL option is incorrect because the two gases are not produced in equal volumes.

The overall reaction for electrolysis of acidified water is \(2H_2O(l) \rightarrow 2H_2(g) + O_2(g)\). Oxygen is produced at the anode and hydrogen at the cathode. At the same temperature and pressure, gas volume is proportional to the number of moles, so the volume ratio of hydrogen to oxygen is \(2:1\). Therefore, the oxygen collected at the anode has the lower volume. This difference is due to the stoichiometric ratio in the equation, not to molar mass.

Pure water has a very low concentration of ions due to self-ionisation, so it is a poor conductor of electricity. Adding a dilute acid increases the number of mobile cations and anions that carry charge through the solution. Therefore, electric current passes more easily. Option B is incorrect because acid does not convert water molecules into electrons; conduction in the solution occurs through ions.

Decomposition means breaking one compound into simpler substances, and it requires an energy input. In electrolytic decomposition, electrical energy supplied through an electric current causes the compound to split, as in electrolysis of water. In thermal decomposition, heat energy causes the splitting, as when calcium carbonate is heated. Thus option A correctly identifies the difference; the other choices describe neither process accurately.

Iron is above copper in the reactivity series, so it can displace copper from copper sulphate. The single-displacement equation is Fe + CuSO₄ → FeSO₄ + Cu. Copper separates and deposits as a reddish-brown coating on the nail, while aqueous iron(II) sulphate gives the solution a pale green colour. Therefore option A is correct. The observations prove that new substances form, so adsorption and the other reaction descriptions are unsuitable.

Option C is a double displacement and precipitation reaction. In aqueous solution, Na⁺ and Ba²⁺ exchange partners with SO₄²⁻ and Cl⁻. Ba²⁺ combines with SO₄²⁻ to produce insoluble barium sulphate, BaSO₄, which appears as a precipitate: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl. Option A is decomposition, B is single displacement, and D is combination, so C is unambiguous.

The displacement rule states that a more reactive metal can remove a less reactive metal from its salt solution. Since metal A displaces metal B, A lies above B in the reactivity series and is more reactive. The fact that B cannot displace A confirms the same order. Therefore option A is correct. Equal reactivity would not produce this one-way displacement, and the statement gives no basis for calling either substance a gas.

When steam is passed over hot iron, magnetite, \(\mathrm{Fe_3O_4}\), and hydrogen are formed. The correct balanced equation is \(\mathrm{3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2}\). Therefore, writing \(\mathrm{FeO}\) as the product is incorrect. Unlike option B, hydrogen from steam is released as hydrogen gas.

Option B is a redox reaction. The oxidation state of Zn changes from \(0\) to \(+2\), so Zn is oxidised. In \(\mathrm{CuSO_4}\), Cu changes from \(+2\) to \(0\), so Cu is reduced. In contrast, option C is a neutralisation reaction, in which oxidation states do not change.

In \(\mathrm{CuO}\), copper changes from oxidation state \(+2\) to \(0\) in the product \(\mathrm{Cu}\). A decrease in oxidation number indicates reduction, so \(\mathrm{CuO}\) is reduced. In contrast, hydrogen in \(\mathrm{H_2}\) changes from \(0\) to \(+1\) in \(\mathrm{H_2O}\), so \(\mathrm{H_2}\) is oxidised.

In a precipitation reaction, the insoluble solid separates from the solution and settles down; it is represented by a downward arrow, atex[↓atex[. An upward arrow, atex[↑atex[, indicates the evolution of a gas, whereas atex[Δatex[ and atexνatex[ indicate the presence of heat and light, respectively.