Chemical Reactions and Equations

Calcium oxide reacts with water to form only one product, calcium hydroxide: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Heat is released, so this is an exothermic combination reaction. Unlike option B, it neither forms two products nor absorbs heat.

A reaction in which one compound breaks down into two or more simpler substances is a decomposition reaction. When heat is supplied to cause this decomposition, it is called thermal decomposition. For example, on heating, calcium carbonate breaks down into calcium oxide and carbon dioxide. Photolytic decomposition requires light energy rather than heat.

Silver chloride and silver bromide are photosensitive salts. On exposure to light, they undergo photodecomposition and form metallic silver, for example: \(2\mathrm{AgCl}\xrightarrow{\text{light}}2\mathrm{Ag}+\mathrm{Cl}_2\). Dark-coloured bottles reduce exposure to light and therefore prevent this decomposition. This is not mainly to prevent thermal decomposition or oxidation by air.

Electrolysis of water is represented by 2H₂O(l) → 2H₂(g) + O₂(g). Reduction occurs at the cathode, so hydrogen is produced there. The balanced equation shows a 2:1 ratio: two volumes of hydrogen are formed for every one volume of oxygen. Oxygen is released at the anode. Nitrogen and chlorine are not products of pure water electrolysis.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). The iron(II) sulphate, \(FeSO_4\), formed gives the solution a pale green colour. Therefore, option A is correct. Copper(II) sulphate solution is blue and is consumed in the reaction.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The released copper deposits on the iron nail as a reddish-brown layer. Iron does not deposit; instead, it reacts and forms iron sulphate in the solution.

In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution. Therefore, if metal A removes metal B from B’s salt solution, A is more reactive than B. Metals with equal reactivity do not undergo such a displacement reaction.

On mixing the solutions, a double-displacement reaction occurs: \(\mathrm{Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Barium sulphate, \(\mathrm{BaSO_4}\), is insoluble in water, so it appears as a white precipitate. Evolution of a brown gas is not a feature of this reaction.

In a precipitation reaction, mixing two aqueous solutions produces an insoluble solid. Here,
\(\mathrm{BaCl_2(aq)+Na_2SO_4(aq)\rightarrow BaSO_4(s)+2NaCl(aq)}\).
\(\mathrm{BaSO_4}\) is an insoluble white solid in water, so it separates from the solution as a precipitate. Evolution of a gas is not the defining feature of a precipitation reaction.

In aqueous solutions of barium chloride and sodium sulphate, ions exchange: \(\mathrm{BaCl_2(aq)+Na_2SO_4(aq)\rightarrow BaSO_4(s)\downarrow+2NaCl(aq)}\). Insoluble barium sulphate is formed as a solid precipitate. Hence, this is both a double displacement reaction and a precipitation reaction. In contrast, burning magnesium is a combination/oxidation reaction, not an exchange of ions.

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Oxygen is removed from copper(II) oxide, so \(\mathrm{CuO}\) is reduced to copper. At the same time, hydrogen gains oxygen to form water and is therefore oxidised. Since reduction and oxidation occur together, this is a redox reaction. Option D is not correct because precipitation usually refers to the formation of an insoluble solid when two aqueous solutions are mixed.

Option A is a single-displacement reaction because iron, being more reactive than copper, replaces copper from copper sulfate: Fe + CuSO₄ → FeSO₄ + Cu. Option B is a combination reaction because two reactants form one product. Option C is a photochemical decomposition reaction, while option D is a double-displacement reaction involving exchange of ions and formation of barium sulfate.

Iron is more reactive than copper. Therefore, it displaces copper from copper sulphate solution. This is a displacement reaction:

\(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\)

The iron sulphate formed makes the solution green, while displaced copper deposits as a reddish-brown coating on the nail. Option B is incorrect because copper is less reactive than iron and cannot displace iron.

Addition of hydrogen to a substance is generally called reduction. For example, when hydrogen reacts with copper(II) oxide, oxygen is removed and copper is formed: \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). In contrast, oxidation generally involves addition of oxygen or removal of hydrogen.

Rusting is the oxidation of iron in the presence of oxygen and water. An airtight, dry container greatly reduces the access of oxygen and water vapour to the iron, so the rate of rusting decreases. Option B is incorrect because iron does not lose its reactivity; the conditions required for rusting are simply reduced.

In galvanisation, the zinc coating prevents iron from coming into direct contact with oxygen in air and moisture. When the coating is removed, iron is exposed and can be oxidised in the presence of oxygen and water, causing rusting. Therefore, option A is correct. Option B is incorrect because a removed zinc coating does not reform on its own.

Oils and fats slowly undergo oxidation on exposure to oxygen in air. This produces substances that give the food an unpleasant smell and taste; this is called rancidity. Nitrogen is generally much less reactive, so option B is not the correct reason.

Rancidity occurs when oils and fats are oxidised by oxygen, causing unpleasant smell and taste in food. Airtight packaging and nitrogen filling reduce exposure to oxygen, so oxidation and rancidity are slowed. Option B is the opposite: greater exposure to oxygen would promote rancidity.

A temperature change can provide evidence that energy has been transferred during a chemical reaction. Exothermic reactions release heat and may raise the temperature, whereas endothermic reactions absorb heat and may lower it. Thus, option A is correct. Temperature change is useful evidence but is not, by itself, an absolutely exclusive test, because some physical processes can also involve heat transfer; the other options make incorrect absolute claims.

Evolution of gas indicates that a gaseous product is being formed. If the container becomes warm without external heating, heat is being released by the reaction; therefore, the reaction is exothermic. In contrast, an endothermic reaction absorbs heat from its surroundings and would not normally make the container warm.

In an exothermic reaction, heat flows from the reacting system to the surroundings, so the surroundings may become warmer. In an endothermic reaction, the system absorbs heat from the surroundings. Option B reverses these processes; formation of a precipitate is not a necessary feature of either type of reaction.

In photosynthesis, atoms from carbon dioxide and water are rearranged to form new substances such as glucose and oxygen. Since chemical bonds change and new substances are produced, it is a chemical change. It is not merely a change of state, because the substances do not just change their physical form; new substances are formed.

In aerobic respiration, glucose reacts with oxygen to form new substances such as carbon dioxide and water, while releasing energy: \(\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + energy}\). Formation of new substances is the key feature of a chemical reaction. Option B describes only a physical change, in which no new substances are formed.

Digestion and curd formation are both chemical changes because the original substances are converted into substances with different compositions and properties. Digestive enzymes break complex food molecules into simpler molecules. During curd formation, bacterial activity converts lactose and changes milk proteins, producing curd with new characteristics. Neither process is merely a change of state or shape, so options B, C and D do not describe the similarity.

Melting of ice is a physical change because ice changes only into liquid water and no new substance is formed. In contrast, electrolysis of water uses electrical energy to produce hydrogen and oxygen gases from water. The formation of new substances shows that electrolysis is a chemical change. Therefore, option B is incorrect because electrolysis is not merely a change of state.