Chemical Reactions and Equations

In a displacement reaction, a more reactive metal can displace a less reactive metal from its salt solution. Zinc is above copper in the reactivity series, so copper cannot displace zinc from zinc sulphate solution. If option B were true, displacement would be possible; however, copper is actually less reactive than zinc.

Zinc is more reactive than copper, so it displaces copper from copper sulphate solution:
\(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\)
The released copper is deposited as a reddish-brown solid on the zinc surface. Zinc sulphate is formed in the solution, so it is not the deposited solid.

The blue colour of copper sulphate solution is due to copper(II) ions. Zinc is more reactive than copper, so it displaces copper from copper sulphate in a displacement reaction: \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). The concentration of blue copper(II) ions decreases, so the blue colour fades. Sulphate ions remain in the solution and are not responsible for the colour change.

On heating, lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \rightarrow 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). The \(\mathrm{NO_2}\) formed is a brown gas, so it produces the brown fumes. \(\mathrm{O_2}\) is also produced, but it is colourless.

When lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide, and oxygen: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). The breakdown of one compound into two or more simpler substances by heat is called a thermal decomposition reaction. In a combination reaction, two or more reactants form one product, whereas here one reactant produces several products.

When copper powder is heated in air, it combines with oxygen and forms black copper(II) oxide: 2Cu + O₂ → 2CuO. The black CuO coating changes the appearance of the copper powder. Copper(I) oxide, Cu₂O, is red, copper carbonate is not formed under these conditions, and ordinary heating in air does not mean that copper melts; therefore option A is unambiguous.

In the reaction
\(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\), oxygen is removed from copper(II) oxide and copper is formed. Removal of oxygen from a substance is called reduction; therefore, \(\mathrm{CuO}\) is reduced. In contrast, hydrogen gains oxygen to form water, so it is oxidised.

In the reaction \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\), hydrogen gains oxygen from copper(II) oxide to form water. Gain of oxygen is oxidation, so hydrogen is oxidised. The oxidation state of hydrogen increases from \(0\) to \(+1\) in water. In contrast, copper changes from \(+2\) in \(\mathrm{CuO}\) to \(0\) in copper metal, so copper is reduced.

In a precipitation reaction, mixing two aqueous solutions may produce an insoluble solid that separates from the solution. This solid is called a precipitate. In contrast, combustion usually involves reaction with oxygen and produces heat and light; formation of an insoluble solid is not its main feature.

The reaction is \(\mathrm{Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Here, \(\mathrm{Na^+}\) and \(\mathrm{Ba^{2+}}\) exchange anionic partners, and insoluble white \(\mathrm{BaSO_4}\) forms as a precipitate. Therefore, it is a double displacement, specifically a precipitation, reaction. In a single displacement reaction, a more reactive element replaces another element from a compound; that does not happen here.

A balanced chemical equation follows the law of conservation of mass. Therefore, the number of atoms of each element is the same on the reactant and product sides. Hence, the number of oxygen atoms is equal on both sides. Physical states such as solid, liquid, or gas do not change the atom count.

During digestion, enzymes convert complex food substances into simpler new substances. For example, starch is broken down into simpler sugars and proteins into amino acids. Since the chemical composition of food changes and new substances are formed, digestion is a chemical change. In a physical change, only the size, state, or appearance changes while the chemical composition remains the same. Therefore, digestion is not merely a change of state or a reversible change.

In a burning candle, wax near the wick first melts and vaporises, but the wax vapour combusts with oxygen to form new substances such as carbon dioxide and water vapour. Formation of new substances is the main feature of a chemical change. Therefore, the burning process of a candle is mainly a chemical change. In contrast, melting of wax is only a physical change, or change of state, because no new substance is formed during melting.

When rice is cooked, heat and water cause permanent changes in its properties and structure. Cooked rice cannot be changed back into raw rice by simple methods, so it is a chemical change. In a physical change, the chemical nature of the substance remains unchanged. Exam tip: A change that is not easily reversible and produces new properties is generally a chemical change.

When ice melts, only its physical state changes from solid to liquid. Its chemical composition remains
$
\mathrm{H_2O}$

, so no new substance is formed. Therefore, melting is a physical change, not a chemical reaction. Gas evolution or precipitate formation can be signs of some chemical reactions, but neither occurs when ice melts.

Magnesium burns in oxygen to form white magnesium oxide: \(2Mg + O_2 \rightarrow 2MgO\)। MgO is a basic oxide and can react with water to form basic magnesium hydroxide: \(MgO + H_2O \rightarrow Mg(OH)_2\)। In contrast, an amphoteric oxide reacts with both acids and bases, whereas MgO is not generally classified as amphoteric.

Calcium oxide reacts with water to form calcium hydroxide: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Heat is released to the surroundings during this process, so it is an exothermic reaction. In contrast, an endothermic reaction absorbs heat from the surroundings.

In a decomposition reaction, one compound breaks down into two or more simpler substances. Energy, often in the form of heat, light, or electricity, is supplied to break bonds; therefore, such reactions are generally endothermic. In contrast, exothermic reactions release energy to the surroundings.

Combustion is a chemical reaction in which a fuel reacts with oxygen and usually releases heat and sometimes light. Thus, oxygen is generally necessary for burning in air, making option A correct. Carbon dioxide and water vapour do not support ordinary combustion, while nitrogen is largely unreactive under these conditions and mainly dilutes the oxygen in air.

Bubbles appearing during a reaction usually provide evidence that a gas is being produced or released, so option A is correct. For example, an acid reacting with a carbonate can form carbon dioxide, which escapes as bubbles. A precipitate is an insoluble solid seen as particles or cloudiness, whereas melting and evaporation are physical changes and do not generally explain reaction bubbles.