Chemical Reactions and Equations

The error is treating compound formation as unrelated to oxidation. Oxidation includes addition of oxygen, so magnesium is oxidised when it changes from elemental Mg to MgO. Its oxidation number rises from 0 to +2. Oxygen changes from 0 to −2 and is reduced. Option C incorrectly assigns magnesium’s oxidation to oxygen, while B and D contradict the redox definitions. Hence option A is correct.

In a balanced equation, the number of atoms of each element is the same on both sides. In \(2H_2 + O_2 \rightarrow 2H_2O\), there are 4 hydrogen atoms and 2 oxygen atoms on each side. In option B, there are 2 oxygen atoms on the left but only 1 on the right, so it is not balanced.

Magnesium burns in oxygen from air to form white magnesium oxide: \(2Mg + O_2 \rightarrow 2MgO\). Since two reactants combine to form one product, this is a combination reaction. In a decomposition reaction, one compound breaks into simpler substances, which does not occur here.

Calcium oxide reacts with water to form calcium hydroxide: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Heat is released during this reaction, so the container becomes hot. Since two reactants combine to form one product, it is a combination reaction; because heat is released, it is exothermic. In contrast, a decomposition reaction involves one compound breaking down into two or more substances.

On heating, lead nitrate decomposes to form 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 brown fumes are nitrogen dioxide, \(\mathrm{NO_2}\). Since one compound breaks down into simpler substances on heating, this is a thermal decomposition reaction, not a combination reaction.

Silver chloride is photosensitive. On exposure to sunlight, it undergoes photochemical decomposition: 2AgCl → 2Ag + Cl₂. The finely divided silver produced is grey, so option A correctly explains the colour change. Dissolving in water does not produce grey silver, and the other options contain chemically impossible products or incorrectly identify the process.

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 moles. Therefore, hydrogen occupies twice the volume of oxygen, so \(H_2:O_2 = 2:1\). The ratio \(1:2\) is the reverse ratio.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. In this displacement reaction, pale-green iron sulphate forms and copper is deposited as a reddish-brown coating on the nail: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Therefore, the blue solution fades and appears green. Option B is incorrect because copper is less reactive than iron and cannot displace iron.

Zinc is above copper in the reactivity series, so it displaces copper from copper sulphate solution. The reaction is \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). The blue colour of the solution fades and copper may deposit on the zinc strip. Option B is incorrect because copper is less reactive than zinc.

On heating, lead nitrate decomposes according to 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide (NO₂) is a brown gas, whereas oxygen (O₂) is colourless. Therefore, the student confused the two gaseous products, and option A gives the correct correction. The other statements reverse or misidentify their colours and formulas.

The reaction is \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). The \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions combine to form insoluble white \(\mathrm{BaSO_4}\), while \(\mathrm{Na^+}\) and \(\mathrm{Cl^-}\) ions remain in solution. It is a double displacement reaction because ions are exchanged, and a precipitation reaction because an insoluble solid forms. It is not a displacement reaction, since no element replaces another element from its compound.

The reaction is CuO + H₂ → Cu + H₂O. Oxygen is removed from copper(II) oxide and combines with hydrogen to form water. Removal of oxygen is called reduction, so CuO is reduced to copper and option A is correct. Hydrogen gains oxygen and is oxidised; this simultaneous reduction and oxidation makes the reaction a redox process.

In CuO + H₂ → Cu + H₂O, hydrogen combines with oxygen removed from copper(II) oxide and becomes water. Gain of oxygen is oxidation, so hydrogen is oxidised and option A is correct. Copper(II) oxide loses oxygen and is reduced. Displacement describes a reaction pattern, while neutralisation involves an acid and a base, neither of which identifies hydrogen’s process here.

Gain of oxygen is oxidation, while loss of oxygen is reduction. When these two processes occur simultaneously in the same reaction, the reaction is called a redox reaction; therefore, option A is correct. Precipitation produces an insoluble solid, neutralisation involves an acid and a base, and displacement involves replacement of one element by another.

Rusting of iron is an oxidation process that requires both oxygen from air and water or moisture. Moisture provides a thin water layer that supports electrochemical reactions, producing hydrated iron(III) oxide, called rust. Therefore, neither air alone nor moisture alone is considered sufficient for rusting.

Silver chloride undergoes photodecomposition in sunlight, producing grey metallic silver and chlorine gas. The balanced equation is \(2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)\). Option B incorrectly shows chlorine as atomic \(Cl\), whereas chlorine is produced as the diatomic gas \(Cl_2\).

The oils and fats in chips can react with oxygen and become oxidised, causing rancidity and an unpleasant taste or smell. Nitrogen is comparatively unreactive; it displaces oxygen in the packet and reduces oxidation of the oil. In contrast, increasing the oxygen content would promote oxidation.

Rancidity occurs when oils and fats in food undergo oxidation and form substances with an unpleasant smell and taste. Antioxidants slow this oxidation, so food does not spoil as quickly. Unlike option B, antioxidants do not increase the rate of oxidation.

During the conversion of milk into curd, bacterial action changes the components of milk and forms new substances; the change is not easily reversible. Hence, it is a chemical change. In contrast, melting ice is only a change of state, and sugar can be recovered from water, so these are physical changes.

When ice melts, only its state changes from solid to liquid; no new substance is formed, so it is a physical change. Near the wick, wax vapour burns to form new substances such as carbon dioxide and water vapour, so burning is a chemical change. Melting of wax is physical, but burning of wax is chemical.

In a precipitation reaction, mixing two aqueous solutions produces a solid that is insoluble in the solvent. This separated solid is called a precipitate. A constant temperature does not identify the reaction type, whereas the formation of an insoluble solid is a clear indicator of precipitation.

In combustion, a fuel reacts with oxygen to form products and releases energy to the surroundings as heat, often with light. Release of heat makes the reaction exothermic. Option B describes an endothermic reaction, in which heat is absorbed from the surroundings.

In respiration, glucose is oxidised and energy is released. Similarly, in combustion, a fuel is oxidised and energy is released as heat and often light. Therefore, both are exothermic oxidation processes. However, respiration is controlled and occurs slowly in cells, whereas combustion is generally much faster.

Balancing follows the law of conservation of mass: the number of atoms of every element must be equal on both sides. A chemical formula represents the fixed composition and identity of a substance, so changing its subscripts would create a different substance. Coefficients change only the number of molecules or formula units. Therefore option A is correct; the other statements misunderstand formulae or coefficients.

According to the law of conservation of mass, atoms are neither created nor destroyed in a chemical reaction. Therefore, in a balanced equation, the number of atoms of every element is equal on the reactant and product sides. A greater number of atoms on either side would indicate an unbalanced equation.