Chemical Reactions and Equations

During the first step, copper combines with oxygen from air: 2Cu + O₂ → 2CuO. Addition of oxygen is oxidation, so copper is oxidised and the product becomes black CuO. In the second step, hydrogen reacts with CuO and removes its oxygen: CuO + H₂ → Cu + H₂O. Removal of oxygen is reduction, so option A correctly describes both changes; the other options ignore hydrogen’s role or misidentify the reaction.

Chewing is mainly a physical change because it breaks food into smaller pieces while its chemical composition essentially remains the same. In digestion, enzymes convert complex substances such as carbohydrates, proteins, and fats into simpler substances such as glucose, amino acids, and fatty acids with glycerol. Therefore, digestion involves chemical change. Option B is incorrect because digestion is not limited to reducing the size of food particles.

In a displacement reaction, a more reactive element displaces a less reactive element from its compound. For example, \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). Zinc is more reactive than copper, so it displaces copper from copper sulfate. Option B describes a double-displacement reaction, in which ions of two compounds exchange places.

A colour change can be a possible sign of a chemical reaction, but by itself it is not conclusive proof that a new substance has formed. Colour may also change due to light, mixing of substances, or other physical changes. Therefore, look for further evidence such as gas evolution, precipitate formation, temperature change, or new properties. Option A is incorrect because many chemical reactions do produce a colour change. Exam tip: Use more than one observation to confirm the formation of a new substance.

A colour change can be a sign of a chemical reaction, but it is not necessary in every chemical reaction. Formation of a gas may suggest that a new substance has formed, so it can indicate a chemical reaction. However, bubbles alone are not conclusive because a physical process such as boiling can also produce bubbles. Check additional observations, such as temperature change, precipitate, odour, or the identity of the gas. Exam tip: Do not treat a single observation as conclusive evidence.

Rusting is the slow oxidation of iron. It requires both oxygen and moisture, producing hydrated iron(III) oxide, \(Fe_2O_3\cdot xH_2O\). No \(H_2\) gas is evolved. Option B is incorrect because balancing coefficients cannot introduce the required \(O_2\) or remove the incorrect product \(H_2\).

\(\text{(aq)}\) specifically means the species is dissolved in water (aqueous solution). \(\text{(l)}\) simply means the species is present as a liquid, without stating that it is dissolved in water. So \(\text{(aq)}\) is a liquid form in aqueous medium, while \(\text{(l)}\) is liquid state in general.

Magnesium reacts with oxygen in air to form magnesium oxide: \(2Mg + O_2 \rightarrow 2MgO\). Since a new substance is formed, this is a chemical change. However, it does not involve mixing aqueous solutions to produce an insoluble solid, so it is not a precipitation reaction. In contrast, option A forms insoluble \(BaSO_4\), option B forms insoluble \(AgCl\), and option D forms insoluble \(PbI_2\); therefore, these are precipitation reactions.

On heating, lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). The energy is supplied by heat, not by electric current, so it is not electrolytic decomposition. In contrast, acidified water in A, molten sodium chloride in B, and copper chloride solution in C decompose because of electric current.

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.

In Al, the oxidation number is 0, while in Al₂O₃ it becomes +3; aluminium loses electrons and is oxidised. Iron in Fe₂O₃ has oxidation number +3 and changes to elemental iron with oxidation number 0, so it gains electrons and is reduced. This is a redox reaction in which aluminium acts as the reducing agent and iron(III) oxide as the oxidising agent. Therefore option A is correct.

The law of conservation of mass applies when the complete system is considered. In an open container, a gaseous product can leave before weighing, so the measured system contains less material and its apparent mass decreases. The gas has not been destroyed; it is simply outside the chosen system boundary. The reaction may still be chemical and has produced a product. Hence option B is correct.

A chemical reaction rearranges existing atoms into new substances; it does not create or destroy atoms. Therefore, each element must have the same number of atoms among the reactants and products. This atom balance expresses the law of conservation of mass because the total mass contributed by each type of atom is retained. Energy may change, but balancing coefficients are not primarily used to show energy, colour or temperature conservation. Option B is correct.

In hydrogen peroxide, oxygen has oxidation number −1. During decomposition, some oxygen atoms become −2 in H₂O, so they are reduced, while other oxygen atoms become 0 in O₂, so they are oxidised. The same compound therefore undergoes simultaneous oxidation and reduction; this is disproportionation. The other choices are displacement, combination, and double-displacement reactions, not disproportionation.

Carbon dioxide normally suppresses fire by reducing contact between fuel and oxygen, but burning magnesium is an important exception. At high temperature magnesium removes oxygen from carbon dioxide: 2Mg + CO₂ → 2MgO + C. The reaction supplies magnesium with oxygen and produces carbon, so the ribbon can continue burning. CO₂ is not combustible, and it does not simply decompose into oxygen.

This reaction is a single-displacement reaction because zinc, being more reactive than copper, replaces copper from copper sulphate: Zn + CuSO₄ → ZnSO₄ + Cu. It is also a redox reaction: Zn changes from oxidation state 0 to +2 and is oxidised, while Cu²⁺ changes to Cu and is reduced. Hence A is the most complete classification. It is not combination, thermal decomposition, or double displacement.

The conservation-of-mass law applies to a complete, closed system. In an open container, the gas produced may escape into the surroundings before the final weighing. The balance then measures only the remaining material, not the escaped gas, so the observed mass is lower even though total mass has been conserved. Therefore A is correct. Mass is not destroyed, the reaction need not stop, and a chemical reaction does not become physical merely because gas escapes.

A subscript gives the fixed ratio of atoms in one molecule or formula unit. For example, H₂O and H₂O₂ are different substances because their atomic compositions differ. Changing a subscript while balancing would therefore change the reactant or product itself, not merely its amount. The correct method is to change coefficients placed before formulas, which alter the number of particles without changing identity. Hence A is correct; B, C and D do not describe the consequence.