Chemical Reactions and Equations

In a displacement reaction, only a more reactive metal displaces 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. Hence, no reaction occurs. Option A is incorrect because copper is less reactive than zinc. Exam tip: Compare the metals in the reactivity series to predict a displacement reaction.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The reaction is \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Since a more reactive element replaces a less reactive element from its compound, this is a displacement reaction. In a double displacement reaction, ions from two compounds exchange partners, which does not occur here.

In a double displacement reaction, the cations and anions of two ionic compounds exchange partners to form new compounds. Its general form is \(AB + CD \rightarrow AD + CB\). Burning a metal is an example of a combination/oxidation reaction, not a double displacement reaction.

Iron is more reactive than copper, so it displaces copper from copper sulfate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The iron sulfate formed makes the solution light green, and displaced copper deposits as a reddish-brown coating on the nail. Unlike option B, less reactive copper cannot displace iron from its salt solution.

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Hydrogen combines with oxygen removed from copper oxide and forms water. Addition of oxygen is oxidation, so hydrogen is oxidised. In contrast, copper oxide is reduced because it loses oxygen. Exam tip: Identify oxidation by checking which substance gains oxygen.

In a redox reaction, oxidation and reduction always occur together. One substance loses electrons and is oxidised, while another substance gains those electrons and is reduced. Options B and C are changes of state, which do not necessarily involve transfer of electrons.

The addition of oxygen to a substance is the classical definition of oxidation. For example, magnesium combines with oxygen to form magnesium oxide: 2Mg + O2 → 2MgO. Thus option B is correct. Reduction generally involves removal of oxygen or addition of hydrogen, while melting and filtration are physical processes and do not describe the chemical change caused by oxygen addition.

Removal of oxygen from a substance is called reduction. For example, in
\(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\), oxygen is removed from copper(II) oxide, so
\(\mathrm{CuO}\) is reduced. In contrast, oxidation involves the addition of oxygen to a substance.

In the classical school-level definition, addition of hydrogen to a substance is reduction. For example, hydrogen can remove oxygen from a metal oxide, converting it to the metal. Therefore option D is correct. Oxidation is commonly associated with addition of oxygen or removal of hydrogen. Precipitation forms an insoluble solid from a solution, and evaporation changes liquid into vapour; neither describes hydrogen addition.

Oxidation can be described as addition of oxygen or removal of hydrogen from a substance. Therefore, when hydrogen is removed, the process is generally identified as oxidation, making option A correct. Reduction is the opposite in this classical framework: it commonly involves addition of hydrogen or removal of oxygen. Double displacement and precipitation describe particular reaction patterns, not the specific removal of hydrogen.

Rusting is a slow chemical reaction in which iron reacts with oxygen in the presence of moisture, forming hydrated iron(III) oxide, commonly called rust. It is therefore an example of slow oxidation, so option B is correct. Evaporation and freezing are physical changes, while dissolving a precipitate does not describe the formation of rust or the reaction of iron with oxygen and water.

Rusting requires iron to be in contact with both oxygen and water or moisture. A coat of paint forms a protective barrier on the iron surface, so air and moisture cannot easily reach the metal and rusting is reduced. Simply making the surface harder does not prevent rusting.

A displacement reaction occurs when a more reactive element replaces a less reactive element from its compound. In option A, zinc is above copper in the reactivity series, so Zn removes Cu from CuSO₄ and forms ZnSO₄ while copper is released. Option B is a combination reaction, option C is a decomposition reaction, and option D is a double-displacement reaction.

This is a single displacement reaction. Iron is above copper in the reactivity series, so \(Fe\) displaces copper from \(CuSO_4\), forming \(FeSO_4\) and \(Cu\). In a double displacement reaction, ions of two compounds exchange partners; here, one metal replaces another metal from its salt.

Chips contain oil or fat, which can react with oxygen in air and become oxidised. This causes rancidity, producing an unpleasant smell and taste. Nitrogen is relatively unreactive; it reduces the amount of oxygen in the packet and thereby slows oxidation of the oil. Formation of a precipitate refers to separation of an insoluble solid from a solution, so it is not relevant here.

Rancidity is the unpleasant smell and taste that develops when oils and fats undergo oxidation. Antioxidants react preferentially with oxidising substances or interrupt oxidation chains, thereby slowing this process and extending the food’s shelf life. Therefore option C is correct. Option A describes antimicrobial action, option B would increase spoilage, and option D is related to drying rather than antioxidant protection.

Rancidity mainly results from oxidation of unsaturated oils and fats by oxygen in the air. An airtight container limits the entry and contact of oxygen, so the oxidation rate decreases and unpleasant smells and tastes develop more slowly. Thus option B is correct. Heating would generally speed many oxidation processes, while precipitation and conversion into metal have no connection with rancidity.

Ice and liquid water are the same substance, H₂O. During melting, the particles gain energy and change from an ordered solid state to a liquid state, but their chemical composition and identity remain unchanged. The process is reversible by freezing, so it is a physical change. Option C is correct; no new gas, precipitate, or substance is produced.

When milk changes into curd, bacterial action changes components of the milk and curd with new properties is formed. Curd differs from milk in taste, smell, and texture, so this is a chemical change. A change of state alone, such as water freezing into ice, is not a chemical change because no new substance is formed.

In respiration, glucose undergoes complete oxidation in the presence of oxygen and releases energy: \(\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy}\). Because energy is released, respiration is an exothermic reaction. Options B and C are true features of respiration, but they do not explain why the reaction is exothermic; the release of energy does.