Chemical Reactions and Equations

In the classical description of redox processes, removal of hydrogen from a substance is oxidation, while addition of hydrogen is reduction. The question asks for these processes in that exact order: first hydrogen is removed, so oxidation occurs; then hydrogen is added, so reduction occurs. Consequently, option D is correct. Option A contains the same two terms but reverses their order, and the other options name unrelated reaction types.

Rusting is an electrochemical corrosion process that requires both water or moisture and oxygen. In moist open air, iron receives a continuous supply of moisture and oxygen, so rust formation is greatest. A dry airtight bottle lacks sufficient moisture and fresh oxygen; an oil coating blocks contact with both, and a nitrogen-filled container lacks oxygen. Therefore option A is the only condition that strongly supports rusting.

Rusting requires iron to be in contact with both oxygen and water (moisture). A coating of paint or oil forms a protective barrier on the iron surface and prevents air and moisture from reaching the metal. Therefore, option B is correct. Heating and drying may remove moisture temporarily, but unlike paint or oil, it does not provide a lasting protective coating.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The reddish-brown coating is copper, and the light-green solution is due to iron sulphate. This is not a combination reaction because the reactants do not combine to form a single product.

Oils and fats in oily food react with oxygen in air and undergo oxidation. This oxidation produces an unpleasant smell and taste; the process is called rancidity. Heat increases the reaction rate, so oily food becomes rancid faster in warm, open air. Option B is incorrect because warmth does not make fats unreactive; it can increase the rate of oxidation.

In this reaction, the two reactants, magnesium and oxygen, combine to form only one product, \(\mathrm{MgO}\). Therefore, it is a combination reaction. In a displacement reaction, an element replaces another element present in a compound, for example \(\mathrm{Zn}+\mathrm{CuSO}_4\rightarrow\mathrm{ZnSO}_4+\mathrm{Cu}\). No compound containing a replaceable element is present here.

Zinc is more reactive than copper, so it displaces copper from copper sulphate: \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). As colourless zinc sulphate forms, the blue colour fades, and displaced copper deposits as a reddish-brown coating on zinc. In contrast, this reaction does not produce a gas or a white precipitate.

The oxidation number of Sn in \(\mathrm{SnCl_2}\) increases from \(+2\) to \(+4\), so \(\mathrm{SnCl_2}\) is oxidised. It donates electrons and reduces \(\mathrm{Fe^{3+}}\) in \(\mathrm{FeCl_3}\) to \(\mathrm{Fe^{2+}}\). Therefore, \(\mathrm{SnCl_2}\) is the reducing agent, whereas \(\mathrm{FeCl_3}\) is the oxidising agent.

Iron is more reactive than copper. Therefore, iron displaces copper from copper sulphate and forms iron sulphate. The green solution is iron sulphate, \(\mathrm{FeSO_4}\), and the reddish-brown coating is copper metal. The reaction is \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). It is a displacement reaction because a more reactive metal replaces a less reactive metal from its compound. In a double displacement reaction, ions of two compounds exchange partners, which does not occur here.

Iron is more reactive than copper, so it displaces copper from copper sulfate solution. In this reaction, iron normally forms \(Fe^{2+}\) ions; therefore, the salt formed is ferrous sulfate, \(FeSO_4\): \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). Option A correctly refers to the relative reactivity, but \(Fe_2(SO_4)_3\) contains iron in the \(Fe^{3+}\) state and is not the product of this usual displacement reaction.

In \(Fe_2O_3\), iron has oxidation state \(+3\), which becomes \(0\) in the product \(Fe\). A decrease in oxidation state indicates reduction; therefore, iron(III) oxide is reduced. In contrast, aluminium changes from \(0\) to \(+3\), so it is oxidised.

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 \(H_2\).

(aq) means that a substance is dissolved in water and is present as an aqueous solution, for example, NaCl(aq). (l) means that the substance itself is in the liquid state, for example, H_2O(l). Thus, (l) does not mean dissolved in water; it indicates the physical liquid state of the substance.

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.