Chemical Reactions and Equations

Removal of oxygen from substance A is reduction, while addition of oxygen to substance B is oxidation. Because these two complementary processes occur simultaneously in one reaction, the overall reaction is classified as a redox reaction. Option B describes only oxidation and option C only reduction; neither represents the complete reaction. Neutralisation requires acid-base interaction.

At this level, oxidation is identified by the addition of oxygen, whereas reduction is identified by the removal of oxygen. The required sequence is therefore addition of oxygen first and removal of oxygen second, which is exactly option A. Option B reverses the order. The processes in option D are physical changes, not oxidation-reduction definitions.

In option A, the ions of two aqueous compounds exchange partners: Ba²⁺ combines with SO₄²⁻ to form insoluble BaSO₄(s), while Na⁺ and Cl⁻ remain in solution. This is both a double displacement reaction and a precipitation reaction. Option B is single displacement, C is combination, and D is neutralisation without a solid precipitate.

In \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\), the oxidation numbers of calcium \((+2)\), carbon \((+4)\), and oxygen \((-2)\) remain unchanged on both sides. Therefore, neither oxidation nor reduction occurs, so it is not a redox reaction. In contrast, in option B, Zn changes from \(0\) to \(+2\) and \(\mathrm{Cu^{2+}}\) changes from \(+2\) to \(0\), making it a redox reaction.

Ions exchange between aqueous \(\mathrm{AgNO_3}\) and \(\mathrm{NaCl}\), producing insoluble white \(\mathrm{AgCl(s)}\) as a precipitate; therefore, this is a precipitation and double-displacement reaction. The oxidation numbers remain unchanged: \(\mathrm{Ag}\) is \(+1\), \(\mathrm{Cl}\) is \(-1\), \(\mathrm{Na}\) is \(+1\), and \(\mathrm{N}\) is \(+5\). Hence, no oxidation or reduction occurs, so it is not a redox reaction. The presence of two reactants does not make it a combination reaction, since two products are formed.

Zinc is more reactive than iron. Even if the coating is scratched, zinc oxidises preferentially and supplies electrons that prevent or slow the oxidation of the exposed iron. This is called sacrificial protection. Option C is incorrect because air and moisture can reach a scratch; the protection mainly results from zinc being more reactive than iron.

Rancidity is the deterioration of oils and fats when they react with oxygen, usually producing unpleasant-smelling and bad-tasting substances. This chemical process is oxidation. Therefore option A is correct. Freezing is a physical change, salt precipitation is unrelated to the spoilage of fats, and metal displacement is a different type of chemical reaction.

Ghee contains fats that can react with oxygen and become rancid. Keeping it open increases contact with atmospheric oxygen, while warmth generally speeds up the oxidation reaction. Consequently, rancidity develops more rapidly. Option A gives both relevant factors; nitrogen formation, conversion into a metal, and precipitate dissolution do not explain oxidation of ghee.

The balanced equation for electrolysis of water is \(2H_2O \rightarrow 2H_2 + O_2\). At the same temperature and pressure, gas volumes are proportional to their moles, so \(H_2:O_2 = 2:1\). Therefore, 15 mL of \(O_2\) at the anode corresponds to 30 mL of \(H_2\) at the cathode. Option B notes that water contains both elements but does not give the correct volume ratio.

Antioxidants slow oxidation reactions in food, whereas airtight packaging reduces the food's contact with oxygen. Thus, both help prevent or reduce oxidation of fats and oils and hence rancidity. In contrast, forming a precipitate is associated with reactions in solutions, not with preventing oxidation of food.

If the container and its surroundings feel colder during a reaction, the reacting system is absorbing thermal energy from the surroundings. Such a heat-absorbing reaction is called endothermic. Therefore option A is the best explanation. A reaction that releases heat would warm the surroundings, while colour change alone does not describe the energy transfer.

Heat written on the product side shows that the reaction releases heat to the surroundings. Emission of light is also a sign of energy release. Therefore, the reaction is exothermic. In contrast, an endothermic reaction absorbs heat, so heat is written on the reactant side.

The governing concept is decomposition: one compound splits into two or more simpler substances. Because heat supplies the energy required for this breakdown, the reaction is specifically called thermal decomposition. For example, calcium carbonate decomposes on heating: CaCO₃ → CaO + CO₂. A combination reaction forms one product from several reactants, while double-displacement and precipitation involve exchange of ions, so option A is correct.

In combustion, oxidation of a fuel releases energy as heat and often light. In respiration, glucose undergoes controlled oxidation and releases energy for cellular activities. Therefore, both are exothermic processes. In contrast, a process that absorbs energy is endothermic.

Iron is more reactive than copper, so it displaces copper from copper sulfate solution: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). Here, \(Fe^0 \rightarrow Fe^{2+} + 2e^-\) is oxidation, while \(Cu^{2+} + 2e^- \rightarrow Cu\) is reduction. The reddish-brown coating is copper metal. Unlike option B, iron loses electrons rather than gaining them.

Chewing breaks food into smaller pieces; its main effect is a change in size and surface area, so it is mainly a physical change. During digestion, enzymes convert complex food substances into new, simpler, absorbable substances, so it is a chemical change. Option B is incorrect because digestion is not merely a reduction in size.

During curd formation, bacterial action produces new substances such as lactic acid and the properties of milk change; therefore, it is a chemical change. In contrast, when water freezes to form ice, its chemical composition remains \(\mathrm{H_2O}\); only its state changes, so it is a physical change. A colour change or precipitate formation is not an essential feature of either process.

When water is added to a copper sulfate solution, its concentration decreases, so the blue colour may become lighter. This dilution is a physical change and does not necessarily form a new substance. In contrast, in option B, the formation of an insoluble precipitate is additional evidence of a new substance being formed.

A careful scientific conclusion must rely on evidence without claiming more than the observation proves. Evolution of a gas may indicate that a new substance has formed, so it can be evidence of a chemical reaction. However, absence of colour change does not rule out a reaction, because many reactions show no visible colour change. The observation alone cannot prove that the change is physical or specifically rusting; therefore option A is best.

In a chemical equation, a downward arrow \(\downarrow\) is used for an insoluble solid product that separates from a solution and settles down; this is called a precipitate. An upward arrow \(\uparrow\) is used for a gas evolved during the reaction. Option B reverses the meanings of these two symbols.