Chemical Reactions and Equations

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Black copper(II) oxide loses oxygen and changes into reddish-brown copper. Loss of oxygen is reduction, so copper(II) oxide is reduced. In contrast, oxidation of copper would involve copper gaining oxygen to form an oxide.

Hydrogen combines with oxygen removed from copper(II) oxide and forms water. By the classical definition, gain of oxygen is oxidation, so hydrogen is oxidised and option B is correct. Copper(II) oxide loses oxygen and becomes copper; therefore CuO is reduced. Hydrogen is the reducing agent because it causes CuO to lose oxygen, not the oxidising agent. No precipitate is involved in this reaction.

Oxidation and reduction occur together in a redox reaction. Loss of oxygen is reduction, while gain of oxygen is oxidation. Therefore, if one substance loses oxygen and another gains it during the same chemical change, the process is called a redox reaction, making option B correct. Precipitation concerns formation of an insoluble solid, whereas evaporation and melting are physical changes and do not describe oxygen transfer.

A redox reaction involves transfer of electrons. The substance that loses electrons is oxidised, and the substance that gains electrons is reduced. Electron loss and electron gain occur simultaneously in the same reaction, so option B is correct. A colour change may sometimes indicate a reaction, but it is not the definition of oxidation or reduction.

In the hydrogen-based definitions of redox chemistry, removal of hydrogen from a substance is oxidation, while addition of hydrogen is reduction. Thus the required order is oxidation followed by reduction, so option A is correct. Option B reverses the two processes. Precipitation, decomposition, combination, and displacement describe other reaction patterns and do not define the gain or loss of hydrogen in this context.

A paint coating acts as a barrier between iron and the environment. At a scratch, iron becomes exposed to oxygen from air and moisture. In their presence, iron can undergo oxidation to form hydrated iron(III) oxide, or rust. Option B is incorrect because paint does not produce oxygen; the scratch merely removes the protective barrier.

In option A, calcium oxide and water combine to form a single product, calcium hydroxide, so it is a combination reaction. Heat is released when quicklime forms slaked lime; therefore, the reaction is also exothermic. In contrast, option B is a photodecomposition reaction in which one compound breaks down into simpler substances in sunlight.

Rancidity is mainly caused by oxidation of unsaturated oils and fats when they react with oxygen in air. The oxidation products, including unpleasant-smelling compounds, change the original taste and odour. Hence option A is correct. Reduction is the opposite type of redox change, neutralisation requires an acid-base reaction, and displacement with a metal is not the normal cause of rancidity. Antioxidants and airtight storage can slow this oxidation.

Ghee contains fats. Oxygen in open air oxidises these fats, and warmth increases the rate of this reaction. This leads to rancidity, which can produce an unpleasant smell and taste. Nitrogen is relatively unreactive, so option B is not the cause. Exam tip: Link spoilage of oils and fats with oxidation and rancidity.

Oils and fats react with oxygen in air and undergo oxidation, causing rancidity. Filling the packet with nitrogen greatly reduces the available oxygen, so oxidation occurs more slowly and the food remains fresh for longer. Unlike option C, nitrogen does not reduce or saturate oils and fats under normal packaging conditions.

Rancidity is the development of an unpleasant smell and taste in fats or oils because they undergo oxidation. Antioxidants react preferentially with oxidising agents or interrupt oxidation chains, while airtight packaging limits the contact of food with oxygen. Thus both methods slow oxidation and reduce rancidity. Galvanisation, electrolytic decomposition and photolytic decomposition are different processes and do not describe this food-preservation effect.

A cooling container may indicate that the process has absorbed heat from its surroundings, so it may be endothermic. Gas evolution has also been observed, but these two observations alone cannot identify the exact type of chemical reaction. For example, gas evolution does not by itself prove that a reaction is exothermic, so option B is not justified.

Heat written on the products side shows that heat is released during the reaction. A warm reaction vessel also indicates that heat is being transferred to the surroundings, so the reaction is exothermic. In contrast, an endothermic reaction absorbs heat from the surroundings, and heat is written on the reactants side.

Heat written on the reactant side shows that the reaction absorbs heat, so it is endothermic. The breakdown of one compound into two or more products is the defining feature of a decomposition reaction. Therefore, it is an endothermic decomposition reaction. In contrast, a combination reaction usually forms one product from two or more reactants.

In respiration, glucose is oxidised in many enzyme-controlled steps. Therefore, energy is released gradually rather than suddenly as heat and light, and part of it is captured in ATP. In contrast, combustion is usually a rapid oxidation process in which energy is released quickly, mainly as heat and sometimes light.

Photosynthesis is an endothermic process in which light energy is used to store chemical energy in glucose formed from carbon dioxide and water. In contrast, cellular respiration oxidises glucose and releases energy, which is used to form ATP. Option B is incorrect because photosynthesis does not release energy from glucose; it stores energy in glucose.

When food is chewed only by teeth, it is broken into smaller pieces without changing its chemical composition; therefore, this is mainly a physical change. During enzyme-mediated digestion, complex food molecules such as starch and proteins are chemically converted into simpler substances. Thus, digestion is a chemical change. Option B is incorrect because digestion does not merely reduce particle size; it also forms new, simpler substances.

During conversion of milk into curd, bacterial action produces new substances such as lactic acid and permanently changes the properties of milk. Hence, it is a chemical change. In contrast, freezing water changes only its state; ice is still water and returns to water on melting. Thus, ice formation is a physical change, so option A is correct.

A colour change can be an indication of a chemical reaction, but it is not conclusive by itself. For example, mixing coloured substances or dissolving a coloured substance can change the colour without forming a new substance. Confirmation of a chemical reaction requires evidence of a change in composition or formation of a new substance. Option B is incorrect because colour change does not always prove a chemical change.

In a chemical equation, an upward arrow is written when a gaseous product is evolved and escapes from the reaction mixture. A downward arrow is used when an insoluble solid separates from a solution; this solid is called a precipitate. Therefore the respective meanings are gas and precipitate. The reverse order in option B confuses the two symbols, while heat, light, liquid and solid are not their standard meanings.