Chemical Reactions and Equations

A common visible indicator of a chemical reaction is the evolution of a gas. If a gaseous product is formed in the liquid mixture, it escapes as bubbles, so option A is the best answer. Melting changes the physical state of a solid, a precipitate is an insoluble solid that settles or remains suspended, and a colour change alone does not specifically explain bubbles. Therefore B, C, and D do not fit.

The governing concept is an endothermic energy change. If the container becomes cooler during a reaction, the reacting system may have absorbed heat from the surroundings, lowering the surroundings’ temperature. Therefore option A is correct. A reaction that releases heat would generally warm its surroundings, while burning and metal deposition are specific processes that cannot be concluded merely from cooling. The word “can” appropriately indicates an inference, not proof of one particular reaction.

The governing concept is the direction of energy transfer. In an exothermic reaction, the chemical system releases energy to its surroundings; that energy may appear as heat and, in some reactions, as light. Because both heat and light are released here, option A is correct. An endothermic reaction absorbs energy, while displacement and double-displacement describe reactant replacement patterns, not necessarily whether heat or light is released.

In school chemical equations, an upward arrow is used to indicate that a gaseous product has been evolved and escapes from the reaction mixture. Thus option A correctly explains the symbol’s purpose. The arrow is not a general rule that every solid or liquid moves upward, and a precipitate is normally represented as an insoluble solid, often by a downward arrow when appropriate. Therefore options B, C, and D are incorrect.

Iron is more reactive than copper, so it displaces copper from copper sulfate: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The liberated copper forms a reddish-brown coating on the nail, and the iron sulfate formed makes the solution light green. In contrast, a silver-like coating and a colourless solution, as in option B, are not expected.

This is a displacement reaction because the more reactive metal zinc displaces copper from its salt solution: \(Zn + CuSO_4 \rightarrow ZnSO_4 + Cu\). In a double displacement reaction, ions of two compounds are exchanged rather than one metal displacing another metal.

When a word equation is converted into a chemical equation, the correct chemical formulae of reactants and products are written first. The next task is to add coefficients so that the number of atoms of each element is equal on both sides. This follows the law of conservation of mass. Removing reactants or products would not make the equation correct.

According to the law of conservation of mass, matter is neither created nor destroyed in a chemical reaction. Therefore, in a balanced equation, the number of atoms of each element must be the same on the reactant and product sides. Having only the same total number of atoms is not sufficient, because the number of atoms of a particular element could still change.

A colour change can be an indication of a chemical reaction, but it is not always conclusive. For example, dissolving a coloured substance in water or mixing coloured substances can change the observed colour without necessarily forming a new substance. Therefore, other observations, such as precipitate formation, gas evolution, temperature change, or the nature of reactants and products, should also be considered. Option D is incorrect because many reactions show a colour change without producing a gas.

The governing criterion is whether the chemical composition changes. In a physical change, a substance may be cut, bent, crushed, reshaped, or changed in state while its composition remains unchanged. Since only the shape changes and no new substance forms, option A is correct. A chemical change forms substances with new compositions; redox and precipitation are particular chemical processes and therefore do not describe this situation.

During digestion, enzymes chemically convert complex food molecules into simpler substances. For example, starch is converted into simpler substances such as glucose. The formation of new substances identifies digestion as a chemical reaction. In contrast, merely cutting or chewing food into smaller pieces is mainly a physical change because no new substances are formed.

Respiration is an exothermic chemical process because glucose is oxidised in cells, generally represented as glucose plus oxygen forming carbon dioxide and water, with energy released for cellular work. The release of energy is the reason for the classification, so option A is correct. Freezing is unrelated to respiration, oxygen does not form a precipitate in this process, and carbon dioxide does not turn into a metal; therefore B, C, and D are impossible distractors.

During photosynthesis, plants absorb energy from sunlight to form glucose from carbon dioxide and water. Because an external input of energy is required, it is classified as an energy-absorbing, or in the school context endothermic, process. In contrast, option B describes release of energy as heat, which is a feature of an exothermic process.

In a displacement reaction, a more reactive element replaces a less reactive element from its compound. In options B, C, and D, Fe, Zn, and Mg respectively displace another metal from its salt. In option A, \(\mathrm{Ba^{2+}}\) combines with \(\mathrm{SO_4^{2-}}\) to form a \(\mathrm{BaSO_4}\) precipitate, while the ions exchange partners. Therefore, it is a double-displacement reaction, not a displacement reaction.

Calcium oxide reacts with water to form a single product, calcium hydroxide: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Hence, it is a combination reaction. Heat is released during this reaction, so it is also exothermic. In contrast, on heating, calcium carbonate breaks into simpler substances, so it is a decomposition reaction.

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 on the nail is copper, and the iron sulphate formed makes the solution green. This is not a combination reaction because the reactants do not combine to form a single product.

Oils and fats can undergo oxidation in the presence of oxygen, causing rancidity and producing an unpleasant smell and taste. Flushing the packet with nitrogen and sealing it airtight greatly reduces contact with oxygen, so rancidity is slowed down. In contrast, filling the packet with oxygen or keeping it in open air increases oxidation.

The governing observation is the formation of an insoluble solid from a reaction mixture. Such an insoluble product is called a precipitate, and a reaction that forms it in solution may be identified as a precipitation reaction. Option A is deliberately phrased as “may” because colour and insolubility alone do not establish the complete reaction type with certainty. Combustion, photolytic decomposition, and a purely physical change cannot be concluded from this observation.

A change in the properties of substances indicates that new substances have formed, so a chemical reaction has occurred. In a closed system, equal total mass before and after the reaction demonstrates the law of conservation of mass. Option B is incorrect because no reaction, or merely a change of position, would not produce substances with new properties.

Iron is more reactive than copper. Therefore, it displaces copper from copper sulphate solution and forms pale-green iron(II) sulphate:
\[\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\]
Thus, the solution turns green, while copper is deposited as a reddish-brown coating on the nail. Copper oxide is not formed in this displacement reaction.