Chemical Reactions and Equations

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.

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.

A displacement reaction occurs only when the free metal is more reactive than the metal present in the salt solution. Copper (Cu) is less reactive than zinc (Zn), so Cu cannot displace Zn from \(\mathrm{ZnSO_4}\). In contrast, in option A, zinc is more reactive than copper and therefore displaces Cu from \(\mathrm{CuSO_4}\).

On mixing the solutions, ions exchange: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Barium sulphate is insoluble in water, so it appears as a white solid precipitate. Gas evolution or a flame is not characteristic of this double-displacement precipitation reaction.

On heating, calcium carbonate undergoes thermal decomposition:
\(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\)
Therefore, calcium oxide and carbon dioxide are formed. Option A is incorrect because calcium hydroxide and water are associated with the reaction of calcium oxide with water, not with heating calcium carbonate alone.

On heating, lead nitrate breaks down into simpler substances: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). The \(\mathrm{NO_2}\) produced is a brown gas. Since heat causes one compound to decompose, this is a thermal decomposition reaction; in a combination reaction, substances combine to form one product.

The formula of water is \(\mathrm{H_2O}\), so each water molecule contains two hydrogen atoms and one oxygen atom. The balanced electrolysis equation is \(2\mathrm{H_2O} \rightarrow 2\mathrm{H_2} + \mathrm{O_2}\). Therefore, under the same conditions, the volume of hydrogen produced is twice the volume of oxygen. Option A is incorrect because water does not contain equal numbers of hydrogen and oxygen atoms.

Silver chloride is photosensitive. In sunlight, it undergoes photodecomposition: \(2\mathrm{AgCl}(s) \xrightarrow{sunlight} 2\mathrm{Ag}(s)+\mathrm{Cl}_2(g)\). The silver formed is grey, so silver chloride is protected from light. Option B is incorrect because silver chloride does not form silver oxide on exposure to light.

Use the oxygen-transfer rule for redox reactions. CuO loses oxygen and becomes Cu, so copper(II) oxide is reduced. H₂ gains that oxygen and becomes H₂O, so hydrogen is oxidised. Because reduction and oxidation occur simultaneously, the reaction is a redox reaction. Therefore option B is correct; option A reverses both changes, while C and D incorrectly assign the same process to both reactants.

The governing concept is the energy change in a chemical reaction. Calcium carbonate undergoes thermal decomposition only when heated: CaCO3 + heat → CaO + CO2. Therefore, heat is a reactant and must be supplied, making option B correct. Adding water to calcium oxide, burning fuel, and respiration are generally exothermic processes that release heat rather than require continuous heat input.

In a chemical equation, substances and energy shown on the product side are formed or released during the reaction. If heat appears among the products, the reaction gives out heat to its surroundings and is therefore exothermic. Option B is correct. An endothermic reaction absorbs heat, while precipitation and electrolysis describe other reaction features, not the heat direction.

The position of heat in an equation indicates the energy transfer. When heat is written on the reactant side, the reaction must absorb that heat in order to proceed. Such a reaction is called endothermic, so option B is correct. An exothermic reaction places released heat on the product side; combustion is often exothermic, and precipitation refers to solid formation.