Chemical Reactions and Equations

A careful scientific conclusion must be based only on the observations. Gas evolution suggests that a gaseous product may have formed, often indicating a chemical reaction, while cooling of the container suggests that heat was absorbed from the surroundings. These observations support option A, but they do not identify the exact reaction or prove rusting. The other options either ignore an observation or make an unjustified claim.

In a thermochemical equation, heat written on the product side means that the reaction releases heat to the surroundings. The surroundings, including the container, therefore become warm. Such a reaction is called exothermic, so option A is correct. An endothermic reaction would absorb heat and generally cause cooling. Electrolytic decomposition and precipitation describe different reaction features and do not follow from the temperature observation.

Iron is above copper in the reactivity series, so it displaces copper from copper sulphate: Fe + CuSO₄ → FeSO₄ + Cu. The newly formed iron sulphate solution is pale green, while copper appears as the reddish-brown coating on the nail. Thus A correctly explains both observations. The reaction is not combination, decomposition by heating, or neutralisation because no acid-base reaction is involved.

The reactivity series shows that iron is more reactive than copper. Therefore iron displaces copper from copper sulphate according to Fe + CuSO₄ → FeSO₄ + Cu. Ferrous sulphate gives the solution a light green colour, and copper forms the reddish-brown coating. Hence B is correct. A reverses the reactivity relationship, while C and D ignore the formation of new substances.

On heating, sodium hydrogen carbonate, NaHCO₃, undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The carbon dioxide produced forms bubbles in the batter, and these bubbles expand during heating, making the cake rise and become porous. Therefore option A is correct. Hydrogen is not produced in this decomposition, while chlorine and nitrogen are not products of sodium hydrogen carbonate heating.

The governing concept is the use of characteristic gas tests. Hydrogen burns rapidly and produces a ‘pop’ when a burning splint is brought near the gas. Carbon dioxide does not give that test; it turns limewater milky because calcium carbonate forms. Thus option A correctly pairs each gas with its diagnostic test. Smell and colour are unreliable here, and reversing the tests gives option B, which is incorrect.

The governing concept is the general basic character of many metal oxides. A basic oxide reacts with an acid in a neutralisation reaction to form salt and water; for example, CuO + 2HCl → CuCl2 + H2O. Some metal oxides also react with water to form hydroxides. Therefore option A gives the defining reason. Option B describes acidity, while C and D are factually incorrect generalisations.

The electrolysis of concentrated sodium chloride solution, called the chlor-alkali process, produces sodium hydroxide, hydrogen, and chlorine. At the cathode, water is reduced to hydrogen and hydroxide ions; at the anode, chloride ions are oxidised to chlorine. The sodium ions combine with hydroxide ions in solution to form NaOH. Therefore option A is correct, while the other gas pairs are not products of brine electrolysis.

The chlor-alkali process is the electrolysis of concentrated brine, or aqueous sodium chloride. At the cathode, water molecules gain electrons in a reduction reaction and form hydrogen gas and hydroxide ions: 2H2O + 2e− → H2 + 2OH−. Chlorine is formed at the anode, not the cathode, while carbon dioxide and ammonia are not products. Therefore option A is correct.

In combustion, a combustible substance generally reacts with oxygen and releases heat, often with light. Therefore, oxygen is the correct answer. Carbon dioxide does not support combustion, which is why it is used in fire extinguishers. Exam tip: Remember that combustion needs both a fuel and oxygen from air.

In a precipitation reaction, ions from two aqueous solutions combine to form an insoluble solid called a precipitate. Therefore, option A is correct. Formation of a gas is characteristic of a gas-evolution reaction, not a precipitation reaction. Exam tip: If mixing solutions produces a visible solid, identify it as a precipitation reaction.

Oxidation involves the addition of oxygen to a substance or the removal of hydrogen from it, so option A is correct. Removal of oxygen or addition of hydrogen is reduction, making option B incorrect. For example, copper reacts with oxygen to form copper oxide. Exam tip: remember that gain of oxygen indicates oxidation.

In reduction, oxygen is removed from a substance or hydrogen is added to it, so option A is correct. Option B describes oxidation in the usual Class 10 sense. For example, removal of oxygen from copper(II) oxide forms copper. Exam tip: remember that removal of oxygen indicates reduction.

In a double displacement reaction, the cations and anions of two ionic compounds exchange partners to form two new compounds. Its general form is \(AB + CD \rightarrow AD + CB\). Therefore, option A is correct. Option B describes a displacement reaction, where one element replaces another. Exam tip: Identify a double displacement reaction when ions of two compounds interchange.

A chemical equation is balanced to obey the law of conservation of mass. In a balanced equation, the number of atoms of every element is the same on the reactant and product sides, so total mass remains unchanged. Balancing does not aim to increase products or reduce reactants. Exam tip: change only coefficients while balancing, never the subscripts in chemical formulae.

In a displacement reaction, a more reactive element removes a less reactive element from its compound. For example, zinc displaces copper from copper sulfate: \(Zn + CuSO_4 \rightarrow ZnSO_4 + Cu\). Option B describes a double displacement reaction, while option C is a decomposition reaction. Exam tip: an element higher in the reactivity series can displace an element below it from its salt solution.