Chemical Reactions and Equations

In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution. Since zinc displaces copper from copper sulphate, zinc is more reactive than copper. In contrast, copper cannot displace zinc from zinc sulphate because copper is less reactive.

In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution. Since A displaces B, A is more reactive than B. Similarly, B is more reactive than C. Thus, the reactivity order is A > B > C, so metal A is the most reactive. Option B is more reactive than C, but it is less reactive than A.

The reaction is \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). The \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions form insoluble white \(\mathrm{BaSO_4}\), producing a visible precipitate. In contrast, sodium chloride remains dissolved in water and does not form a precipitate.

In a double displacement reaction, ions from two ionic compounds exchange partners. If one of the products is insoluble in water, it separates as a solid precipitate: \(AB(aq)+CD(aq)\rightarrow AD(s)+CB(aq)\). Therefore, this is a double displacement reaction with precipitation. In contrast, a single displacement reaction usually involves an element replacing another element from a compound.

In CuO + H₂ → Cu + H₂O, hydrogen combines with oxygen from copper(II) oxide and becomes part of water. By the school-level definition, addition of oxygen is oxidation, so hydrogen is oxidised. At the same time, CuO loses oxygen and changes to copper, so CuO is reduced. Water is a product, not the substance being oxidised, making option B correct.

Copper oxide contains oxygen chemically combined with copper. When it changes into copper, that oxygen is removed from the compound, for example in CuO + H₂ → Cu + H₂O. Removal of oxygen is the traditional school-level definition of reduction. Addition of oxygen describes oxidation, while hydrogen removal and precipitate formation are not the defining features here. Therefore, option A is correct.

Loss of oxygen is called reduction, whereas gain of oxygen is called oxidation. Since reduction and oxidation occur simultaneously in this reaction, it is a redox reaction. For example, when oxygen is removed from a metal oxide, that oxide is reduced while the substance receiving the oxygen is oxidised.

Reduction can be described in several equivalent ways. At this level, it includes removal of oxygen, addition of hydrogen, gain of electrons, or a decrease in oxidation number. Among the given choices, addition of hydrogen is the direct reduction criterion, so option C is correct. Oxygen addition indicates oxidation, hydrogen removal indicates oxidation, and dissolving in water alone does not identify a redox change.

Oxidation is a process in which oxygen is added to a substance or hydrogen is removed from it. Therefore, a substance is oxidised when oxygen is added to it. In contrast, addition of hydrogen or removal of oxygen is reduction; a decrease in temperature alone does not indicate oxidation.

Rusting is an electrochemical process that requires both oxygen and water. A thin layer of moisture allows ions to move, accelerating oxidation of iron and formation of rust. In contrast, moisture does not block oxygen; iron rusts faster in moist air than in dry air.

In CuO + H₂ → Cu + H₂O, copper in CuO changes from oxidation state +2 to 0, so it is reduced. Hydrogen changes from oxidation state 0 to +1 in water, so it is oxidised. Since oxidation and reduction occur together, this is a redox reaction and option A is correct. The other equations are combination, double displacement, or decomposition reactions without simultaneous oxidation and reduction.

Rusting requires iron to come into contact with oxygen and moisture in air. In galvanisation, iron is coated with zinc, whereas in painting it is coated with paint. Both coatings act as barriers between the iron surface and air and moisture, thereby preventing rusting. Option B is incorrect because paint is not a coating of a reactive metal.

In an open bottle, oil remains in contact with oxygen in air. Oxidation can produce unpleasant smell and taste in the oil; this process is called rancidity. Galvanisation is the coating of iron with zinc, so it is unrelated to keeping oil exposed to air.

The governing concept is oxidation and rancidity. Oils and fats in chips react slowly with oxygen, producing unpleasant smell and taste. Nitrogen is comparatively unreactive and displaces much of the oxygen from the packet, so the oil has less contact with oxygen and oxidation becomes slower. Therefore option A is correct. Nitrogen does not make oxygen precipitate or convert it into a metal.

The governing concept is prevention of oxidation. Oils and fats react with oxygen during storage, and this process can produce rancid odours, unpleasant flavours, and reduced food quality. Antioxidants react preferentially with reactive species or interrupt the oxidation chain, thereby slowing spoilage. Hence option A is correct. They do not decompose water, rust iron, or necessarily form a precipitate.

Rancidity is caused by the oxidation of fats and oils, which produces an unpleasant smell and taste in food. More oxygen provides greater opportunity for oxidation, and a higher temperature increases the rate of this reaction. Galvanisation is the coating of iron with zinc to prevent rusting, so it is not a food-spoilage problem.

An endothermic reaction absorbs heat from its surroundings, so the surroundings lose thermal energy and the reaction vessel may feel cooler. Thus option A is the appropriate observation. A hot vessel generally indicates an exothermic process that releases heat. A bright flame and burning food are also commonly associated with combustion, which is exothermic, not endothermic.

In an exothermic reaction, heat is released from the reaction mixture to the surroundings. Therefore, when no external heat is supplied, a rise in the temperature of the mixture and container strongly indicates heat release. Option B may indicate an endothermic reaction, whereas gas evolution or a colour change may show that a reaction is occurring but do not by themselves identify the direction of heat flow.

When one compound breaks down to form two or more products, the reaction is a decomposition reaction. Since heat is absorbed during the reaction, it is endothermic. Hence, it is an endothermic decomposition reaction. In a combination reaction, two or more reactants usually form a single product, so option D does not fit.

In the equation, two reactants, \(A\) and \(B\), combine to form a single product, \(C\); therefore, it is a combination reaction. Since heat appears on the product side, heat is released, so the reaction is exothermic. In a decomposition reaction, one compound generally breaks down into two or more products.