Chemical Reactions and Equations

Electrolysis of water is a decomposition reaction in which electric current breaks water into its constituent elements: \(2H_2O \rightarrow 2H_2 + O_2\). Thus, hydrogen and oxygen gases are produced, so option A is correct. Option D includes hydrogen, but nitrogen is not a constituent of water.

Rusting is a corrosion process in which iron is slowly oxidised in the presence of oxygen and water. Moist air supplies both necessary conditions at the surface of the grill, so rust is most likely to form there. Dry oxygen lacks moisture, and oxygen-free water lacks the oxidising gas needed for ordinary rusting. Carbon dioxide alone does not provide the complete required conditions. Thus option C is correct.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The reaction is: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Therefore, iron sulphate is formed in the solution and copper is deposited on the nail. Zinc sulphate cannot form because zinc is not present in the reaction.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The displaced copper deposits on the iron nail as a reddish-brown coating. Blue is the colour of the copper sulphate solution, not of the coating on the nail. Reaction: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\).

In a metal displacement reaction, a free metal replaces another metal from its compound only if the free metal is more reactive. For example, zinc displaces copper from copper sulphate because zinc is higher in the reactivity series. Lustre, colour, and mass do not determine whether displacement can occur. The essential property is comparative chemical reactivity, so option A is correct.

In a combination reaction, two or more reactants combine to form one product. In \(2Mg + O_2 \rightarrow 2MgO\), magnesium and oxygen form magnesium oxide, so it is a combination reaction. In contrast, option B shows one compound breaking into two products and is therefore a decomposition reaction.

An insoluble solid formed when two aqueous solutions are mixed is called a precipitate. Therefore, this is a precipitation reaction. For example, mixing silver nitrate and sodium chloride solutions produces a white precipitate of silver chloride. In a neutralisation reaction, an acid and a base generally form salt and water; formation of an insoluble solid is not its defining feature.

This is a double-displacement reaction: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). The \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions form insoluble white \(\mathrm{BaSO_4}\), so barium sulphate is the precipitate. Sodium chloride remains dissolved in water.

In a double displacement reaction, the cations and anions of two compounds exchange partners to form new compounds. Its general form is \(AB + CD \rightarrow AD + CB\). For example, in \(Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl\), ions are exchanged; heat or colour is not exchanged.

On heating, hydrogen removes oxygen from copper(II) oxide to form water, and copper(II) oxide is reduced to copper. The equation is \(\mathrm{CuO + H_2 \xrightarrow{heat} Cu + H_2O}\). Therefore, copper and water are formed. Copper sulphate cannot form because no source of sulfate ions is present in the reaction.

According to the oxygen-based definition of redox reactions, removal of oxygen from a substance is reduction. Thus, when oxygen is removed from copper(II) oxide, CuO is reduced, commonly producing copper in a suitable reaction. Oxidation is the addition of oxygen, neutralisation involves acid and base reacting, and displacement describes replacement of one element by another. Therefore option B is correct.

The reaction is CuO + H₂ → Cu + H₂O. Hydrogen combines with the oxygen removed from copper(II) oxide, so it changes into water. At the same time, CuO loses oxygen and is reduced to copper, while hydrogen gains oxygen and is oxidised. Oxygen itself is not produced, and hydrogen peroxide is not the product under these conditions. Hence option A is correct.

In the classical oxygen-transfer definition, addition of oxygen to a substance is oxidation. Therefore, the substance receiving oxygen is said to be oxidised. For example, magnesium gains oxygen to form magnesium oxide and is consequently oxidised. Reduction means removal of oxygen, neutralisation is an acid-base process, and precipitation is formation of an insoluble solid. Thus option A is correct.

Removal of oxygen from a substance is called reduction in the oxygen-based description of redox reactions. For example, copper(II) oxide can lose oxygen and change into copper, so the oxide is reduced. Oxidation is the opposite oxygen change, namely addition of oxygen. Neutralisation concerns acid-base reactions, while displacement concerns replacement of one element by another. Therefore option B is correct.

The error is treating compound formation as unrelated to oxidation. Oxidation includes addition of oxygen, so magnesium is oxidised when it changes from elemental Mg to MgO. Its oxidation number rises from 0 to +2. Oxygen changes from 0 to −2 and is reduced. Option C incorrectly assigns magnesium’s oxidation to oxygen, while B and D contradict the redox definitions. Hence option A is correct.

Rusting is a chemical corrosion process in which iron reacts gradually with oxygen in the presence of moisture and forms hydrated iron(III) oxide, commonly called rust. Since iron combines with oxygen, the process is classified as oxidation. Freezing and melting are physical changes of state, while filtration separates substances without forming a new compound. Therefore, oxidation is the correct choice, so option A is correct.

Rusting is the oxidation of iron in the presence of oxygen and moisture. A paint coating forms a barrier on the iron surface and prevents air and water from reaching the iron. Therefore, rusting is prevented or greatly slowed. Painting does not change the melting point of iron.

Galvanisation is the process of protecting iron from rusting by coating it with zinc. Zinc prevents air and moisture from reaching the iron and can help protect the iron even if the coating is scratched. Copper, silver, and lead are not the standard coating metals used in galvanisation.

Rancidity is the deterioration of oils and fats when they react with oxygen, usually forming substances with an unpleasant smell and taste. This is an example of oxidation and can be slowed by refrigeration, airtight storage, or antioxidants. Corrosion applies mainly to the gradual damage of metals, precipitation forms an insoluble solid, and displacement involves replacement of one element by another. Therefore A is correct.

Rancidity occurs when oxygen reacts with fats and oils in food. Cooling lowers the average kinetic energy of particles and generally slows the rate of chemical reactions, so oxidation proceeds more slowly. The food does not lose its mass or its fat merely because it is cooled, and the container volume is unchanged. Therefore the reduced quantity is the rate of oxidation, making option A correct.