Chemical Reactions and Equations

If mixing two aqueous solutions causes a reaction that forms an insoluble solid, the solid separates from the solution. Such a solid is called a precipitate. A solute is the substance that dissolves in a solvent, whereas a suspension is a mixture containing insoluble particles; the solid produced and separated during the reaction is a precipitate.

The reaction is \(\mathrm{Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Barium ions, \(\mathrm{Ba^{2+}}\), combine with sulphate ions, \(\mathrm{SO_4^{2-}}\), to form insoluble barium sulphate, which appears as a white precipitate. In contrast, sodium chloride remains soluble in water.

In a double displacement reaction, the cations and anions of two compounds exchange partners to form two 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. Option C describes a single displacement reaction, in which one element replaces another element in a compound.

Combustion is a chemical reaction in which a fuel reacts with oxygen and releases energy, usually as heat and sometimes as light. Thus, oxygen is essential for ordinary burning and option B is correct. Nitrogen is present in air but normally does not support combustion, helium is inert, and chlorine is not the gas required for common fuel combustion.

In combustion, a substance reacts with oxygen and generally releases energy as heat and light. Because energy is released to the surroundings, combustion is an exothermic reaction. In contrast, an endothermic reaction absorbs energy from the surroundings.

During respiration, glucose breaks down and releases the chemical energy stored in it. Cells use this energy for their activities, so energy is the correct answer. Oxygen is generally used as a reactant in aerobic respiration; it is not released as energy. Exam tip: Remember respiration as an energy-releasing (exothermic) process.

A chemical reaction that releases heat energy to the surroundings is called an exothermic reaction; therefore, option B is correct. In contrast, an endothermic reaction absorbs heat energy from the surroundings. For example, the combustion of fuel is generally an exothermic reaction.

A reaction that absorbs heat from its surroundings is called an endothermic reaction, so option A is correct. In contrast, an exothermic reaction releases heat to the surroundings. Exam tip: associate “heat absorbed” with endothermic and “heat released” with exothermic.

A common definition of oxidation is the addition of oxygen to a substance, so option A is correct. For example, magnesium reacts with oxygen to form magnesium oxide. In contrast, removal of oxygen or addition of hydrogen indicates reduction; the oxidation number also decreases during reduction.

Reduction is commonly defined as the removal of oxygen from a substance or the addition of hydrogen to it. Therefore, option B is correct. In the electronic view of redox reactions, reduction also means gain of electrons. Oxygen addition, hydrogen removal, and electron loss describe oxidation rather than reduction.

A reaction in which oxidation and reduction occur simultaneously is called a redox reaction. Oxidation involves loss of electrons, whereas reduction involves gain of electrons. In contrast, a precipitation reaction generally forms an insoluble solid and does not necessarily involve oxidation and reduction.

On heating, the reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Oxygen is removed from copper oxide to form water, so \(\mathrm{CuO}\) is reduced to copper. Oxidation is incorrect because oxygen is not added to copper oxide; it is removed. Exam tip: removal of oxygen indicates reduction.

When copper is heated in air, it combines with oxygen and forms black copper(II) oxide. The balanced equation is 2Cu + O₂ → 2CuO, showing that the product contains copper and oxygen. Copper(I) oxide is generally red, while copper sulphate and copper carbonate are not the products of this direct heating reaction in air.

Rusting is a slow corrosion process in which iron is oxidised in the presence of both oxygen and water or moisture. The product is hydrated iron(III) oxide, commonly called rust. Therefore, option A is correct. Oxygen alone cannot normally produce rust on dry iron, and moisture alone without an oxygen supply is also insufficient.

During rusting, iron reacts slowly with oxygen in the presence of water or moisture and forms hydrated iron(III) oxide. This corrosion product is called rust and appears as a reddish-brown, flaky coating, so option A is correct. It is a solid deposit, not a blue liquid or green gas; the appearance also does not match a bright white solid.

In galvanisation, a thin layer of zinc is coated on iron or steel. This layer prevents air and moisture from reaching the iron, reducing rusting. Zinc is more reactive than iron, so it can oxidise preferentially and provide some protection even if the coating is scratched. Coating iron with copper is not called galvanisation.

Rancidity is the deterioration of oils and fats when they react with oxygen, producing unpleasant tastes and smells. Therefore, option B is correct. Measures such as adding antioxidants, storing food in airtight containers, refrigeration, and keeping it away from light can slow this oxidation. Rusting concerns metals, precipitation forms an insoluble solid, and neutralisation involves acid and base.

The oils and fats in chips can react with oxygen in air and become oxidised, causing rancidity. Filling the packet with nitrogen reduces the amount of oxygen present. Nitrogen is relatively unreactive, so it slows the oxidation of oils. Carbon dioxide may be used in the packaging of some foods, but nitrogen is commonly used in packets of chips.

Oils and fats in food can react with oxygen in air and become rancid. This spoilage is called rancidity. Antioxidants slow down oxidation, helping food retain its taste and quality for longer. Option C is incorrect because faster oxidation would increase rancidity.

A precipitation reaction occurs when two aqueous solutions react and produce an insoluble solid. This solid separates from the solution and is called a precipitate, so option A is correct. A substance that remains completely dissolved is not a precipitate. Although some reactions produce gases or liquids, those products do not define precipitation reactions.