Chemical Reactions and Equations

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.

A colour change during a chemical reaction often indicates the formation of a new substance, because the product may have properties different from those of the reactants. It does not always mean a physical change, nor does it require the temperature to decrease. Exam tip: Remember colour change as a possible sign of a chemical reaction, along with gas evolution, precipitate formation, and temperature change.

When a gas is formed in a chemical reaction, it often escapes from the mixture or solution as bubbles. Therefore, the appearance of bubbles is a common sign of gas evolution. In contrast, filtration is a method of separating a solid from a liquid, not an indication that a gas has formed.

The correct answer is B: (g). In a chemical equation, the symbol written in brackets after a formula identifies the physical state of that substance. The letter g stands for gas or gaseous state, so CO₂(g) indicates gaseous carbon dioxide. Option A, (s), means solid. Option C, (l), means liquid. Option D, (aq), means aqueous, showing that the substance is dissolved in water rather than simply being a gas. A quick memory cue is to match the first letter of the English state names: s for solid, l for liquid, and g for gas. The aqueous symbol is different because aq refers to water-based dissolution, not to a fourth ordinary state like gas.

The correct answer is A: (l). The symbols in brackets after chemical formulas show physical states. The lowercase l means liquid, so H₂O(l) represents water in its liquid form. Option B, (s), means solid, such as a solid salt. Option C, (g), means gas, such as gaseous oxygen. Option D, (aq), means aqueous: a substance dissolved in water. Although an aqueous solution is usually liquid in appearance, (aq) is more specific than (l), because it tells us that water is the solvent. Therefore, the symbols must not be treated as interchangeable. The memory cue “l for liquid” helps, but notice that it is the letter l, not the numeral 1.

The correct answer is D: (aq). The abbreviation aq comes from aqueous and means that the substance has dissolved in water, forming an aqueous solution. For example, NaCl(aq) indicates sodium chloride dissolved in water. Option A, (s), identifies a solid. Option B, (l), identifies a liquid but does not specifically tell us that the liquid is a solution in water. Option C, (g), identifies a gas. Thus (aq) is the most precise symbol for the condition asked in the question. A common confusion is to choose (l) because an aqueous solution is liquid in appearance; however, (aq) gives the additional and important information that water is the solvent.

When balancing an equation, chemical formulae and subscripts must not be changed because doing so changes the substance. \(H_2O\) is water, whereas \(H_2O_2\) is hydrogen peroxide. The equation is balanced by using coefficients only: \(2H_2 + O_2 \rightarrow 2H_2O\). It has 4 hydrogen atoms and 2 oxygen atoms on each side. Changing \(O_2\) to \(O\) is also incorrect because it changes the chemical formula of oxygen.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The ferrous sulphate formed makes the blue solution light green, and displaced copper deposits on the nail. Option B is incorrect because a displacement reaction occurs between iron and copper sulphate.

During digestion, enzymes convert complex food components such as carbohydrates, proteins, and fats into simpler new substances. Digestion is a chemical change because new substances are formed and their properties differ from those of the original food. Option B describes a physical change, such as breaking food into smaller pieces, in which no new substance is formed.

Milk changes into curd through bacterial fermentation, mainly involving the conversion of lactose into lactic acid. New substances form, the composition changes, and the properties of curd differ from those of milk; therefore, it is a chemical change and option B is correct. It is not merely a change of state, shape, or physical appearance, and the process is not readily reversible.

In a chemical reaction, new substances are formed. Evidence may include a colour change, gas evolution, a temperature change, or formation of a precipitate. Therefore, option A is correct. For example, a precipitate indicates formation of a new insoluble substance, whereas the shape of the container does not prove that a reaction has occurred.