Chemical Reactions and Equations

A change in smell can indicate that a new substance with different properties has formed during the reaction. A change of state alone, such as freezing or boiling of water, does not usually form a new substance. Exam tip: Change in colour or smell, temperature change, gas evolution, and precipitate formation are common signs of a chemical reaction.

Rusting is the reaction of iron with oxygen in the presence of water or moisture. A paint coating forms a barrier between the iron surface and air and moisture, thereby preventing rust formation. In contrast, painting does not raise iron's melting point or increase its chemical reactivity.

The process of applying a protective zinc coating to iron is called galvanisation. Zinc blocks air and moisture from reaching the iron and, even if the coating is scratched, zinc oxidises preferentially and protects the iron. Electroplating is a general method of depositing a metal coating, whereas coating iron specifically with zinc to prevent rusting is galvanisation. Exam tip: If you see ‘iron coated with zinc’, choose galvanisation.

Rusting is an oxidation process because iron reacts with oxygen from air in the presence of moisture to form hydrated iron(III) oxide (rust). In reduction, oxygen is removed from a substance or it gains electrons; in rusting, iron undergoes oxidation instead.

Rancidity is the spoilage of food containing oils or fats when these substances react with oxygen from the air. This oxidation produces unpleasant smells and tastes, so option A is correct. Freezing water, dissolving salt, and melting sugar are physical changes; none explains the characteristic oxidation and bad odour associated with rancid food.

Rancidity occurs when oils and fats in food react with oxygen and become oxidised. This can produce an unpleasant smell and taste. An airtight container reduces the food’s contact with air, especially oxygen. Therefore, the rate of oxidation decreases and rancidity is delayed. Unlike option B, increasing the oxygen available to food would promote oxidation of its oils and fats rather than prevent rancidity.

The oils and fats in chips can oxidise in the presence of oxygen in air, producing an unpleasant smell and taste. This process is called rancidity. Filling the packet with nitrogen reduces the oxygen available, so oxidation of the oils and fats slows down. Rusting involves oxidation of iron, whereas oils in chips undergo rancidity.

A substance dissolved in water forms an aqueous solution, and its state is represented by the symbol (aq). Therefore, option A is correct; for example, sodium chloride in solution may be written as NaCl(aq). The other symbols describe different states: (s) means solid, (g) means gas, and (l) means liquid, not specifically an aqueous solution.

In a chemical equation, the physical state of a substance is written in brackets after its chemical formula. The symbol \((s)\) represents the solid state; for example, \(\mathrm{NaCl(s)}\). Therefore, option A is correct. The symbol \((g)\) represents a gas, \((l)\) represents a liquid, and \((aq)\) represents a substance dissolved in water. State symbols make the actual physical conditions of reactants and products clear.

The governing concept is the use of state symbols in chemical equations. The symbol (l), from the word “liquid,” represents a substance in the liquid state, so option A is correct. The distractors describe different conditions: (s) means solid, (g) means gas, and (aq) means that the substance is dissolved in water as an aqueous solution. Therefore (aq) is not the general symbol for every liquid.

In a chemical equation, an upward arrow shows that a gas is evolved during the reaction. For example, hydrogen gas is produced when zinc reacts with a dilute acid. A downward arrow is generally used for the formation of a precipitate, so option B is not correct. Exam tip: remember—upward arrow for gas, downward arrow for precipitate.

A downward arrow (↓) in a chemical equation indicates that an insoluble solid has formed and settled out of the solution. This solid is called a precipitate, so option A is correct. An upward arrow (↑), where used, indicates gas evolution; heat is shown by reaction conditions, and an aqueous state is represented by (aq), not by a downward arrow.

A skeletal chemical equation contains the correct chemical formulae of both reactants and products, but the number of atoms of each element may be unequal on the two sides. It must therefore be balanced to obey the law of conservation of mass. Options B and C are incorrect because a skeletal equation includes the formulae of both reactants and products.

A word equation gives the names of the reactants and products. The first step is to write their correct chemical formulae; for example, hydrogen is H₂ and oxygen is O₂. The equation is balanced afterwards by changing coefficients to make the number of atoms equal on both sides. Changing subscripts in a formula is not the correct way to balance an equation.

While balancing a chemical equation, only the coefficients written before formulae are changed. This makes the number of atoms of every element equal on the reactant and product sides, satisfying the law of conservation of mass. For example, the 2s in \(2H_2 + O_2 \rightarrow 2H_2O\) are coefficients. Changing a subscript is not allowed because changing \(H_2O\) to \(H_2O_2\) changes water into a different substance.

According to the law of conservation of mass, matter is neither created nor destroyed in a chemical reaction. Therefore, in a closed system, the total mass of the reactants equals the total mass of the products. Option B is incorrect because the actual mass does not decrease; in an open container, the measured mass may appear lower if a gas escapes.

A rise in temperature during a reaction usually indicates that heat is being released by the reaction. Such a reaction is called an exothermic reaction. In an endothermic reaction, heat is absorbed, so the temperature generally tends to decrease. Exam tip: Associate release of heat or a temperature rise with an exothermic reaction.

A fall in the temperature of the reaction mixture generally means that the reaction is absorbing heat from its surroundings. Such reactions are called endothermic reactions. In contrast, exothermic reactions release heat and generally cause a temperature rise, so option B is incorrect. Formation of a gas or a metal is not a necessary indication of a temperature decrease.

A colour change is only one possible sign of a chemical reaction; it is not compulsory. Many gases, such as hydrogen and carbon dioxide, are colourless, yet bubbling or gas evolution can indicate that a reaction has occurred. Therefore, the absence of a visible colour change does not prove that no reaction took place. Option B is incorrect because not all gases are coloured, while option C is incorrect because temperature change and precipitate formation can also indicate a reaction.

During a chemical reaction, new substances are formed, and observable evidence may include a colour change, gas evolution, a temperature change, or precipitate formation. Therefore, option A is an appropriate indication of a reaction. In contrast, merely knowing the names of the reactants does not show that a reaction has occurred.