Chemical Reactions and Equations

The governing concept is energy transfer during a chemical reaction. When the reacting system gives heat to the surroundings, the reaction is called exothermic and its enthalpy change is negative under the usual convention. Combustion is a familiar example because it releases noticeable heat. Therefore, option A is correct. An endothermic reaction absorbs heat, while displacement and decomposition describe reaction patterns rather than this heat-flow property.

The governing concept is the direction of heat transfer between a reacting system and its surroundings. If the system absorbs heat from the surroundings, the reaction is endothermic; its enthalpy change is positive under the usual convention. Hence option B is correct. An exothermic reaction releases heat, while displacement and neutralisation classify reactions by substance exchange or products, not by absorption of heat.

A fuel reacts with oxygen and releases heat and usually light; this process is called a combustion reaction. For example, methane burns as \(\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}\). Although reactants combine in some combustion reactions, the specific classification for burning a fuel in oxygen is combustion.

The governing concept is a combination and combustion reaction involving oxygen. Magnesium burns with a dazzling white flame and combines with oxygen in air to form white magnesium oxide. The balanced equation is 2Mg + O₂ → 2MgO, so option A is correct. Magnesium chloride would require chlorine, magnesium sulphate would require sulphate ions, and magnesium carbonate is not the direct product of burning magnesium in air.

On exposure to air, a thin layer of magnesium oxide forms on the surface of magnesium ribbon. This coating prevents direct contact between magnesium and oxygen. Cleaning the ribbon with sandpaper removes the oxide layer, so magnesium burns readily and forms magnesium oxide. Cleaning does not increase its melting point or produce oxygen.

The governing concept is the classical definition of oxidation in terms of oxygen and hydrogen. Oxidation means addition of oxygen to a substance or removal of hydrogen from it. For example, conversion of copper to copper oxide involves addition of oxygen. Therefore, option A is correct. Option B defines reduction, while dissolving and melting are physical changes, not necessarily oxidation.

The governing concept is the classical meaning of reduction in a redox reaction. Reduction occurs when oxygen is removed from a substance or hydrogen is added to it. For example, removal of oxygen from a metal oxide can produce the metal. Hence option A is correct. Option B describes oxidation, whereas melting and gas bubbles may be physical or other reaction observations and do not define reduction.

A reaction in which one substance loses electrons and is oxidised while another gains electrons and is reduced is called a redox reaction. Oxidation and reduction occur together. For example, in
\(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\), zinc is oxidised and copper ions are reduced. A neutralisation reaction occurs between an acid and a base; oxidation and reduction are not necessary in it.

Iron reacts with oxygen and moisture to form rust, a hydrated iron oxide. Since rust is a new substance with properties different from iron, rusting is a chemical change. In contrast, a physical change alters only the form or state of a substance without producing a new substance.

Rusting of iron requires both oxygen and water (moisture). Air supplies oxygen, while moisture helps the reaction occur; therefore, iron rusts when exposed to air and moisture. Heat alone is not sufficient for rusting. Exam tip: Remember the two essential conditions for rusting—oxygen and water.

Oils and fats slowly undergo oxidation when exposed to oxygen in air, producing an unpleasant smell and taste. This process is called rancidity. Corrosion is the deterioration of metals due to environmental action, so it does not apply to oils and fats.

Rancidity occurs when fats and oils undergo oxidation, producing an unpleasant smell and taste. Antioxidants slow down oxidation and are therefore used to prevent rancidity. Preservatives mainly inhibit the growth of microorganisms; they do not directly prevent oxidation of fats and oils.

Chips contain oils that can react with oxygen and undergo oxidation, causing rancidity and an unpleasant smell and taste. Nitrogen is relatively unreactive; it reduces the amount of oxygen in the packet and prevents oxidation. Option B is incorrect because added moisture makes chips lose their crispness.

Bacteria convert lactose in milk into lactic acid. This coagulates milk proteins and forms curd, which has properties different from those of milk. Curd cannot be converted back into milk by simple physical methods. Therefore, this is a chemical change; in a physical change, no new substance is formed.

A colour change in a chemical reaction can indicate the formation of a new substance because the products may have properties different from those of the reactants. A change only in shape, state, or position does not usually form a new substance; these can be physical changes. Exam tip: Colour change, gas evolution, temperature change, and precipitate formation are common signs of a chemical reaction.

In respiration, glucose is oxidised to form carbon dioxide, water, and energy: \(\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + energy}\). Because energy is released, respiration is an exothermic reaction. In contrast, an endothermic reaction absorbs energy from the surroundings.

Natural gas consists mainly of methane. On burning in oxygen, methane forms carbon dioxide and water and releases heat and light: \(\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}\). A reaction in which a substance undergoes rapid oxidation in oxygen with release of energy is called combustion. Hence, this is a combustion reaction. In contrast, a decomposition reaction involves one compound breaking into simpler substances; here methane reacts with oxygen rather than breaking down.

The governing convention is the use of state symbols in chemical equations. A symbol written in parentheses after a formula shows the physical state of that substance. The symbol (s) means solid, so option A is correct; for example, CaCO₃(s) represents solid calcium carbonate. The other symbols are (l) for liquid, (g) for gas, and (aq) for a substance dissolved in water.

In a chemical change, a new substance with different properties is formed. Therefore, formation of a new substance indicates a chemical change. In contrast, melting of ice is only a change of state: it remains water and no new substance is produced. Evaporation of water and breaking of glass are also physical changes.

The governing convention is that state symbols identify the physical condition of each substance in a chemical equation. The symbol (aq) means aqueous: the substance is dissolved in water. Thus option D is correct; NaCl(aq), for example, represents sodium chloride dissolved in water. In contrast, (s) means solid, (l) means liquid, and (g) means gas, so none of those indicates an aqueous solution.