Chemical Reactions and Equations

At the introductory level, oxidation can be identified as the addition of oxygen to a substance. For example, magnesium combines with oxygen to form magnesium oxide, so magnesium is oxidised. In broader definitions, oxidation may also mean loss of electrons or removal of hydrogen, but those are not the simple oxygen-based criterion asked here. Removal of oxygen is reduction, and freezing is a physical change.

In reduction, oxygen is removed from a substance. For example, when oxygen is removed from copper(II) oxide to form copper, copper(II) oxide is reduced. In contrast, oxidation involves the addition of oxygen. Release of heat or evolution of a gas may occur in some reactions, but neither is a definite sign of reduction.

A decomposition reaction follows the pattern \(AB \rightarrow A + B\): one compound breaks into two or more simpler substances. For example, on heating, calcium carbonate decomposes as \(CaCO_3 \rightarrow CaO + CO_2\). This is the reverse structural pattern of combination, which forms one product from multiple reactants. Displacement involves replacement by an element, and double displacement involves exchange between two compounds.

Rusting of iron is an oxidation process that requires both oxygen and water. In the presence of water, oxygen reacts with iron to form hydrated iron(III) oxide, commonly called rust. Carbon dioxide may affect the rate of corrosion, but it is not essential for rusting.

The reaction is \(CaO + H_2O \rightarrow Ca(OH)_2\). Two reactants, quicklime and water, combine to form only one product, calcium hydroxide; therefore it is a combination reaction. The number of products is the useful test here. It is not decomposition because nothing breaks apart, not displacement because no element replaces another, and not double displacement because two ionic compounds do not exchange ions.

A chemical change produces one or more new substances with different properties. During rusting, iron reacts with oxygen and moisture to form iron oxides, commonly represented as hydrated iron(III) oxide, so the original iron is chemically changed. Melting ice changes only the physical state, dissolving sugar usually allows recovery by evaporation, and tearing paper changes size and shape without forming a new substance.

Rancidity is the spoilage of oils and fats caused mainly by their reaction with oxygen. Oxidation produces unpleasant-smelling and unpleasant-tasting substances, which is why stale fried food or old oily foods may become unacceptable. Preventive methods include airtight storage, refrigeration, and adding suitable antioxidants. Precipitation forms an insoluble solid, neutralisation involves acid and base, and distillation separates liquids by boiling points.

Rancidity occurs when fats and oils in food react with oxygen to form substances with an unpleasant smell and taste. Antioxidants slow this oxidation, so rancidity occurs later. In contrast, increasing the rate of oxidation would make rancidity happen faster.

Chips contain fats and oils that can oxidise on contact with oxygen in air. This causes rancidity and gives the chips an unpleasant taste and smell. Nitrogen is relatively unreactive; it reduces the amount of oxygen in the packet and therefore helps prevent oxidation. It is not filled to increase moisture.

Iron is more reactive than copper, so it displaces copper from copper sulphate: \\(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\\). The iron sulphate formed makes the solution pale green, and displaced copper appears as a reddish-brown deposit on the nail. Unlike option B, the solution does not become colourless after the reaction.

Combustion is a chemical reaction in which a fuel reacts rapidly with oxygen and releases heat, often with light. Therefore oxygen is the gas generally required to support burning. Nitrogen is abundant in air but does not normally support combustion, while helium is inert and chlorine is not the usual gas identified as the supporter of ordinary fuel burning. Thus option B is the unambiguous answer.

In combustion, a fuel reacts with oxygen to form new products such as carbon dioxide and water. Energy is required to break bonds in the reactants, but more energy is released when new, more stable bonds form in the products. Therefore, heat is released overall, making combustion exothermic. Option B describes the opposite energy balance, which would correspond to an endothermic reaction.

If the temperature of the reaction mixture rises, the reaction is generally releasing heat to the surroundings. Such a reaction is called exothermic. In contrast, an endothermic reaction absorbs heat and may cause a temperature decrease unless heat is continuously supplied from outside.

A fall in temperature during a reaction generally indicates that the reacting system is absorbing heat from its surroundings. Such a reaction is called endothermic. Therefore, option A is correct. Release of heat would usually raise the temperature, while light emission and rusting are possible reaction features but do not specifically explain a temperature decrease.

An exothermic reaction releases energy to the surroundings, and the released heat may be written among the products. In option C, methane burns in oxygen and produces carbon dioxide, water, and heat, so it is exothermic. Options A, B, and D require heat, electricity, or sunlight as energy input and represent endothermic processes.

The governing concept is the notation of physical states in chemical equations. A lowercase l inside parentheses, written as (l), indicates that the substance is liquid; therefore option B is correct. The symbol (s) denotes a solid, (g) denotes a gas, and (aq) denotes a substance dissolved in water. The distinction is important because an aqueous solution is not simply the same as an undissolved liquid.

In a chemical equation, \((g)\) indicates that a substance is in the gaseous state, so option C is correct. \((aq)\) indicates an aqueous solution, meaning the substance is dissolved in water, not that it is a gas. Exam tip: Remember the state symbols: \((s)\) for solid, \((l)\) for liquid, \((g)\) for gas, and \((aq)\) for aqueous solution.

The governing concept is the notation for physical state and solution medium. When a substance is dissolved in water, the equation marks it as aqueous with (aq), so option D is correct. The other symbols indicate different conditions: (s) is solid, (l) is liquid, and (g) is gas. Thus (aq) specifically communicates water-based dissolution, not merely that the substance is flowing or liquid.

The arrow \(\uparrow\) next to a product in a chemical equation indicates that the product appears as an insoluble solid (a precipitate).

In a chemical equation, a downward arrow (↓) commonly indicates that an insoluble solid has formed and settled out of a solution. This solid is called a precipitate, so option B is correct. A gas is usually shown by an upward arrow (↑), whereas heat and light are indicated by words or suitable symbols rather than by a downward arrow.

During digestion, enzymes convert complex food substances into new, simpler substances through chemical reactions. These new substances have properties different from the original food, so digestion is a chemical change. In contrast, chewing food into smaller pieces changes only its size and is a physical change.

In option A, both sides contain 2 magnesium atoms and 2 oxygen atoms: \(2Mg + O_2 \rightarrow 2MgO\). While balancing an equation, only coefficients are changed, not subscripts in chemical formulae. For example, option B changes \(MgO\) to \(MgO_2\), which changes the substance itself.

A colour change, gas evolution, a temperature change, and precipitate formation are common signs that new substances may have formed; therefore, they provide good evidence of a chemical reaction. In contrast, melting or boiling alone is usually a physical change because no new substance is formed.

The governing concept is the classification of reactions by the number of reactants and products. In a decomposition reaction, one compound breaks into two or more simpler substances, represented as AB → A + B. Therefore, the described reaction is decomposition, so option A is correct. Combination has several reactants but one product, while displacement reactions involve replacement or ion exchange.

A combination reaction is identified by the pattern A + B → AB: two or more reactants join to form one product. For example, 2Mg + O₂ → 2MgO shows magnesium and oxygen forming magnesium oxide. Thus option A is correct. Decomposition has one reactant and several products, whereas displacement and double displacement involve replacement or exchange rather than simple formation of one product.