Chemical Reactions and Equations

Oils and fats slowly undergo oxidation on exposure to oxygen in air. This produces substances that give the food an unpleasant smell and taste; this is called rancidity. Nitrogen is generally much less reactive, so option B is not the correct reason.

Rancidity occurs when oils and fats are oxidised by oxygen, causing unpleasant smell and taste in food. Airtight packaging and nitrogen filling reduce exposure to oxygen, so oxidation and rancidity are slowed. Option B is the opposite: greater exposure to oxygen would promote rancidity.

A temperature change can provide evidence that energy has been transferred during a chemical reaction. Exothermic reactions release heat and may raise the temperature, whereas endothermic reactions absorb heat and may lower it. Thus, option A is correct. Temperature change is useful evidence but is not, by itself, an absolutely exclusive test, because some physical processes can also involve heat transfer; the other options make incorrect absolute claims.

Evolution of gas indicates that a gaseous product is being formed. If the container becomes warm without external heating, heat is being released by the reaction; therefore, the reaction is exothermic. In contrast, an endothermic reaction absorbs heat from its surroundings and would not normally make the container warm.

In an exothermic reaction, heat flows from the reacting system to the surroundings, so the surroundings may become warmer. In an endothermic reaction, the system absorbs heat from the surroundings. Option B reverses these processes; formation of a precipitate is not a necessary feature of either type of reaction.

In photosynthesis, atoms from carbon dioxide and water are rearranged to form new substances such as glucose and oxygen. Since chemical bonds change and new substances are produced, it is a chemical change. It is not merely a change of state, because the substances do not just change their physical form; new substances are formed.

In aerobic respiration, glucose reacts with oxygen to form new substances such as carbon dioxide and water, while releasing energy: \(\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + energy}\). Formation of new substances is the key feature of a chemical reaction. Option B describes only a physical change, in which no new substances are formed.

Digestion and curd formation are both chemical changes because the original substances are converted into substances with different compositions and properties. Digestive enzymes break complex food molecules into simpler molecules. During curd formation, bacterial activity converts lactose and changes milk proteins, producing curd with new characteristics. Neither process is merely a change of state or shape, so options B, C and D do not describe the similarity.

Melting of ice is a physical change because ice changes only into liquid water and no new substance is formed. In contrast, electrolysis of water uses electrical energy to produce hydrogen and oxygen gases from water. The formation of new substances shows that electrolysis is a chemical change. Therefore, option B is incorrect because electrolysis is not merely a change of state.

When an iron nail is placed in blue copper sulphate solution, iron displaces copper from copper sulphate. Green iron sulphate and copper are formed: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Thus, the colour change is accompanied by the formation of new substances. In contrast, folding paper is only a physical change and produces no new substance.

An insoluble solid formed on mixing two aqueous solutions is called a precipitate. Its formation shows that ions have reacted to produce a new insoluble substance, so the reaction is a precipitation reaction. Option B is the opposite situation, because a precipitate does not dissolve in the solution.

State symbols specify the physical states of substances: \((s)\) means solid, \((l)\) liquid, \((g)\) gas, and \((aq)\) a substance in aqueous solution. For example, \(\mathrm{H_2O(l)}\) is liquid water, whereas \(\mathrm{H_2O(g)}\) is water vapour. These symbols do not indicate the reaction time or rate.

In the conventional notation used in school chemical equations, an upward arrow (↑) beside a product indicates that a gas is evolved during the reaction. For example, acid reacting with a carbonate may produce carbon dioxide gas, shown with this symbol. A downward arrow (↓), not an upward arrow, is commonly used to indicate an insoluble precipitate. The symbols do not directly represent heat absorption or dissolution.

A downward arrow (↓) in a chemical equation conventionally shows that an insoluble solid has formed from a solution and separated as a precipitate. For example, mixing suitable soluble salts can produce solid barium sulfate, written with a downward arrow. An upward arrow (↑) is commonly used for gas evolution, while heat and light require separate conditions or descriptions rather than this precipitation symbol.

The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction. Atoms are rearranged into new substances, so the total mass of the reactants equals the total mass of the products in a closed system. This is why chemical equations are balanced. Options B, C and D contradict the law by claiming that mass disappears, products have no mass, or reactant mass is always doubled.

In a balanced chemical equation, the number of atoms of each element is the same on the reactant and product sides. If the atoms of any element are not equal, the law of conservation of mass is not satisfied, so the equation is called an unbalanced chemical equation. In contrast, a balanced equation has equal atoms of every element on both sides.

When converting a word equation into a chemical equation, the first essential step is to write the correct formula for every reactant and product. Formulae must reflect the proper symbols, valencies and subscripts; a wrong subscript changes the substance itself. After that, coefficients can be added to balance atoms without changing formulae. Reactants are conventionally placed on the left and products on the right, so options B, C and D are incorrect.

The chemical formulae of reactants and products must not be changed during balancing, because changing a formula changes the substance itself. Only coefficients placed before formulae are changed so that the number of atoms of each element becomes equal on both sides. For example, \(\mathrm{H_2 + O_2 \rightarrow H_2O}\) is balanced as \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\); the formula \(\mathrm{H_2O}\) has not been changed.

Metals have different reactivities. A more reactive metal can displace a less reactive metal from its salt solution. For example, zinc displaces copper from copper sulfate solution. In contrast, copper cannot displace zinc from zinc sulfate solution because copper is less reactive than zinc.

In a double displacement reaction, two ionic compounds exchange ions and form two new compounds. Its general form is \(AB + CD \rightarrow AD + CB\). In contrast, in a single displacement reaction, one element displaces another element from a compound.