Chemical Reactions and Equations

In respiration, food substances such as glucose undergo controlled oxidation and release energy. In combustion, a fuel undergoes relatively rapid oxidation and releases energy. Thus, both are energy-releasing oxidation processes. Option B is incorrect because respiration does not require a flame.

Photosynthesis requires an energy input to form energy-rich glucose from carbon dioxide and water. Plants absorb energy from sunlight to drive this reaction; therefore, sunlight is written above the arrow and the process is energy absorbing. Option C is incorrect because sunlight is not a catalyst: a catalyst generally remains unchanged after a reaction, whereas light is the source of energy for this process.

During digestion, enzymes convert complex food substances such as carbohydrates, proteins, and fats into simpler new substances. The formation of new substances is the characteristic of a chemical change. In contrast, chewing only breaks food into smaller pieces, changing mainly its size and shape without forming a new substance. Therefore, option B is incorrect.

Iron is above copper in the reactivity series. Therefore, iron displaces copper from copper sulphate solution:


\(
\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}
\)

The iron sulphate formed gives the solution a light-green colour, while displaced copper deposits as a reddish-brown coating on the nail. Option D is incorrect because dilution alone cannot produce a solid copper deposit.

A colour change may indicate a chemical reaction, but by itself it does not prove that a new substance has formed. A change in colour can also be observed during physical processes such as mixing or dilution. Therefore, additional evidence, such as gas evolution, precipitate formation, temperature change, or a suitable test for a new substance, is needed. Option B is incorrect because many chemical reactions do not evolve a gas.

Evolution of a gas is an observable sign of a chemical reaction. A cooling container suggests that the reaction may be taking heat from its surroundings, so it may be endothermic. However, these observations alone do not establish that combustion or rusting definitely occurred.

In the usual notation of school chemical equations, a downward arrow placed beside a product indicates that a precipitate has formed. A precipitate is an insoluble solid that separates from a solution during the reaction, so option B is correct. Gas evolution is commonly shown with an upward arrow, while light and heat are not the substances represented by this symbol.

The upward arrow next to a product is a conventional state or observation symbol showing that a gas is evolved during the reaction. The gas may be noticed as bubbles escaping from the mixture, so option A is correct. A downward arrow is used for a precipitate in the usual school notation. Freezing and melting are phase changes, not the meaning of this reaction symbol.

In a chemical equation, (aq) means aqueous: the substance is dissolved in water, so option A is correct. The other state symbols have specific meanings: (s) denotes a solid, (l) denotes a liquid, and (g) denotes a gas. Therefore option B incorrectly calls a solid gaseous, option C reverses liquid and solid, and option D reverses gas and liquid.

A skeletal equation shows the correct chemical formulae of reactants and products, but the number of atoms may not be equal on the two sides. In a balanced equation, the number of atoms of each element is the same on the reactant and product sides, in accordance with the law of conservation of mass. For example, \(\mathrm{H_2 + O_2 \rightarrow H_2O}\) is skeletal, whereas \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\) is balanced. Only coefficients are changed during balancing; chemical formulae are not changed.

In a chemical reaction, the chemical composition of the reactants changes and new substances are formed. These substances have properties different from those of the original substances, so their formation is a clear indication of a chemical change. In contrast, changing only the position, shape, or container does not form a new substance; these are physical changes.

In \(2Mg + O_2 \rightarrow 2MgO\), magnesium and oxygen combine to form only one compound, MgO; therefore, it is a combination reaction. Magnesium changes its oxidation state from 0 to +2, while oxygen changes from 0 to −2. Thus, magnesium is oxidised and oxygen is reduced, so the reaction is also a redox reaction. Option D is also a redox reaction, but it is a displacement reaction rather than a combination reaction.

The reactivity series places iron above copper, so iron displaces copper from copper sulphate: Fe + CuSO₄ → FeSO₄ + Cu. Reddish-brown copper deposits on the nail, while pale green FeSO₄ forms in solution and reduces the blue colour. Thus A is correct. Silver deposition, no reaction, or rapid gas bubbles do not describe this displacement reaction.

In \(H_2O_2\), oxygen has oxidation number \(-1\). One portion becomes \(-2\) in water, \(H_2O\), so it is reduced; another portion becomes 0 in \(O_2\), so it is oxidised. Therefore, this is a disproportionation reaction. In option B, zinc is oxidised and \(Cu^{2+}\) is reduced, but they are different reactants.

In a precipitation reaction, ions from two aqueous solutions combine to form an insoluble solid that separates from the solution. Therefore, option A is the clear indication of precipitation. Option B represents combustion/combination, whereas option C represents decomposition by electrolysis.

The law of conservation of mass states that total mass remains constant in a chemical reaction when matter is not allowed to enter or leave the system. A closed container retains reactants and any gaseous products, so measuring and comparing the total mass before and after the reaction provides the best test. Therefore A is correct; colour and smell are qualitative observations, not mass measurements.

According to the law of conservation of mass, mass is neither created nor destroyed in a chemical reaction. Since no substance leaves the closed container, the total mass of the products equals the total mass of the reactants. Therefore, the mass of the products is 12 g. The 24 g option is incorrect because a reaction does not double the mass.

Subscripts show the number of atoms of each element in a chemical formula. Changing the subscript 2 in \(\mathrm{H_2O}\) changes the composition, so the substance is no longer water. An equation is balanced by changing coefficients placed before formulae, not by changing subscripts within them.

The balanced equation for forming water is
0(
02 ext{H}_2+ ext{O}_2
ightarrow2 ext{H}_2 ext{O}
0)
0. The 2 water molecules on the product side contain 2 oxygen atoms altogether. One
0(
0 ext{O}_2
0)
0 molecule also contains 2 oxygen atoms, so 1 oxygen molecule is required on the reactant side. Using two oxygen molecules would not balance the oxygen atoms.

Each \(\mathrm{MgO}\) unit contains one magnesium atom. Therefore, \(2\mathrm{MgO}\) contains two magnesium atoms in total. To conserve atoms on both sides, the reactant side must contain \(2\mathrm{Mg}\). Oxygen is written as \(\mathrm{O}_2\), but this does not change the number of magnesium atoms required.