Chemical Reactions and Equations

During photosynthesis, carbon dioxide and water are converted into glucose and oxygen in the presence of light and chlorophyll. The formation of new substances is the key feature of a chemical reaction. In contrast, the change of water into water vapour is only a change of state, and no new substance is formed.

In a double displacement reaction, ions of two compounds exchange partners. In \(AgNO_3\) and \(NaCl\), the ions rearrange to form \(AgCl\) and \(NaNO_3\), and insoluble \(AgCl\) is produced as a precipitate. Therefore, option C is correct. In contrast, in option A, zinc displaces hydrogen from hydrochloric acid, so it is a single displacement reaction.

In a photolytic decomposition reaction, a compound breaks down using light energy. Silver bromide decomposes in light to form silver and bromine: \(2\mathrm{AgBr} \xrightarrow{\text{light}} 2\mathrm{Ag} + \mathrm{Br}_2\). Therefore, option A is correct. Option B is a combination reaction, whereas option C is a displacement reaction.

In an electrolytic decomposition reaction, an electric current causes a compound to break down into simpler substances. When electric current is passed through acidified water, it decomposes into hydrogen and oxygen: \(2H_2O(l) \xrightarrow{\text{electric current}} 2H_2(g)+O_2(g)\). Therefore, option A is correct. Option B is a photolytic decomposition reaction caused by light, not by electricity.

In a thermal decomposition reaction, a compound breaks down into simpler substances on heating. When calcium carbonate is heated, it forms calcium oxide and carbon dioxide:
\(\mathrm{CaCO_3 \xrightarrow{heat} CaO + CO_2}\). Therefore, option A is correct. Electrolysis of water requires electrical energy, whereas the decomposition of silver chloride is caused by light.

A colour change can indicate a chemical reaction because newly formed substances may have a different colour. However, colour change alone is not conclusive, since colour may also change because of physical factors such as light, heating, or mixing. Other observations, such as evolution of a gas, formation of a precipitate, or a temperature change, should also be considered. Option B is incorrect because a colour change does not always prove a chemical reaction.

Chemical reactions may show different observable signs, and no single sign must appear in every reaction. Evolution of a gas can indicate that a new gaseous substance has formed, even if the colour remains unchanged. Therefore option A is the correct conclusion. Colour change is only one possible indicator, and gas evolution is not automatically a physical change; its cause and the substances involved must be considered.

In a balanced chemical equation, the number of atoms of each element is equal on both sides, so it follows the law of conservation of mass. Only the coefficients written before chemical formulae are changed during balancing. Changing a chemical formula changes the identity and composition of a substance; therefore, option A is incorrect.

A thin layer of magnesium oxide forms on the surface of magnesium ribbon due to its reaction with oxygen in air. This layer prevents direct contact between magnesium and oxygen, so the ribbon does not burn readily. Rubbing removes the oxide layer and allows magnesium to burn easily. Increasing the mass is not the purpose of cleaning.

In a displacement reaction, a more reactive element replaces a less reactive element in a compound, such as Zn + CuSO₄ → ZnSO₄ + Cu. In a double displacement reaction, two ionic compounds exchange ions, often forming a precipitate, water, or gas, as in AgNO₃ + NaCl → AgCl + NaNO₃. Thus option A correctly states the fundamental difference.

Rusting of iron requires both water (moisture) and oxygen. Moist open air provides both moisture and oxygen, so an iron object is most likely to rust there. In contrast, a layer of oil or paint prevents iron from coming into contact with air and moisture.

Rancidity occurs when oils and fats undergo oxidation, producing an unpleasant smell and taste. An airtight container reduces contact with oxygen, and a cool place slows the rate of oxidation. In contrast, open air or sunlight can increase rancidity because of oxygen and heat.

In a double displacement reaction, the cations and anions of two compounds exchange partners. Its general form is \(AB + CD \rightarrow AD + CB\). For example, in \(Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl\), the ions exchange partners. In contrast, a combination reaction generally forms one product from reactants rather than exchanging ions.

In a balanced chemical equation, the number of atoms of each element must be the same on the reactant and product sides. Here, there are 2 oxygen atoms on the left and 1 on the right, so the equation is unbalanced. Option B is incorrect because oxygen atoms, too, must be equal on both sides of a balanced equation.

In a precipitation reaction, two aqueous solutions react to form an insoluble solid. The state symbols
(aq
) and
(s
) show the aqueous substances and the solid precipitate, making it clear that the product has separated from the solution. State symbols may also be written in a combustion reaction, but they do not specifically show precipitate formation.

Oxidation and reduction are complementary processes. In a redox reaction, one substance loses electrons or gains oxygen and is oxidised, while another substance gains electrons or loses oxygen and is reduced. These changes occur simultaneously because electrons transferred by one species are accepted by another. Therefore option A is correct; a precipitate is not required, and redox changes are chemical rather than merely physical.

When water is added to a coloured solution, the colour becomes lighter because the solution is diluted. Its concentration changes, but no new substance is formed, so this is a physical change. A change in colour or colour intensity alone is not definite proof of a chemical reaction. In contrast, a chemical reaction produces new substances.

In photolytic decomposition, light energy helps break chemical bonds in a compound. For example, silver chloride decomposes in sunlight: \(2\mathrm{AgCl}\xrightarrow{\text{sunlight}}2\mathrm{Ag}+\mathrm{Cl}_2\). In contrast, electrolytic decomposition uses electrical energy, whereas thermal decomposition uses heat energy.

Bubbles may indicate the formation of a gas, and a cold container may indicate absorption of heat. Therefore, option A correctly uses cautious wording such as “may.” The observations alone cannot establish a specific change such as rusting with certainty.

The unbalanced equation is \(\mathrm{Mg + O_2 \rightarrow MgO}\). There are 2 oxygen atoms on the left, so a coefficient of 2 is placed before \(\mathrm{MgO}\): \(\mathrm{Mg + O_2 \rightarrow 2MgO}\). There are then 2 magnesium atoms on the right, so placing 2 before magnesium gives the balanced equation \(\mathrm{2Mg + O_2 \rightarrow 2MgO}\). Therefore, the correct coefficient is 2; using 1 would leave the oxygen atoms unbalanced.