Chemical Reactions and Equations

In the presence of light, silver chloride decomposes to form metallic silver and chlorine gas: \(2AgCl \xrightarrow{sunlight} 2Ag + Cl_2\). The metallic silver formed causes the grey colour. Since light causes the decomposition, it is photolytic decomposition. In contrast, thermal decomposition is caused by heat, while electrolytic decomposition is caused by electric current.

In the presence of light, silver chloride undergoes photodecomposition: \(2\mathrm{AgCl}\xrightarrow{\text{light}}2\mathrm{Ag}+\mathrm{Cl_2}\). The metallic silver formed is grey, so AgCl appears grey. Chlorine is also produced, but it does not cause the grey colour.

During electrolysis of water, hydrogen is produced at the cathode and oxygen at the anode. Their volume ratio is hydrogen : oxygen = 2 : 1, so oxygen is the gas formed in the lower volume, at the anode. The cathode produces the larger volume of hydrogen. Exam tip: Remember: H₂ at the cathode and O₂ at the anode during electrolysis of water.

The blue colour of aqueous copper sulphate is mainly due to hydrated copper(II) ions, \(\mathrm{Cu^{2+}}\). When these ions are converted into copper metal during a reaction, their concentration in the solution decreases and the blue colour fades. Iron(II) ions may form and change the solution’s appearance, but the direct reason for loss of blue colour is the decrease in \(\mathrm{Cu^{2+}}\) ions. Exam tip: Link the colour of a solution with the metal ion responsible for it.

In a displacement reaction, a solid metal can displace the metal present in a salt solution only if it is more reactive. Since metal A is less reactive than metal B, A cannot remove B from B’s salt solution. Therefore, no displacement reaction occurs. Option B would be possible only if A were more reactive than B. Exam tip: A metal higher in the reactivity series displaces a metal lower in the series from its salt solution.

For example, in
\(BaCl_2(aq)+Na_2SO_4(aq)\rightarrow BaSO_4(s)+2NaCl(aq)\), the ions exchange partners, so it is a double displacement reaction. The \(BaSO_4\) formed is an insoluble white solid in water; therefore, its formation is called precipitation. A combination reaction usually forms one product from the reactants, so option A is not suitable. Exam tip: When mixing aqueous solutions produces an insoluble solid, identify that solid as a precipitate.

In the reaction \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\), hydrogen takes oxygen from copper oxide to form water. Gain of oxygen is oxidation; therefore, hydrogen is oxidised. In terms of electrons, hydrogen’s oxidation number changes from 0 to +1, meaning that it loses electrons. Option C is incorrect because gaining electrons is reduction. Exam tip: Identify oxidation by oxygen gain or electron loss.

Iron rusts only when both oxygen and water (moisture) are available. Moisture acts as a medium for the electrochemical reaction, while oxygen participates in the oxidation of iron. The product is hydrated iron(III) oxide, called rust. Option C contains oxygen but lacks the necessary moisture in dry air, so rusting does not occur. Exam tip: Painting, oiling, or greasing prevents rust by blocking air and moisture from reaching iron.

Paint forms a protective barrier between iron and air and moisture. When the layer is scratched, the exposed iron can contact oxygen and water, so oxidation begins and rust may form. The scratch itself does not produce rust; it removes the protective coating and allows the required conditions. Exam tip: Remember that rusting needs both oxygen and moisture.

Rusting is the oxidation of iron in the presence of oxygen and water or moisture. An oil layer forms a protective barrier on the iron surface, reducing the access of air and moisture; therefore, rusting decreases. Option B is incorrect because a catalyst increases the reaction rate, whereas oil prevents the required reactants from reaching the iron surface. Exam tip: Paint, grease and oil prevent rusting by forming barrier coatings.

Oils and fats, especially unsaturated fats, undergo oxidation on exposure to oxygen in air. This produces compounds with an unpleasant smell and taste, causing rancidity. Hydrolysis may break down fats, but oxidation is the main cause of usual rancidity in oils and fats. Exam tip: Nitrogen is filled in chip packets to reduce oxidation.

Oils and fats react with oxygen in air and undergo oxidation. This produces an unpleasant smell and taste, called rancidity. A higher temperature increases the reaction rate, so oily food spoils faster in warm open air. Nitrogen is relatively unreactive and does not convert oils into sweet substances. Exam tip: Connect spoilage of fats and oils with oxidation and rancidity.

Oils and fats can react with oxygen in air and become oxidised. This produces an unpleasant smell and taste, called rancidity. Nitrogen filling displaces oxygen from the packet, so oxidation of the oils slows down. Converting oils into solid fats is hydrogenation, not the effect of nitrogen packaging. Exam tip: Associate nitrogen packing of oily foods with preventing oxidation and rancidity.

Gas evolution may indicate the formation of a gaseous product during a chemical reaction. A cold container suggests that the process may have absorbed heat from its surroundings and may therefore be endothermic. Hence, option A is the most careful conclusion. Option B is incorrect because a heat-releasing reaction would generally tend to make the container or surroundings warmer. Exam tip: When evidence is limited, prefer cautious statements such as “may” over statements claiming certainty.

When heat is written on the product side of a chemical equation, the reaction releases heat. The hot container further indicates that energy is being transferred out of the reaction mixture. Therefore, the reaction is exothermic. In an endothermic reaction, heat is shown on the reactant side because the reaction absorbs heat from the surroundings. Exam tip: Heat shown with the products indicates an exothermic reaction.

Iron is more reactive than copper, so it displaces copper from CuSO₄: Fe + CuSO₄ → FeSO₄ + Cu. FeSO₄ causes the green colour, while copper deposits on the nail. A combination reaction forms one product from reactants. Exam tip: use the reactivity series to identify displacement reactions.

During respiration, glucose undergoes controlled, stepwise oxidation with the help of enzymes. Energy is released gradually, and a useful portion is captured in ATP; hence, respiration is an energy-releasing chemical process. Combustion also involves oxidation, but energy is released very rapidly, mainly as heat and light. Option C is incorrect because respiration is exothermic, not endothermic. Exam tip: Remember respiration as “slow, controlled oxidation” and combustion as “rapid oxidation.”

In photosynthesis, plants use light energy to make glucose from carbon dioxide and water. The energy is therefore stored in the chemical bonds of glucose. During cellular respiration, glucose is oxidised and energy is released as ATP and heat. Hence, option A is correct. Option C is a close distractor but is incorrect because respiration does not require an external energy input such as light; it uses the energy stored in glucose. Exam tip: Remember photosynthesis as energy storage and respiration as energy release.

Formation of a new substance is clear evidence of a chemical reaction, and gas evolution is a common observation during such reactions. In a closed container, no matter enters or leaves, so the total mass of reactants and products remains unchanged. Therefore, A is correct. B is incorrect because a physical change does not form a new substance. Exam tip: For a reaction in a closed system, compare total mass before and after the reaction using the law of conservation of mass.

A skeletal equation gives the correct chemical formulae of reactants and products, but the number of atoms of each element may not be equal on the two sides. In a balanced equation, only the coefficients are adjusted to make the number of atoms of every element equal on both sides. Thus, it obeys the law of conservation of mass. Option C is incorrect because chemical formulae are also written in a skeletal equation. Exam tip: while balancing, do not change subscripts in formulae; change only coefficients.

The chemical formula of water is \(H_2O\). Each water molecule contains 2 hydrogen atoms and 1 oxygen atom, making 3 atoms altogether. Therefore, three water molecules contain \(3 \times 3 = 9\) atoms. Option A, 6, counts only the hydrogen atoms in three water molecules, not all atoms. Exam tip: Add the atoms shown in the formula and multiply by the number of molecules.

In electrolysis of water, hydrogen gas is formed at the cathode and oxygen gas at the anode. From the balanced equation \(2H_2O \rightarrow 2H_2 + O_2\), the volume ratio of hydrogen to oxygen is \(2:1\). Therefore, the greater volume of hydrogen is obtained at the cathode. Oxygen forms at the anode, but its volume is half that of hydrogen. Exam tip: Remember: \(H_2\) at the cathode and \(O_2\) at the anode.

This is a double-displacement precipitation reaction. The \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions combine to form insoluble barium sulphate as a white precipitate: \(\mathrm{BaCl_2(aq)+Na_2SO_4(aq)\rightarrow BaSO_4(s)+2NaCl(aq)}\). Hence, the products are barium sulphate and sodium chloride. Option B contains sulphite rather than sulphate, and sulphite ions are not present in the reactants. Exam tip: In such questions, identify the insoluble salt to find the precipitate product.

Removal of oxygen from a substance is reduction, so substance A undergoes reduction. Removal of hydrogen is oxidation, so substance B undergoes oxidation. Therefore, the correct sequence is reduction and oxidation. Option B gives the two processes in reverse order. Exam tip: Addition of oxygen or removal of hydrogen indicates oxidation.

The zinc coating forms a barrier between iron and air and moisture, preventing the oxygen and water needed for rusting from reaching the iron. If the coating is scratched, zinc, being more reactive than iron, corrodes first and provides sacrificial protection. Option B is incorrect because zinc does not convert iron into zinc. Exam tip: Remember that galvanisation gives both barrier and sacrificial protection.