Chemical Reactions and Equations

Rancidity occurs when fats and oils undergo oxidation. In warm, open air, oily food has a continuous supply of oxygen, and higher temperature increases the rate of oxidation. Therefore, rancidity increases fastest in option A. In contrast, nitrogen-filled packets reduce oxidation by displacing oxygen.

Rusting requires iron to be in contact with both oxygen and moisture. Leaving iron exposed to moist air increases this contact and therefore promotes rusting. In contrast, paint and oil form protective barriers on iron, while galvanising coats iron with zinc to protect it from rusting.

Calcium carbonate requires continuous heating to decompose: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). Here, \(\Delta\) indicates that heat is a necessary condition for the reaction, not a product. In contrast, the reaction of calcium oxide with water and the combustion of fuel are exothermic reactions that release heat.

The governing concept is energy change in a chemical reaction. If heat appears on the product side, the reaction releases heat to its surroundings, so it is exothermic and option A is correct. In an endothermic reaction, heat is absorbed and is written with the reactants. Precipitation and displacement describe reaction patterns, not the direction of heat transfer.

The governing idea is whether a reaction absorbs or releases energy. Heat written on the reactant side is consumed during the reaction, so the process is endothermic and option A is correct. An exothermic reaction releases heat and places it among the products. Displacement and precipitation identify chemical patterns, but they do not answer the energy-change question.

On heating, lead nitrate undergoes thermal decomposition. The white lead nitrate changes into yellow lead oxide, and brown nitrogen dioxide gas is evolved. Thus, both a colour change and gas evolution are observed. The reaction is: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). In contrast, zinc reacting with dilute hydrochloric acid mainly shows evolution of hydrogen gas, not a clear colour change.

Electrolysis of water produces hydrogen and oxygen gases. The volume of hydrogen is twice the volume of oxygen, so their volume ratio is 2:1. This indicates the combining ratio of hydrogen and oxygen in water. In contrast, burning magnesium mainly forms magnesium oxide and does not produce gases in this ratio.

When water freezes, only its state changes from liquid to solid; both water and ice have the chemical formula \(\mathrm{H_2O}\). Thus, no new substance is formed, so this is a physical change. In contrast, rusting produces a new substance such as iron oxide, making it a chemical change.

The governing concept is identifying evidence of a chemical reaction. A colour change, release of heat, and formation of a gas are observable indicators that new substances may have formed. Because several indicators occur together, option A is the strongest conclusion. A physical change usually does not produce such combined evidence, while options C and D contradict the observations.

The first step in balancing a chemical equation is to count the atoms of every element on the reactant and product sides. By conservation of mass, the number of atoms of each element must be the same on both sides. This is achieved by changing coefficients, not the subscripts in chemical formulae. Colour or smell is not relevant to balancing an equation.

Subscripts in a chemical formula specify the fixed number of atoms of each element. For example, changing
\(\mathrm{H_2O}\) to
\(\mathrm{H_2O_2}\) changes water into hydrogen peroxide, a different substance. Only coefficients are changed during balancing because they change the number of molecules without changing the identity of a substance.

In the balanced equation \(2\mathrm{Mg}+\mathrm{O}_2\rightarrow2\mathrm{MgO}\), the coefficient of \(\mathrm{Mg}\) is 2, so there are 2 magnesium atoms on the reactant side. Since \(\mathrm{O}_2\) contains 2 oxygen atoms, \(2\mathrm{MgO}\) is required on the product side; therefore, 1 magnesium atom would not balance the equation.

The balanced equation is \(2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}\). Since \(\mathrm{O_2}\) contains two oxygen atoms, \(2\mathrm{H_2O}\) must be written on the product side. This also balances the four hydrogen atoms. Writing only one water molecule would leave the oxygen atoms unbalanced.

The reaction is represented as CaO + H₂O → Ca(OH)₂ + heat. Formation of calcium hydroxide from quicklime and water releases considerable heat, so it is an exothermic reaction and option A is correct. Option B describes an endothermic change, while the reaction does not primarily produce light or electrical energy as stated in options C and D.

In the reaction
\(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\), two reactants, calcium oxide and water, combine to form the single product calcium hydroxide. A reaction in which two or more reactants form one product is called a combination reaction. In contrast, option B describes a decomposition reaction, in which one substance breaks down into multiple products.

On heating, lead nitrate undergoes thermal decomposition: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. The nitrogen dioxide produced is a reddish-brown gas, so option A is correct. Oxygen is also formed in the reaction but is colourless. Hydrogen and carbon dioxide are not products of lead nitrate decomposition.

On heating, lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). Therefore, lead oxide, \(\mathrm{PbO}\), is the solid product, whereas nitrogen dioxide and oxygen are gases. Lead chloride cannot form because no chloride source is present in the reaction.

In sunlight, silver chloride decomposes: \(2\mathrm{AgCl}(s)\xrightarrow{\text{sunlight}}2\mathrm{Ag}(s)+\mathrm{Cl}_2(g)\). The metallic silver formed is grey, so the white silver chloride gradually appears grey. Chlorine gas is also produced, but metallic silver is the main cause of the colour change.

Silver chloride and silver bromide are photosensitive salts. On exposure to light, they decompose to form metallic silver, which appears dark grey or black and helps form a photographic image. For example: \(2\mathrm{AgBr}\xrightarrow{\text{light}}2\mathrm{Ag}+\mathrm{Br}_2\). Option C is incorrect because these salts are not chemically stable in light; they undergo a chemical change when exposed to it.

The governing concept is electrolysis and electrode reactions. In acidified-water electrolysis, oxidation occurs at the anode, producing oxygen; reduction at the cathode produces hydrogen. The overall equation is 2H₂O → 2H₂ + O₂, so option A is correct. Hydrogen is the cathode gas, while nitrogen and chlorine are not produced under these conditions.