Chemical Reactions and Equations

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.

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.

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.