Chemical Reactions and Equations

In a balanced chemical equation, the number of atoms of each element is the same on the reactant and product sides. This represents the law of conservation of mass because atoms are neither created nor destroyed in a chemical reaction. In contrast, reaction rate depends on factors such as temperature, concentration, and catalysts; balancing an equation does not increase the rate.

Coefficients are written before chemical formulae and indicate the number of units of each substance involved. They are adjusted to make the number of atoms of every element equal on both sides of an equation, in accordance with the law of conservation of mass. In contrast, changing a subscript within a chemical formula changes the identity of the substance, so option B is incorrect.

Reactants are the starting substances that undergo chemical change and form products. In a chemical equation, they are conventionally written on the left side of the arrow; for example, in 2H₂ + O₂ → 2H₂O, hydrogen and oxygen are reactants. The new substances on the right are products, while a substance that changes reaction rate is a catalyst.

In a chemical reaction, reactants change to form new substances. These new substances are called products. In a chemical equation, products are written on the right-hand side of the arrow. Option B describes reactants, the substances present at the beginning of a reaction, whereas option C describes a catalyst.

On heating, calcium carbonate decomposes: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). The breaking of one compound into two or more simpler substances by heat is called a thermal decomposition reaction. In contrast, in a combination reaction, two or more substances form a single product.

The decomposition of water by electric current is represented by \(2H_2O \xrightarrow{\text{electric current}} 2H_2 + O_2\). A decomposition reaction caused by electrical energy is called an electrolytic decomposition reaction, so option A is correct. Thermal decomposition requires heat, whereas photolytic decomposition requires light energy.

A decomposition reaction in which a compound breaks down in the presence of light is called photolytic decomposition. For example, silver chloride decomposes in sunlight: \(2\mathrm{AgCl} \xrightarrow{\text{sunlight}} 2\mathrm{Ag} + \mathrm{Cl}_2\). In contrast, thermal decomposition requires heat, while electrolytic decomposition is caused by electric current.

Silver bromide is a light-sensitive compound. On exposure to light, it undergoes photodecomposition to form silver and bromine: \(2AgBr \xrightarrow{\text{light}} 2Ag + Br_2\). The silver formed is grey, producing a visible colour change. Option C refers to formation of silver oxide by oxidation, not to photodecomposition.

A combination reaction occurs when two or more reactants join to produce one product. For example, 2Mg + O₂ → 2MgO has two reactants and one product, magnesium oxide. A decomposition reaction has the reverse pattern, with one compound producing multiple products. Displacement and double-displacement reactions involve replacement or ion exchange rather than simple formation of one product.

A decomposition reaction is one in which a single compound breaks into two or more simpler substances. For example, on heating, calcium carbonate decomposes as CaCO₃ → CaO + CO₂. This is the reverse pattern of combination, where several reactants form one product. Displacement and double-displacement reactions involve replacement or exchange between substances, not the breakdown of one compound alone.

In a displacement reaction, a more reactive metal can displace a less reactive metal from its salt solution. Zinc is above copper in the reactivity series, so copper cannot displace zinc from zinc sulphate solution. If option B were true, displacement would be possible; however, copper is actually less reactive than zinc.

Zinc is more reactive than copper, so it displaces copper from copper sulphate solution:
\(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\)
The released copper is deposited as a reddish-brown solid on the zinc surface. Zinc sulphate is formed in the solution, so it is not the deposited solid.

The blue colour of copper sulphate solution is due to copper(II) ions. Zinc is more reactive than copper, so it displaces copper from copper sulphate in a displacement reaction: \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). The concentration of blue copper(II) ions decreases, so the blue colour fades. Sulphate ions remain in the solution and are not responsible for the colour change.

On heating, lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \rightarrow 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). The \(\mathrm{NO_2}\) formed is a brown gas, so it produces the brown fumes. \(\mathrm{O_2}\) is also produced, but it is colourless.

When lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide, and oxygen: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). The breakdown of one compound into two or more simpler substances by heat is called a thermal decomposition reaction. In a combination reaction, two or more reactants form one product, whereas here one reactant produces several products.

When copper powder is heated in air, it combines with oxygen and forms black copper(II) oxide: 2Cu + O₂ → 2CuO. The black CuO coating changes the appearance of the copper powder. Copper(I) oxide, Cu₂O, is red, copper carbonate is not formed under these conditions, and ordinary heating in air does not mean that copper melts; therefore option A is unambiguous.

In the reaction
\(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\), oxygen is removed from copper(II) oxide and copper is formed. Removal of oxygen from a substance is called reduction; therefore, \(\mathrm{CuO}\) is reduced. In contrast, hydrogen gains oxygen to form water, so it is oxidised.

In the reaction \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\), hydrogen gains oxygen from copper(II) oxide to form water. Gain of oxygen is oxidation, so hydrogen is oxidised. The oxidation state of hydrogen increases from \(0\) to \(+1\) in water. In contrast, copper changes from \(+2\) in \(\mathrm{CuO}\) to \(0\) in copper metal, so copper is reduced.

In a precipitation reaction, mixing two aqueous solutions may produce an insoluble solid that separates from the solution. This solid is called a precipitate. In contrast, combustion usually involves reaction with oxygen and produces heat and light; formation of an insoluble solid is not its main feature.

The reaction is \(\mathrm{Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Here, \(\mathrm{Na^+}\) and \(\mathrm{Ba^{2+}}\) exchange anionic partners, and insoluble white \(\mathrm{BaSO_4}\) forms as a precipitate. Therefore, it is a double displacement, specifically a precipitation, reaction. In a single displacement reaction, a more reactive element replaces another element from a compound; that does not happen here.

A balanced chemical equation follows the law of conservation of mass. Therefore, the number of atoms of each element is the same on the reactant and product sides. Hence, the number of oxygen atoms is equal on both sides. Physical states such as solid, liquid, or gas do not change the atom count.

During digestion, enzymes convert complex food substances into simpler new substances. For example, starch is broken down into simpler sugars and proteins into amino acids. Since the chemical composition of food changes and new substances are formed, digestion is a chemical change. In a physical change, only the size, state, or appearance changes while the chemical composition remains the same. Therefore, digestion is not merely a change of state or a reversible change.

In a burning candle, wax near the wick first melts and vaporises, but the wax vapour combusts with oxygen to form new substances such as carbon dioxide and water vapour. Formation of new substances is the main feature of a chemical change. Therefore, the burning process of a candle is mainly a chemical change. In contrast, melting of wax is only a physical change, or change of state, because no new substance is formed during melting.

When rice is cooked, heat and water cause permanent changes in its properties and structure. Cooked rice cannot be changed back into raw rice by simple methods, so it is a chemical change. In a physical change, the chemical nature of the substance remains unchanged. Exam tip: A change that is not easily reversible and produces new properties is generally a chemical change.

When ice melts, only its physical state changes from solid to liquid. Its chemical composition remains
$
\mathrm{H_2O}$

, so no new substance is formed. Therefore, melting is a physical change, not a chemical reaction. Gas evolution or precipitate formation can be signs of some chemical reactions, but neither occurs when ice melts.