Chemical Reactions and Equations

An exothermic reaction releases energy to the surroundings, and the released heat may be written among the products. In option C, methane burns in oxygen and produces carbon dioxide, water, and heat, so it is exothermic. Options A, B, and D require heat, electricity, or sunlight as energy input and represent endothermic processes.

The governing concept is the notation of physical states in chemical equations. A lowercase l inside parentheses, written as (l), indicates that the substance is liquid; therefore option B is correct. The symbol (s) denotes a solid, (g) denotes a gas, and (aq) denotes a substance dissolved in water. The distinction is important because an aqueous solution is not simply the same as an undissolved liquid.

In a chemical equation, \((g)\) indicates that a substance is in the gaseous state, so option C is correct. \((aq)\) indicates an aqueous solution, meaning the substance is dissolved in water, not that it is a gas. Exam tip: Remember the state symbols: \((s)\) for solid, \((l)\) for liquid, \((g)\) for gas, and \((aq)\) for aqueous solution.

The governing concept is the notation for physical state and solution medium. When a substance is dissolved in water, the equation marks it as aqueous with (aq), so option D is correct. The other symbols indicate different conditions: (s) is solid, (l) is liquid, and (g) is gas. Thus (aq) specifically communicates water-based dissolution, not merely that the substance is flowing or liquid.

An upward arrow
 written with a product indicates that a gas is evolved during the reaction. For example, in

$

ext{Zn} + 2 ext{HCl}

ightarrow

ext{ZnCl}_2 + ext{H}_2
$

, hydrogen gas is evolved. In contrast, a precipitate is conventionally indicated by a downward arrow
.

In a chemical equation, a downward arrow (↓) commonly indicates that an insoluble solid has formed and settled out of a solution. This solid is called a precipitate, so option B is correct. A gas is usually shown by an upward arrow (↑), whereas heat and light are indicated by words or suitable symbols rather than by a downward arrow.

During digestion, enzymes convert complex food substances into new, simpler substances through chemical reactions. These new substances have properties different from the original food, so digestion is a chemical change. In contrast, chewing food into smaller pieces changes only its size and is a physical change.

In option A, both sides contain 2 magnesium atoms and 2 oxygen atoms: \(2Mg + O_2 \rightarrow 2MgO\). While balancing an equation, only coefficients are changed, not subscripts in chemical formulae. For example, option B changes \(MgO\) to \(MgO_2\), which changes the substance itself.

A colour change, gas evolution, a temperature change, and precipitate formation are common signs that new substances may have formed; therefore, they provide good evidence of a chemical reaction. In contrast, melting or boiling alone is usually a physical change because no new substance is formed.

In a balanced equation, the number of atoms of each element is the same on both sides. In \(2H_2 + O_2 \rightarrow 2H_2O\), there are 4 hydrogen atoms and 2 oxygen atoms on each side. In option B, there are 2 oxygen atoms on the left but only 1 on the right, so it is not balanced.

Magnesium burns in oxygen from air to form white magnesium oxide: \(2Mg + O_2 \rightarrow 2MgO\). Since two reactants combine to form one product, this is a combination reaction. In a decomposition reaction, one compound breaks into simpler substances, which does not occur here.

Calcium oxide reacts with water to form calcium hydroxide: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Heat is released during this reaction, so the container becomes hot. Since two reactants combine to form one product, it is a combination reaction; because heat is released, it is exothermic. In contrast, a decomposition reaction involves one compound breaking down into two or more substances.

On heating, lead nitrate decomposes to form 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 brown fumes are nitrogen dioxide, \(\mathrm{NO_2}\). Since one compound breaks down into simpler substances on heating, this is a thermal decomposition reaction, not a combination reaction.

Silver chloride is photosensitive. On exposure to sunlight, it undergoes photochemical decomposition: 2AgCl → 2Ag + Cl₂. The finely divided silver produced is grey, so option A correctly explains the colour change. Dissolving in water does not produce grey silver, and the other options contain chemically impossible products or incorrectly identify the process.

The balanced equation for electrolysis of water is \(2H_2O(l) \rightarrow 2H_2(g) + O_2(g)\). At the same temperature and pressure, gas volumes are proportional to their moles. Therefore, hydrogen occupies twice the volume of oxygen, so \(H_2:O_2 = 2:1\). The ratio \(1:2\) is the reverse ratio.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. In this displacement reaction, pale-green iron sulphate forms and copper is deposited as a reddish-brown coating on the nail: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Therefore, the blue solution fades and appears green. Option B is incorrect because copper is less reactive than iron and cannot displace iron.

Zinc is above copper in the reactivity series, so it displaces copper from copper sulphate solution. The reaction is \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). The blue colour of the solution fades and copper may deposit on the zinc strip. Option B is incorrect because copper is less reactive than zinc.

On heating, lead nitrate decomposes according to 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide (NO₂) is a brown gas, whereas oxygen (O₂) is colourless. Therefore, the student confused the two gaseous products, and option A gives the correct correction. The other statements reverse or misidentify their colours and formulas.

The reaction is \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). The \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions combine to form insoluble white \(\mathrm{BaSO_4}\), while \(\mathrm{Na^+}\) and \(\mathrm{Cl^-}\) ions remain in solution. It is a double displacement reaction because ions are exchanged, and a precipitation reaction because an insoluble solid forms. It is not a displacement reaction, since no element replaces another element from its compound.

The reaction is CuO + H₂ → Cu + H₂O. Oxygen is removed from copper(II) oxide and combines with hydrogen to form water. Removal of oxygen is called reduction, so CuO is reduced to copper and option A is correct. Hydrogen gains oxygen and is oxidised; this simultaneous reduction and oxidation makes the reaction a redox process.