Acids, Bases, and Salts

The governing concept is the cathode reaction in aqueous brine. Water gains electrons at the cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Sodium ions remain in the solution and combine with hydroxide ions to form NaOH, so the region becomes basic. Thus option A is correct. Chlorine forms at the anode, salt does dissolve, and sodium metal is not deposited in water.

The gas released from an acid–metal carbonate reaction is carbon dioxide. Initially, CO₂ reacts with calcium hydroxide in lime water: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, and insoluble calcium carbonate makes the solution milky. With excess CO₂, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂, which is soluble, so the milkiness disappears.

Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonia, the weak base. In water, the ammonium ion hydrolyses and produces hydronium ions, so the solution is acidic: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. Sodium carbonate is generally basic, while sodium chloride and potassium nitrate are salts of strong acids and strong bases and are approximately neutral.

Sodium carbonate is the salt of a strong base, sodium hydroxide, and a weak acid, carbonic acid. The carbonate ion reacts with water and generates hydroxide ions: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻, so its solution is basic. Sodium chloride and potassium chloride are approximately neutral salts, whereas ammonium chloride is acidic because NH₄⁺ produces hydronium ions.

Concentration describes how much acid is present per unit amount of solution, whereas strength describes the degree to which the acid ionises in water. A concentrated weak acid may contain much solute but ionise only partly; a dilute strong acid may ionise almost completely while containing little solute. Therefore, concentration and strength are different ideas, making option A correct.

The observation matches thermal decomposition of lead nitrate: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ on heating. Nitrogen dioxide produces brown fumes, while lead(II) oxide is yellow when hot and appears white on cooling. Therefore option A, lead nitrate, is indicated by both independent clues. Sodium chloride, sodium carbonate, and calcium sulphate do not normally give this characteristic combination of brown nitrogen dioxide fumes and a hot-yellow, cold-white residue.

Hydrogen chloride shows acidic behaviour only after dissolving in water, where it ionises and produces hydronium ions: HCl + H₂O → H₃O⁺ + Cl⁻. Dry HCl gas cannot supply these aqueous ions to dry litmus, so there is no colour change. Moist litmus provides water and turns red; therefore option A is correct.

Carbon dioxide is an acidic oxide. With limited CO₂, sodium hydroxide can form sodium carbonate: 2NaOH + CO₂ → Na₂CO₃ + H₂O. When CO₂ is passed in excess, the carbonate reacts further with carbonic acid and forms soluble sodium hydrogen carbonate: Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃. Thus option A is correct.

Turning red litmus blue shows that the solution contains a base, such as sodium hydroxide. On heating with ammonium chloride, the base releases ammonia: NaOH + NH₄Cl → NH₃ + H₂O + NaCl. Ammonia has a characteristic pungent smell and turns moist red litmus blue. Hydrogen, carbon dioxide and sulphur dioxide do not fit these observations, so A is correct.

The pH relation is [H⁺] = 10⁻ᵖᴴ. Therefore, the ratio of the concentrations is 10⁻² ÷ 10⁻⁵ = 10³ = 1000. Equivalently, the pH difference is three units, and every one-unit decrease in pH means a tenfold increase in hydrogen-ion concentration. Hence the first solution has 1000 times more H⁺ and option A is correct.

Sodium carbonate is a salt of a strong base, sodium hydroxide, and a weak acid, carbonic acid. Its carbonate ions hydrolyse in water and produce hydroxide ions, making the solution alkaline. Phenolphthalein is colourless in acidic or nearly neutral conditions but pink in a sufficiently basic solution. Therefore A is correct; not every salt gives a pink colour.

Quicklime is calcium oxide. It reacts vigorously with water to form slaked lime and release heat: CaO + H₂O → Ca(OH)₂. The resulting limewater becomes milky when carbon dioxide is passed through it because insoluble calcium carbonate forms: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. The combined observations identify quicklime, so option A is correct.

Gypsum is CaSO₄·2H₂O, and controlled heating removes only part of its water of crystallisation to produce plaster of Paris, CaSO₄·½H₂O. If heating is excessive, dehydration can proceed too far and the desired hemihydrate may not be obtained, reducing the material’s useful setting behaviour. Thus option A is correct. The other choices introduce unrelated sodium or chlorine compounds and do not describe gypsum dehydration.

For a sufficiently strong acidic solution, pH = −log[H+]. Tenfold dilution reduces the hydrogen-ion concentration by a factor of 10, so the logarithm changes by one unit and the pH increases by approximately 1. Starting from pH 3, the new value is about pH 4. It does not fall to 2, jump to neutral 7, or become the basic value 12.

Use \(\text{pH}=-\log_{10}[\text{H}^+]\). Then
\[\frac{[\text{H}^+]_{\text{new}}}{[\text{H}^+]_{\text{old}}}=10^{(\text{pH}_{\text{old}}-\text{pH}_{\text{new}})}=10^{(4-2)}=10^2=100.\]
So, when pH goes from \(4\) to \(2\), \([\text{H}^+]\) increases by \(100\) times.

The governing concept is salt hydrolysis. A pH of 3 is distinctly acidic, so the salt produces excess H3O+ ions in water. Salts formed from a strong acid and a weak base, such as ammonium chloride from HCl and NH4OH, commonly give acidic solutions because the conjugate acid of the weak base hydrolyses. A weak-acid/strong-base salt is basic, a strong/strong salt is approximately neutral, and weak/weak salts are not always neutral.

Initially, carbon dioxide reacts with limewater, calcium hydroxide, to form insoluble calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The suspended CaCO₃ causes milkiness. When excess CO₂ is passed, the precipitate reacts further with water and carbon dioxide to form soluble calcium hydrogen carbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. The solid dissolves, so the milkiness disappears and option A is correct.

The governing concept is that neutralisation depends on the reacting amounts of H⁺ and OH⁻, not simply on the measured volumes. Equal volumes can contain different concentrations, and acids or bases can differ in strength and degree of ionisation. If the acid supplies more effective H⁺ than the base can remove through OH⁻, excess acid remains and the mixture stays acidic. Therefore A is the best reason.

The governing concept is the stoichiometry of neutralisation. Equal volumes do not necessarily contain equal numbers of reactive particles because concentration and strength may differ. If the base provides more effective OH⁻ ions than the acid can neutralise with H⁺ ions, hydroxide remains after mixing and the final solution is basic. Thus option A is correct. Equal volume alone fixes neither pH nor neutrality, and neutralisation does produce water.