Acids, Bases, and Salts

The governing concept is the pH scale: a lower pH means a greater hydrogen-ion concentration. At the same formal concentration, the solution with pH 2 has more H⁺ ions than the solution with pH 5, so it shows the stronger acidic effect and is more extensively ionised under the stated comparison. In fact, the difference of three pH units corresponds to about a thousandfold difference in hydrogen-ion concentration. Therefore A is correct.

Electrical conductivity in an aqueous solution is caused by mobile charged particles, especially hydrogen ions and the corresponding negative ions. A highly ionised acid produces a larger concentration of such mobile ions in water, so it carries electric current more effectively. A dry gas, a solid, or a substance producing no ions cannot provide comparable ionic conduction. Therefore, option A is correct.

Amphoteric substances react with both acids and bases. Zinc reacts with a dilute acid to release hydrogen, for example, zinc plus hydrochloric acid forms zinc chloride and hydrogen. It also reacts with concentrated sodium hydroxide solution to form a soluble zincate compound and hydrogen. Thus the observation demonstrates amphoteric behaviour, not merely acidic, neutral, or oxidising behaviour. Option A is correct.

A basic oxide is generally identified by its reaction with acids, but aluminium oxide also reacts with strong bases. With an acid it forms an aluminium salt and water; with a strong base it forms a soluble aluminate. A substance showing both kinds of reaction is called amphoteric. Therefore, describing aluminium oxide as only basic is incomplete, and option A gives the correct reason.

Calcium oxide is a metal oxide and behaves as a basic oxide. Carbon dioxide is a non-metal oxide and behaves as an acidic oxide. They undergo a combination reaction: CaO + CO₂ → CaCO₃. Calcium carbonate is a salt formed when the basic oxide neutralises the acidic oxide. Hence option A correctly describes the reaction; the other choices do not match the reactants or reaction type.

Carbon dioxide is an acidic oxide, so it reacts with the hydroxide ions supplied by sodium hydroxide. With limited carbon dioxide, sodium carbonate may form: 2NaOH + CO₂ → Na₂CO₃ + H₂O; with excess carbon dioxide, sodium hydrogen carbonate can form. In either case, hydroxide ions are consumed, so the solution becomes less basic. Therefore, option A is correct.

Hydrochloric acid and sodium hydroxide undergo neutralisation: HCl + NaOH → NaCl + H₂O. Since sodium hydroxide is a base rather than a carbonate or reactive metal, the reaction normally produces no gas. Carbonates and hydrogencarbonates release carbon dioxide with acids, while zinc can release hydrogen. Therefore option A is the only suitable pair.

A metal carbonate reacts with an acid in three stages conceptually: the carbonate produces carbonic acid, which decomposes into water and carbon dioxide, while the metal ion combines with the acid radical to form a salt. The general equation is acid + metal carbonate → salt + water + CO₂. Active metals usually release hydrogen, whereas oxides and bases generally form salt and water only. Hence option A is correct.

The gas is identified as carbon dioxide because it turns limewater milky by forming insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. When excess CO₂ is passed, the precipitate reacts further to form soluble calcium hydrogencarbonate, so the milkiness disappears. Acids release CO₂ from carbonates and hydrogencarbonates. Thus option A is correct.

The governing concept is the pH scale: at 25°C, pH = 7 is neutral, values below 7 are acidic, and values above 7 are basic. A higher pH corresponds to a lower hydrogen-ion concentration and, in a basic solution, a higher hydroxide-ion concentration. Therefore, among pH 12, 7, 5, and 2, the pH 12 solution has the greatest hydroxide concentration. pH 7 is neutral, while pH 5 and pH 2 are acidic.

The pH scale measures acidity through hydrogen-ion concentration: pH = −log[H⁺]. When pH rises from 3 to 6, the hydrogen-ion concentration decreases greatly, so the solution becomes less acidic. However, pH 6 is still below the neutral value of 7; it therefore remains acidic rather than becoming basic. Option B reverses the pH relationship, option C ignores the value 7, and option D is chemically impossible for an aqueous solution.

A basic solution has pH above 7 because its hydroxide-ion concentration is relatively high. Adding a suitable amount of acid supplies hydrogen ions, which react with hydroxide ions in the neutralisation reaction H⁺ + OH⁻ → H₂O. This reduces the basic effect and can bring the pH close to 7, provided the amount is controlled. More base would raise pH, while removing water or changing salt into a gas does not describe the required neutralisation.

Dilution increases the volume of an acid solution without removing its acidic substance completely. Consequently, the concentration of hydrogen ions decreases and the pH rises. The solution therefore becomes less acidic and moves toward neutrality, but it does not automatically cross pH 7 to become basic. Water does not convert every acid into a base, the ions are not destroyed, and pH 14 is not the inevitable result of dilution. The final pH depends on the acid and the amount of water added.

Dilution of a concentrated acid is highly exothermic, meaning that considerable heat is released when water and acid mix. If a small amount of water is poured into the acid, the water may heat rapidly, boil locally, and eject acid droplets. These droplets can cause serious chemical burns. The safe laboratory rule is to add acid slowly to a larger volume of water while stirring and wearing protection. The other choices contradict the actual thermal behaviour.

Sodium chloride is formed from hydrochloric acid, a strong acid, and sodium hydroxide, a strong base. In water, NaCl dissociates mainly into Na⁺ and Cl⁻ ions, which do not produce a significant excess of H⁺ or OH⁻. Its aqueous solution is therefore approximately neutral and does not change the colours of red or blue litmus paper. It is neither a strong acid nor a strong base, and it does not destroy the indicator under ordinary conditions.

Sodium carbonate is considered a salt of the strong base sodium hydroxide and the weak acid carbonic acid. In water, carbonate ions undergo hydrolysis and produce hydroxide ions: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻. The formation of OH⁻ makes the solution basic, so it can turn red litmus blue. It is not strongly acidic, it does dissociate and react in water, and it is a compound rather than merely a metal.

Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonia, a weak base. In water, the ammonium ion reacts partially with water: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. The hydronium ions make the solution acidic, allowing it to turn blue indicator paper red. It is not sodium hydroxide, is not generally a basic salt, and it dissolves readily enough in water to produce an aqueous solution.

A pH greater than 7 indicates a basic solution containing a greater effective concentration of hydroxide ions than hydrogen ions. Sodium carbonate is a salt of a strong base and a weak acid; carbonate ions hydrolyse in water and generate some OH⁻ ions, so its solution is basic. Sodium chloride is approximately neutral, ammonium chloride is acidic because of ammonium-ion hydrolysis, and pure water is approximately neutral at ordinary conditions. Thus sodium carbonate is the only suitable choice.

A solution with pH less than 7 is acidic. Ammonium chloride is produced from a strong acid, hydrochloric acid, and a weak base, ammonia. In water, NH₄⁺ undergoes partial hydrolysis and forms H₃O⁺ ions, giving the solution an acidic character. Sodium chloride is approximately neutral, while sodium carbonate and washing soda, sodium carbonate decahydrate, give basic solutions because carbonate ions produce OH⁻ through hydrolysis. Therefore ammonium chloride is the correct answer.

The governing concept is water of crystallisation in hydrated salts. Blue copper sulphate is CuSO₄·5H₂O. On heating, it loses its fixed water molecules and becomes white anhydrous CuSO₄. Adding water restores the hydrated form and its blue colour. Therefore option A is correct; the other options do not explain the reversible colour change or involve conversion of copper, sulphate, or acidity.