Acids, Bases, and Salts

The governing concept is the ionisation of acids in water. Hydrochloric acid dissociates almost completely: HCl(aq) → H⁺(aq) + Cl⁻(aq), with H⁺ commonly represented as hydronium, H₃O⁺, in water. Thus option A is correct. Hydroxide ions indicate bases, carbonate ions come from carbonates, and sodium ions are not present in pure hydrochloric acid.

The governing concept is acid strength, which depends on the extent of ionisation in water, not simply on concentration. Hydrochloric acid ionises almost completely, producing a high concentration of H₃O⁺ and Cl⁻ ions. Therefore option B is correct. Partial ionisation describes a weak acid, insolubility is false for its aqueous solutions, and HCl is an acid rather than a base.

The governing concept is the extent of ionisation. Ethanoic acid, CH₃COOH, establishes a reversible equilibrium in water and only a fraction of its molecules release H⁺ ions: CH₃COOH ⇌ H⁺ + CH₃COO⁻. Hence option A is correct. Complete ionisation characterises a strong acid, while the metal and strong-base descriptions are chemically incorrect.

The governing concept is electrical conduction in ionic solutions. At equal concentration, hydrochloric acid ionises almost completely, producing many mobile H₃O⁺ and Cl⁻ ions. Ethanoic acid ionises only partially, so it produces fewer charge carriers. Therefore option A is correct. Conductivity is not evidence that HCl is a base, and it is soluble in water.

The governing concept is the acid–base nature of oxides. Most metal oxides are basic because they react with acids to form salt and water; for example, CuO + 2HCl → CuCl₂ + H₂O. This neutralisation behaviour supports option B. Although some metal oxides may be amphoteric or neutral, the general school-level classification is basic, not acidic or sweet.

The governing concept is the classification of oxides by acid–base behaviour. Many non-metal oxides react with water to form acids; for example, CO₂ + H₂O ⇌ H₂CO₃. Their aqueous solutions can turn blue litmus red, supporting option B. The word “general” is important because a few oxides may be neutral or show special behaviour, but non-metal oxides are commonly acidic.

The governing concept is the acidic behaviour of non-metal oxides. Carbon dioxide reacts with bases such as sodium hydroxide: CO₂ + 2NaOH → Na₂CO₃ + H₂O. It can also form carbonic acid with water. Thus option A correctly identifies its reaction with bases. It does not form metals, turn red litmus blue, or represent common salt, so the other options are unsuitable.

The governing concept is neutralisation between a basic oxide and an acid. Copper(II) oxide reacts as follows: CuO + 2HCl → CuCl₂ + H₂O. Formation of salt and water from an oxide and an acid indicates that the oxide has basic character. Therefore option B is correct. Acidic, neutral and volatile describe different properties and do not explain this neutralisation reaction.

Turmeric is a natural acid–base indicator. Its yellow pigment changes to reddish-brown when it comes in contact with a basic or alkaline substance. Therefore, option B is correct. Acidic and neutral substances generally do not produce this characteristic reddish-brown change, while sugar is not classified as the required alkaline substance in this test.

Phenolphthalein is an acid–base indicator whose colour depends on the nature of the solution. It remains colourless in an acidic medium but changes to pink, or pinkish-red in stronger alkaline conditions, in a basic medium. Thus option C is the standard school-level answer. Red, green, and colourless do not represent its usual basic-solution colour.

Methyl orange is an acid–base indicator used to distinguish acidic and basic media. In an acidic solution, it appears red; in a basic solution, it appears yellow, with orange shades occurring during the transition range. Therefore option B is correct. Yellow describes the basic condition, while blue and green are not its characteristic colours in this test.

Methyl orange changes colour according to the acidity or alkalinity of a solution. It is red in an acidic medium and yellow in a basic medium. Consequently, option B is correct for a basic solution. Red is the plausible distractor because it belongs to the acidic condition, whereas black and white are not the standard colours shown by methyl orange in this indicator test.

Baking soda is the common name of sodium hydrogen carbonate, with the formula NaHCO₃. It is a basic salt and reacts with acids to release carbon dioxide, which explains several of its household uses. Therefore option B is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium sulphate is a different salt.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The principal solid product is sodium carbonate, while carbon dioxide and water are gaseous products under the usual conditions. Hence option A is correct; the other salts do not result from this decomposition.

Baking powder contains baking soda and a mild edible acid. When the mixture becomes wet and is heated, the acid reacts with sodium hydrogen carbonate and produces carbon dioxide: acid + NaHCO₃ → salt + CO₂ + water. The gas forms bubbles that expand and make dough rise. Therefore option A is correct; hydrogen, chlorine, and metal are not produced in this process.

Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The term decahydrate indicates that ten water molecules are associated with each formula unit as water of crystallisation. Therefore option A is correct. Sodium hydrogen carbonate is baking soda, calcium oxychloride is bleaching powder, and sodium chloride is common salt, so each distractor names a different compound.

Hard water contains dissolved calcium and magnesium ions, which can form deposits and react with soap. Washing soda supplies carbonate ions that precipitate these ions as insoluble carbonates, such as calcium carbonate and magnesium carbonate. Removing them reduces hardness, so option A is correct. The process does not depend on hydrogen or chlorine gas and does not make the water acidic.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and forms calcium sulphate hemihydrate, CaSO₄·½H₂O, known as plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Thus option A is correct; the other substances do not undergo this preparation reaction.

Plaster of Paris is prepared by partially heating gypsum, whose formula is CaSO₄·2H₂O. Partial dehydration removes one and a half water molecules, producing CaSO₄·½H₂O, called calcium sulphate hemihydrate. Calcium carbonate, sodium carbonate decahydrate, and sodium chloride are different salts, so option C is correct.

The hardening of plaster of Paris is a hydration reaction. Calcium sulphate hemihydrate, CaSO₄·½H₂O, combines with water and changes back into gypsum, CaSO₄·2H₂O. Gypsum forms an interlocking solid mass, making the paste hard. No sodium chloride, gas, or sugar is produced, so option A is correct.