Acids, Bases, and Salts

The prefix “deca-” means ten, and “hydrate” refers to water associated with a compound. Thus, sodium carbonate decahydrate, Na₂CO₃·10H₂O, contains ten water molecules of crystallisation for each formula unit of sodium carbonate. Therefore, option A is correct. The term does not indicate ten metals or ten acids, and it cannot mean that the crystal has no water. The water is present in a definite chemical ratio.

Hard water contains dissolved calcium and magnesium ions, which react with soap to form insoluble scum and prevent good lather. Washing soda, Na₂CO₃·10H₂O, supplies carbonate ions that precipitate these ions as insoluble carbonates, thereby softening the water. Hence, option B is correct. It does not increase hardness, convert soap into an acid, or change liquid water into a gas. Better lather then makes washing more effective.

Blue hydrated copper sulphate is CuSO₄·5H₂O. On strong heating, it loses its water of crystallisation and becomes white anhydrous CuSO₄. When water is added again, the salt becomes hydrated and the characteristic blue colour returns. Therefore, option C is correct. The change is not caused by formation of common salt, a metal, or chlorine. This reversible colour change demonstrates the importance of water of crystallisation in hydrated salts.

Water of crystallisation is the definite number of water molecules chemically associated with one formula unit of certain salt crystals. For example, washing soda is Na₂CO₃·10H₂O and blue copper sulphate is CuSO₄·5H₂O. Therefore, option B is correct. It is not an uncertain amount present in every salt, nor is it restricted to boiling water. The term also does not mean that the water itself is always acidic.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating to about 373 K, it loses part of its water of crystallisation and forms calcium sulphate hemihydrate, CaSO₄·½H₂O, called plaster of Paris. Thus, option C is correct. Washing soda and baking soda are different sodium salts, while bleaching powder is a chlorine-based substance and is not produced by heating gypsum. Controlled temperature prevents complete dehydration.

The governing concept is the hydration and setting of plaster of Paris (calcium sulphate hemihydrate). It reacts with absorbed water to form gypsum, calcium sulphate dihydrate, which is a hard solid: 2CaSO4·1/2H2O + 3H2O → 2CaSO4·2H2O. Therefore, moisture may make the powder set before it is required. Option D is correct; the other options describe no such storage-related reaction.

The pH of a salt solution is governed mainly by the strengths of the parent acid and base and by the hydrolysis of the salt ions. A strong-acid–strong-base salt such as sodium chloride is nearly neutral, whereas a strong-acid–weak-base salt such as ammonium chloride is acidic, and a weak-acid–strong-base salt such as sodium carbonate is basic. Thus option C is correct; colour, shape, and container size do not determine pH.

Sodium chloride is formed by the neutralisation of hydrochloric acid, a strong acid, with sodium hydroxide, a strong base: HCl + NaOH → NaCl + H2O. In water, NaCl dissociates into Na+ and Cl−, but these ions do not significantly hydrolyse to produce excess H+ or OH−. Consequently, the solution has a pH close to 7, so option A is correct.

Ammonium chloride is regarded at this level as the salt of a strong acid, hydrochloric acid, and a weak base, ammonium hydroxide. In water, NH4+ undergoes hydrolysis and produces hydronium ions: NH4+ + H2O ⇌ NH3 + H3O+. The resulting excess of H3O+ makes the solution acidic, although it is not necessarily strongly acidic. Therefore option C is correct; it is not neutral or basic.

Sodium carbonate is the salt of a weak acid, carbonic acid, and a strong base, sodium hydroxide. In water, the carbonate ion reacts partially with water: CO3^2− + H2O ⇌ HCO3− + OH−. The formation of hydroxide ions gives the solution a basic nature. Hence option C is correct. It is not acidic or neutral, and the solution is certainly ionic and aqueous.

The governing classification is based on the acid-forming behaviour of many non-metal oxides. For example, carbon dioxide reacts with water to form carbonic acid: CO2 + H2O ⇌ H2CO3, while sulphur dioxide forms sulphurous acid. Their aqueous solutions can therefore show acidic properties. Option A is correct. Turning red litmus blue would indicate a base, and the other choices do not describe the defining reaction.

A neutralisation reaction occurs when a basic substance reacts with an acid to produce salt and water. For example, copper(II) oxide reacts with hydrochloric acid: CuO + 2HCl → CuCl2 + H2O. Since metal oxides show this characteristic reaction with acids, they generally exhibit basic nature and are called basic oxides. Thus option B is correct; volatility or neutrality is not demonstrated by this reaction.

An acid-base indicator changes colour over a particular pH transition range. During gradual addition of one solution to another, a stable final colour can indicate that the mixture has reached or passed the indicator’s endpoint, often near the neutralisation point in a suitable titration. It does not prove an explosion, absence of water, or metal formation. Therefore option C is the appropriate conclusion, with the exact pH depending on the indicator used.

Olfactory indicators are substances whose characteristic smell changes, becomes weak, or is masked in acidic or basic media. This smell response provides evidence about the nature of the tested solution without relying on a colour change. Onion, vanilla essence, and clove are commonly cited school-level examples. Thus option B is correct. Permanent black colour, metal formation, and zero temperature are not the identifying basis of these indicators.

Onion juice is classified as an olfactory indicator because onion’s characteristic smell can change or become difficult to detect in different acid-base media. The indicator property is therefore sensed through smell rather than through a typical colour transition. Option A is correct. Onion juice is not a metal, radioactive, or temperature-only indicator; those descriptions do not explain its use in acid-base testing.

Phenolphthalein is an acid–base indicator that becomes pink in a sufficiently basic solution, while it remains colourless in acidic and neutral solutions. Therefore, colourlessness alone cannot prove that the solution is strongly acidic or neutral; it only rules out the usual pink basic range. Hence option B is the most careful conclusion. The other choices either reverse the indicator result or make unrelated claims.

Methyl orange is an acid–base indicator whose colour depends on the acidity of the solution. In an acidic medium it appears red, while in a basic medium it appears yellow. Since the question specifies a basic solution, the correct observation is yellow, so option C is correct. Red belongs to the acidic range; blue is associated with other indicators such as litmus, and black is not the normal methyl-orange colour.

Blue litmus turning red is a characteristic test for an acidic solution. A second independent observation supports the same conclusion: acids react with metal carbonates to form a salt, water, and carbon dioxide gas, for example acid + carbonate → salt + H₂O + CO₂. Thus the solution is acidic, making option D correct. A basic or neutral solution would not normally produce this combined indicator pattern.

The governing concept is the ionisation of acids in water. An acid produces hydrogen ions in aqueous solution; more precisely, these hydrogen ions associate with water to form hydronium ions, H₃O⁺. Therefore the concentration of the acidic hydrogen-ion species increases, making option B correct. Hydroxide ions are associated with basicity, carbonate ions are not produced by every acid, and chloride ions occur only with particular acids such as hydrochloric acid.

The pH scale describes hydrogen-ion activity: neutral water is approximately pH 7, acidic solutions are below 7, and basic solutions are above 7. Dissolving a base in water increases hydroxide-ion concentration and makes the solution alkaline. Thus its pH moves to a value above 7, so option C is correct. It does not move below 7 or remain exactly neutral; the actual value depends on the amount and strength of the base.