Acids, Bases, and Salts

The governing concept is controlled thermal dehydration of a hydrated salt. Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On careful heating, it loses one and a half molecules of water per formula unit and changes into calcium sulphate hemihydrate, CaSO₄·½H₂O, which is plaster of Paris. Therefore, option A is correct; no metal formation or chlorine release is involved.

The governing concept is the role of water of crystallisation in the colour of hydrated salts. Blue copper sulphate crystals are CuSO₄·5H₂O. Heating removes this water and produces white anhydrous CuSO₄, so the colour change demonstrates that the associated water contributes to the blue appearance. Hence option A is correct. The salt does not become gold, and sulphate does not generally turn into a gas.

The governing concept is reversible hydration of copper sulphate. Heating blue CuSO₄·5H₂O removes its water of crystallisation and leaves white anhydrous CuSO₄. When water is added again, the salt becomes hydrated and the characteristic blue colour reappears. Thus option A is correct. Copper is not burning, sulphur is not melting, and the colour change does not require formation of a new acid.

The governing concept is neutralisation and acid–carbonate reaction in baking powder. Sodium hydrogen carbonate is basic and, if used alone in excess, can leave an unpleasant alkaline taste. A mild edible acid reacts with it to produce carbon dioxide, water, and a salt; the gas helps the dough rise, while neutralisation reduces the basic taste. Therefore, option A is correct.

The governing concept is thermal decomposition and the limewater test for carbon dioxide. On strong heating, sodium hydrogen carbonate decomposes: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The released carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate, making the solution milky. Hence option A is correct; the other gases do not give this test.

The governing concept is electrolysis of concentrated aqueous sodium chloride, or brine. At the cathode, water is reduced more readily than sodium ions, producing hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Chlorine forms at the anode, while sodium hydroxide remains in solution. Therefore, option A is correct; sodium metal is not deposited from the aqueous solution.

The governing concept is oxidation at the anode during electrolysis. In brine, chloride ions lose electrons at the anode: 2Cl⁻ → Cl₂ + 2e⁻. This oxidation produces chlorine gas. At the cathode, water is reduced and hydrogen gas forms instead. Thus option A is correct. Chlorine is not produced equally at both electrodes and is not absent from the process.

The governing concept is the industrial use of products obtained from brine electrolysis. The chlor-alkali process produces sodium hydroxide, chlorine, and hydrogen. Sodium hydroxide is an important alkali used in soap manufacture and in the paper industry, including processing plant materials. Therefore, option A is correct. The other choices are not recognised uses of sodium hydroxide and do not describe chemical industries based on this product.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, whose chemical formula is Ca(OH)₂. The reaction produces bleaching powder, commonly represented as CaOCl₂, along with water. Therefore, chlorine is the required gas. Hydrogen, nitrogen, and carbon monoxide do not participate in this standard preparation and are not suitable distractors for the reaction.

Bleaching powder releases active chlorine in moist conditions or in water. This chlorine oxidises and destroys many disease-causing microorganisms, so the substance is used as a disinfectant for water in controlled quantities. Its action is not related to sweetness, conversion of water into oil, or changing minerals into gold. Hence option A correctly identifies the relevant property.

Hard water contains dissolved calcium and magnesium ions, which interfere with soap and form scum. Washing soda, sodium carbonate (Na₂CO₃), reacts with these ions and precipitates them as insoluble carbonates, thereby removing their effect and softening the water. Baking powder, vinegar, and sugar do not provide this water-softening action in the stated context, so option A is correct.

Hard water contains Ca²⁺ and Mg²⁺ ions. These ions react with soap anions to form insoluble calcium or magnesium soaps, commonly called scum, so lather is not produced readily. Washing soda supplies carbonate ions that precipitate the hardness-causing ions as carbonates. Once those ions are removed, soap can form lather normally. Therefore, option A is correct.

On the pH scale, a value of 7 represents a neutral solution at ordinary school-level conditions. Values greater than 7 indicate a basic or alkaline solution, and pH 11 is therefore basic. The change from 7 to 11 shows that the solution has changed from neutral to basic. It does not become acidic, because acidic solutions have pH values below 7.

A pH below 7 indicates acidity, so both pH 3 and pH 6 describe acidic solutions. Moving upward from 3 to 6 means the hydrogen-ion concentration decreases and the acidity becomes weaker, but the final value has not reached neutrality at pH 7. Thus the solution remains acidic, making option A the only scientifically correct conclusion.

Neutralisation is the reaction between an acid and a base in which salt and water are formed. Hydrochloric acid reacts with sodium hydroxide according to HCl + NaOH → NaCl + H₂O. The other pairs represent dissolution or no relevant acid-base reaction under ordinary conditions: sodium chloride and sugar dissolve in water, while copper does not neutralise water. Hence A is correct.

A rise in the temperature of the reacting mixture means that energy has been released as heat to the surroundings. A chemical reaction that releases heat is called an exothermic reaction. Acid-base neutralisation commonly releases heat because hydrogen ions combine with hydroxide ions to form water. The temperature observation therefore supports option A, not the claims that no reaction or only physical mixing occurs.

Dilution of a concentrated acid is strongly exothermic: hydration of acid particles releases considerable heat. If water is poured rapidly into concentrated acid, the small amount of water may heat suddenly, boil, and throw corrosive acid droplets outward. Adding acid slowly to a larger volume of water, with stirring, spreads the heat safely. Thus option A states the correct laboratory safety principle.

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 governing concept is acid strength, which depends on the extent of ionisation in water. A weak acid, such as ethanoic acid, ionises only partially and establishes an equilibrium between molecules and ions. Therefore, option A is correct. Weak does not mean insoluble, neutral, or basic; a weak acid can dissolve and still produce hydrogen or hydronium ions and show acidic properties.

The governing concept is the colour range of an acid–base indicator. Phenolphthalein remains colourless in acidic and approximately neutral conditions, but its structure changes in a sufficiently basic aqueous medium, producing a pink colour. Thus option A is correct. Acidic solutions do not turn it pink, and a neutral salt solution normally gives no pink colour; the dry-acid choice is not a valid indicator test.