Acids, Bases, and Salts

Sodium chloride is produced by neutralisation of hydrochloric acid, a strong acid, and sodium hydroxide, a strong base: HCl + NaOH → NaCl + H₂O. In water, NaCl dissociates into Na⁺ and Cl⁻; neither ion hydrolyses appreciably, so the solution is approximately neutral at ordinary concentration. It does dissolve in water, is not made only of metal, and does not release carbon dioxide.

The governing concept is the distinction between hydrates and hemihydrates. Plaster of Paris is calcium sulphate hemihydrate, represented as CaSO₄·½H₂O. Thus, one formula unit of calcium sulphate is associated with half a water molecule on average. Option A is correct. Gypsum contains two water molecules per calcium sulphate unit, while the other choices describe different or incorrect hydration states.

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.

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.

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.

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.

The governing concept is neutralisation and pH correction. A very low pH indicates that the soil is strongly acidic. Slaked lime, calcium hydroxide, is a basic substance; when added in a suitable amount, it reacts with excess acidity and raises the soil pH toward a more suitable range. Therefore option A is correct. Lemon juice, vinegar, and tamarind water are acidic and would intensify the problem.

The governing concept is pH-dependent demineralisation. Bacteria in dental plaque metabolise sugars left in the mouth and release organic acids. These acids lower the local pH; when it becomes sufficiently low, minerals in enamel dissolve more readily, beginning tooth decay. Option A is correct. Bacteria do not turn teeth or saliva into metals, nor do they fill teeth with chlorine.

The governing concept is acid–base neutralisation in dental care. Oral bacteria can convert sugars into acids, lowering the pH around tooth enamel and encouraging demineralisation. Mildly basic substances in toothpaste help neutralise some of these acids and support a safer oral pH. Thus option A is correct. The other choices would damage teeth, introduce a harmful gas, or worsen the conditions that support decay.

The governing concept is neutralisation of an acid by a mild base. Ant venom commonly contains formic acid, which contributes to the burning sensation at the sting site. Baking soda, sodium hydrogen carbonate, is mildly basic and can react with or neutralise part of the acid, reducing irritation. Therefore option A is correct. The remaining choices either reverse the chemistry or describe impossible transformations.

The governing concept is acid–base neutralisation. In the standard school-level model, a bee sting has an acidic effect, while sodium hydrogen carbonate, NaHCO₃, is a mild base. Applying its solution can neutralise part of the acidic effect and may reduce irritation. Hence option A is correct. The other options incorrectly increase acidity or claim impossible changes to skin and water.

The governing concept is the characteristic reaction of an acid with a metal carbonate. In general, acid + metal carbonate produces salt + water + carbon dioxide; for example, 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂. Therefore option A is correct. An acid with a metal usually produces salt and hydrogen, while simple mixing of salt and water produces no such reaction.

Limewater turns milky when carbon dioxide passes through it because insoluble calcium carbonate forms: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Acids react with carbonates and hydrogen carbonates to release carbon dioxide, for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Therefore option A is correct. The other classes are too restrictive or do not generally produce this gas with acid.

Sodium hydroxide is deliquescent, so it absorbs moisture from air and may become wet or form a solution. It also reacts with atmospheric carbon dioxide, producing sodium carbonate and water: 2NaOH + CO₂ → Na₂CO₃ + H₂O. These changes alter the substance and its concentration, so it should be stored in a tightly closed container. Hence option A is correct; the other statements describe impossible effects.

Plant growth depends on a suitable soil pH and on the availability of nutrients. If soil is already acidic, adding another acidic substance can lower its pH further and may make nutrient uptake or root health worse. Thus option A is the most appropriate conclusion. Neutralisation would require a suitable basic material, such as carefully applied slaked lime; the soil will not automatically become neutral or basic, and pH is not irrelevant.

Phenolphthalein is pink in a basic solution and colourless when the solution is acidic or sufficiently near neutral. The initial pink colour therefore shows that the sample was basic. Adding hydrochloric acid supplies H⁺ ions, which react with OH⁻ ions and reduce the basicity; as the mixture approaches neutralisation, the pink colour disappears. Hence option A is correct, while the other choices reverse indicator behaviour or misread the change.

Highly acidic soil can be corrected by adding a suitable mild base that neutralises excess acidity. Slaked lime is calcium hydroxide, Ca(OH)₂, and is commonly used in controlled amounts for this purpose; it reacts with acids to form salt and water. Therefore option A is the best choice. Vinegar is acidic and would worsen the condition, while sodium chloride is not a useful neutralising agent and ammonium chloride can contribute to soil acidity rather than correct it.