Acids, Bases, and Salts

The governing concept is the use of natural acid–base indicators. Turmeric contains curcumin, which normally appears yellow but turns reddish-brown in a basic medium. Soap solution is generally basic because of its alkaline components. Consequently, option A correctly explains the stain. Soap is not an acid, turmeric does not become a metal, and the colour is not produced by carbon dioxide.

The governing concept is indicator behaviour: an indicator changes colour according to whether the medium is acidic or basic. Red cabbage juice contains anthocyanin pigments whose molecular form changes with pH, so different colours appear in acidic, neutral, and basic solutions. Option A is therefore correct. The juice does not transform substances into gold or gas and is not restricted to testing metals.

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.

The governing pattern is: active metal + dilute acid → salt + hydrogen gas. For example, magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen: Mg + 2HCl → MgCl₂ + H₂. Therefore, option A is correct. Carbon dioxide is normally produced when an acid reacts with a carbonate or hydrogen carbonate, not with an ordinary active metal; water and oxygen or a base and chlorine are not the general products here.

Magnesium and zinc are both metals above hydrogen in the usual reactivity series, so each can displace hydrogen from a dilute non-oxidising acid. The general reaction is metal + acid → salt + H₂; for example, Zn + 2HCl → ZnCl₂ + H₂. Thus hydrogen is common to both reactions, making option A correct. Carbon dioxide would suggest a carbonate, chlorine requires a different chemical process, and ammonia is not produced in this reaction.

Indicators provide the governing evidence. An acidic solution changes blue litmus to red, while phenolphthalein remains colourless in an acidic or sufficiently neutral medium. The two observations together therefore identify the solution as acidic, so option A is correct. A basic solution would turn red litmus blue and generally make phenolphthalein pink. The test does not justify claiming that the solution must specifically be sodium chloride.

The governing concept is the indicator test for a base. A basic solution changes red litmus to blue and turns colourless phenolphthalein pink. Sodium hydroxide dissolves in water to give an alkaline solution, so option A satisfies both observations. Lemon juice and vinegar contain acids and would show the opposite litmus behaviour, while soda water is mildly acidic because dissolved carbon dioxide forms carbonic acid; it would not normally turn phenolphthalein pink.

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.

The observation matches thermal decomposition of lead nitrate: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ on heating. Nitrogen dioxide produces brown fumes, while lead(II) oxide is yellow when hot and appears white on cooling. Therefore option A, lead nitrate, is indicated by both independent clues. Sodium chloride, sodium carbonate, and calcium sulphate do not normally give this characteristic combination of brown nitrogen dioxide fumes and a hot-yellow, cold-white residue.

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.

Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from dilute sulphuric acid: Zn + H₂SO₄ → ZnSO₄ + H₂. The gas evolved is hydrogen, and hydrogen burns rapidly with oxygen in air, producing the characteristic pop sound. Oxygen, nitrogen and chlorine are not products of this metal–acid reaction, so option A is correct.

Hydrogen chloride shows acidic behaviour only after dissolving in water, where it ionises and produces hydronium ions: HCl + H₂O → H₃O⁺ + Cl⁻. Dry HCl gas cannot supply these aqueous ions to dry litmus, so there is no colour change. Moist litmus provides water and turns red; therefore option A is correct.