Acids, Bases, and Salts

Blue litmus turning red is a standard indication that hydrogen ions are present and the solution is acidic. Phenolphthalein is colourless in acidic and neutral media, but it becomes pink in a sufficiently basic solution. Thus the two observations agree with an acidic solution. Basic and strongly basic options conflict with the litmus result, and “metallic” is not a solution nature identified by these indicators.

Sodium hydroxide is a base, so its initial pH is above 7. Adding hydrochloric acid supplies hydrogen ions that neutralise hydroxide ions: H⁺ + OH⁻ → H₂O. The pH therefore falls toward 7 near the equivalence point. If more acid is added after neutralisation, excess hydrogen ions make the solution acidic and the pH drops below 7. Hence A is correct.

Neutralisation depends on the amounts of reactive hydrogen ions and hydroxide ions, not simply on the measured volumes. Equal volumes may contain unequal concentrations, and weak or strong acids and bases may provide different effective ion amounts. For example, the same volume of 1 M acid and 0.1 M base will not neutralise completely. Therefore option A is correct; the other statements deny or misdescribe neutralisation.

Burning coal, petroleum, and other fossil fuels releases sulphur dioxide and nitrogen oxides. In the atmosphere, these gases undergo oxidation and react with water droplets, producing sulphuric acid and nitric acid or related acidic substances. The acids are then carried to the ground with rain. Oxygen does not directly become salt, sodium chloride does not melt into acids, and limewater evaporation is unrelated; therefore A is correct.

The governing concept is the reaction of a metal with a dilute acid. Magnesium reacts with dilute sulphuric acid to form magnesium sulphate and hydrogen: Mg + H₂SO₄ → MgSO₄ + H₂. Hydrogen is identified by bringing a burning splint near the gas; it burns rapidly with a characteristic pop sound. Option A is a test for a basic gas, B is not specific, and D indicates carbon dioxide, so option C is correct.

The governing concept is neutralisation and the logarithmic pH scale. Sodium hydroxide initially makes the solution basic, so its pH is above 7. As hydrochloric acid is added, hydrogen ions reduce the hydroxide concentration and the pH falls. At the equivalence point, equal reacting amounts of strong acid and strong base produce nearly neutral sodium chloride solution, with pH about 7. Therefore C is correct; A and D show the wrong trend.

The governing concept is the basic character of many metal oxides. A basic oxide reacts with an acid in a neutralisation reaction to produce salt and water; for example, CuO + 2HCl → CuCl₂ + H₂O. Some metal oxides also dissolve in water to form bases, although not every metal oxide is soluble. Colour, insolubility, and carbon dioxide formation are not defining criteria. Hence option A is correct.

The governing concept is the acidic behaviour of many non-metal oxides. Such oxides react with bases to form salt and water, just as acids do. For example, carbon dioxide reacts with calcium hydroxide: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Some non-metal oxides may also form acids with water, but insolubility and hydrogen release are not general defining properties. Therefore option A is correct.

The governing concept is safe dilution of a concentrated acid. Mixing acid and water releases considerable heat, so the process is exothermic. Adding acid slowly to a larger volume of water allows the heat to disperse and reduces the chance of sudden boiling and splashing. The reverse procedure can eject hot corrosive droplets and cause serious burns. Water does not solidify the acid, and density or colour is irrelevant. Thus A is correct.

The governing relationship is pH = −log₁₀[H⁺]. Therefore, a lower pH corresponds to a greater hydrogen-ion concentration. A solution of pH 2 has [H⁺] = 10⁻² mol L⁻¹, while pH 5 has 10⁻⁵ mol L⁻¹; the former is 1000 times greater. pH 7 is neutral and pH 9 is basic with a much lower hydrogen-ion concentration. Hence A is correct.

The governing concept is the effect of dilution on hydrogen-ion concentration. Diluting an acidic solution tenfold reduces [H⁺] to one-tenth. Since pH = −log[H⁺], the pH rises by about one unit for an ideal strong-acid solution, but it generally remains below 7 unless the dilution is sufficient or other effects are considered. Thus acidity decreases without automatically becoming basic. Option B is correct; A, C, and D contradict this relationship.

The governing concept is water of crystallisation in hydrated salts. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. When exposed to dry air, it can lose some or all of this bound water, a process called efflorescence. The crystal lattice changes and the crystals may become powdery or less transparent. This is not caused by absorbing oxygen, changing into nitrogen, or releasing hydrogen gas. Therefore option A is correct.

The governing concept is water of crystallisation in hydrated salts. Blue vitriol is hydrated copper sulphate, CuSO4·5H2O. Heating removes its crystallisation water and produces white anhydrous CuSO4. Adding water restores the hydrated form and its blue colour. Thus option A is correct; sodium chloride, basicity, or metallic character cannot explain this reversible colour change.

The governing concept is neutralisation of an acidic irritant. Nettle hairs can inject an irritating substance with an acidic effect, so a mild basic material is the suitable opposite treatment in this school-level context. It can reduce the acidity without adding another strong irritant. Therefore option A is correct; a strong acid would worsen irritation, while neutral salt powder or sugar solution does not provide the needed neutralising action.

The governing concept is the reaction of a strong base with an acidic oxide. Sodium hydroxide absorbs carbon dioxide from air and reacts as 2NaOH + CO2 → Na2CO3 + H2O. Some hydroxide ions are therefore converted into carbonate, reducing the available OH− concentration and the solution’s effective basicity. Thus option A is correct; the other changes do not occur in this situation.

The governing concept is the nature of salts formed by neutralisation. Sodium chloride is produced by HCl, a strong acid, and NaOH, a strong base: HCl + NaOH → NaCl + H2O. Neither ion significantly hydrolyses water, so its aqueous solution is generally neutral, close to pH 7. Therefore option A is correct; the other statements are chemically false or irrelevant.

The governing concept is hydrolysis of salts. Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonium hydroxide, a weak base. In water, the ammonium ion hydrolyses and produces H3O+, making the solution acidic and giving pH below 7. Hence option A is correct. A strong-acid/strong-base salt is generally neutral, while the other choices do not describe NH4Cl.

The governing concept is neutralisation of a basic metal oxide by an acid. Calcium oxide reacts with sulphuric acid according to CaO + H2SO4 → CaSO4 + H2O, producing exactly calcium sulphate and water. Therefore option A is correct. A carbonate would also release carbon dioxide in an acid reaction, while the other listed pairs do not provide the stated products under the given conditions.

Phenolphthalein is pink only in a sufficiently basic solution. Adding hydrochloric acid supplies hydrogen ions, which neutralise hydroxide ions from sodium hydroxide: HCl + NaOH → NaCl + H2O. As the excess alkalinity is removed and the solution approaches neutral or becomes acidic, phenolphthalein loses its pink colour. The other choices do not describe the indicator transition.

Zinc is amphoteric, so it can react with a strong base such as sodium hydroxide. In aqueous solution, zinc forms a soluble zincate species while water is reduced and hydrogen gas is evolved; a simplified equation is Zn + 2NaOH + 2H2O → Na2[Zn(OH)4] + H2. Thus hydrogen is correct, not oxygen, carbon dioxide, or chlorine.

An amphoteric oxide shows both acidic and basic chemical behaviour. Aluminium oxide reacts with acids to form aluminium salts and water, while it reacts with strong bases to form soluble aluminate compounds. This ability to react with both kinds of substances gives the name amphoteric. The other choices incorrectly restrict its reactions or describe an impossible physical state.

Limewater turns milky when carbon dioxide is passed through it, because insoluble calcium carbonate forms: CO2 + Ca(OH)2 → CaCO3 + H2O. Dilute acids react with carbonate ions to release carbon dioxide, for example CO3^2− + 2H+ → CO2 + H2O. Therefore carbonate is the likely ion; chloride and nitrate do not normally give this gas test.

Bleaching powder contains hypochlorite and chloride species and is not completely stable in moist air. Moisture and carbon dioxide can promote decomposition or acidification of its chlorine-containing components, allowing a small amount of chlorine or related active chlorine to escape and produce the characteristic smell. It does not become sodium chloride, absorb oxygen as the main process, or freeze water.

Plaster of Paris is calcium sulphate hemihydrate, CaSO4·1/2H2O. When water is added, it undergoes hydration and reforms gypsum, calcium sulphate dihydrate, CaSO4·2H2O: CaSO4·1/2H2O + 3/2H2O → CaSO4·2H2O. The interlocking gypsum crystals produce a hard mass, which explains its use in casts and moulds. The other products are unrelated.

The chlor-alkali process is the electrolysis of concentrated sodium chloride solution, commonly called brine. At the anode, chloride ions are oxidised to chlorine gas; at the cathode, water is reduced to hydrogen gas and hydroxide ions. The sodium ions combine with hydroxide ions to form sodium hydroxide solution. Therefore option A gives all three correct products. The other options contain substances not formed in this process.