Acids, Bases, and Salts

Initially, carbon dioxide reacts with limewater to form insoluble calcium carbonate, which causes the milky appearance: CO2 + Ca(OH)2 → CaCO3 + H2O. When excess CO2 is passed, the calcium carbonate reacts further with carbon dioxide and water to form soluble calcium hydrogen carbonate, Ca(HCO3)2. The solid precipitate dissolves, so the solution becomes clear again; option D is correct.

An exothermic process releases heat to the surroundings. During dilution, ions of a concentrated acid become hydrated as the acid mixes with water, and considerable heat may be released. This warms the glass container, so option A is correct. An endothermic process would absorb heat and produce cooling; evaporation and crystallisation do not describe the main heat change in this dilution. For safety, acid must be added slowly to water.

Diluting concentrated acid is strongly exothermic because hydration of acid particles releases a large amount of heat. If a small volume of water is poured into the acid, the water can heat rapidly, boil locally and eject corrosive acid droplets. Therefore option B is correct. Water does not become a base, and the acid does not instantly freeze or become neutral. The safer rule is to add acid slowly to a larger volume of water while stirring.

This is an application of neutralisation. Since the bee sting is described as acidic, a mild base can partly neutralise the acid and reduce its irritating effect. Sodium hydrogen carbonate, commonly called baking soda, is mildly basic, so option C is correct. Vinegar, lemon juice and tamarind juice are acidic and would not be suitable for neutralising an acidic sting. In real treatment, the wound should also be handled according to appropriate medical guidance.

Plant roots absorb mineral nutrients most effectively when soil conditions, including pH, are suitable. The governing concept is nutrient availability: extremely acidic or extremely basic soil can change the solubility of minerals and may damage roots, making absorption difficult. Thus option A is correct. Neutral soil is not automatically harmful, and the other choices are oversimplified rather than scientifically reliable conditions.

Magnesium hydroxide is a sparingly soluble base. The governing concept is neutralisation: its hydroxide ions react with excess hydrochloric acid in the stomach, reducing acidity and discomfort. For example, the overall reaction can be represented as Mg(OH)2 + 2HCl → MgCl2 + 2H2O. Therefore option B is correct; the other options do not describe an antacid reaction.

An aqueous solution of sodium chloride is called brine. In the chlor-alkali process, brine undergoes electrolysis. Chlorine is released at the anode, hydrogen at the cathode, and sodium hydroxide remains in the solution, subject to industrial separation and operating conditions. Therefore option B is correct. The other choices are not the name of the industrial electrolysis process that begins with brine.

Hydrogen is one of the products formed during electrolysis of brine in the chlor-alkali process. A major industrial use of hydrogen is hydrogenation, in which hydrogen adds to unsaturated bonds in vegetable oils, often in the presence of a catalyst, converting them into more saturated fats. Therefore option A is correct. The other choices are not standard uses of this hydrogen product.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, whose chemical formula is Ca(OH)₂. In simplified form, chlorine reacts with calcium hydroxide to produce bleaching powder along with other products. Therefore, option B is correct. Hydrogen and oxygen do not provide the required chlorinating action, while carbon dioxide reacts differently and is not used for this preparation. The lime must be dry so that the product forms properly.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it decomposes according to the equation 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The gaseous product asked for is therefore carbon dioxide, so option D is correct. The released carbon dioxide forms bubbles in dough and helps cakes or other baked foods become light and porous. Hydrogen, chlorine, and oxygen are not products of this decomposition.

Baking soda is chiefly sodium hydrogen carbonate, NaHCO₃, whereas baking powder is a mixture containing baking soda and a mild edible acid, commonly such as tartaric acid, along with a suitable drying agent. When moistened and heated, the acid reacts with the bicarbonate and produces carbon dioxide. Therefore, option B is correct. Baking soda alone does not contain the added edible acid, and baking powder is neither common salt nor chemically identical to baking soda.

When baking soda is used in excess, heating can leave sodium carbonate or another alkaline residue if it is not fully neutralised by an acid. Alkaline substances generally produce an unpleasant bitter or soapy taste, so option A is correct. Baking powder includes an edible acid that reacts with the bicarbonate and reduces this alkaline residue. Formation of common salt, freezing of water, and acid rain are unrelated to the taste change in cake.

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.

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 a salt of a strong base (NaOH) and a weak acid (H$_2$CO$_3$). Such salts produce OH$^-$ in water, so the solution is alkaline. Therefore, its pH is greater than 7.

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.