Acids, Bases, and Salts

The gas evolved is carbon dioxide. Initially, CO₂ reacts with lime water: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O, and insoluble calcium carbonate makes the liquid milky. When excess CO₂ is passed, the precipitate reacts further: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂, which is soluble. Thus the milkiness disappears and option B is correct.

Salt hydrolysis determines the nature of a salt solution. Sodium carbonate, formed from a strong base and weak acid, gives a basic solution. Sodium chloride and potassium nitrate are approximately neutral because their ions hydrolyse negligibly. Ammonium chloride comes from the strong acid HCl and the weak base NH₄OH; NH₄⁺ hydrolyses to produce H₃O⁺, so its solution is acidic. Therefore D is correct.

A metal above hydrogen in the reactivity series can generally displace hydrogen from a dilute non-oxidising acid. Since zinc reacts with the same acid and releases hydrogen, while the unnamed metal does not under the stated conditions, the first metal may be below zinc in reactivity. It cannot be identified as sodium, and the acid must contain replaceable hydrogen. Hence A is the valid conclusion.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, so its crystals contain water of crystallisation. On exposure to comparatively dry air, some of this water is lost from the crystal lattice. The crystals become dull and powdery; this process is called efflorescence. Deliquescence involves absorbing moisture and dissolving, while neutralisation and displacement are chemical reaction types. Therefore B is correct.

Plaster of Paris is calcium sulfate hemihydrate, CaSO₄·½H₂O. It is intended to react with added water and set into gypsum, CaSO₄·2H₂O. If exposed to humid air before use, it can slowly absorb moisture and become gypsum prematurely. The partly hydrated material loses its ability to set properly when later mixed for application. Thus option C gives the correct reason; the other products are unrelated.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the cathode, water is reduced and hydrogen gas forms: 2H₂O + 2e⁻ → H₂ + 2OH⁻. At the anode, chloride ions are oxidised to chlorine: 2Cl⁻ → Cl₂ + 2e⁻. The remaining Na⁺ combines with OH⁻ to give sodium hydroxide. Therefore A is correct.

Acid strength depends on the extent of ionisation in water, not simply on concentration. Hydrochloric acid ionises almost completely and is classified as a strong acid. Ethanoic acid ionises only partially and is therefore a weak acid. Option A contains an acid and a base, while C and D do not present a strong-acid/weak-acid pair. Hence hydrochloric acid followed by ethanoic acid in option B is correct.

Dilution decreases the concentration of hydronium ions in an acidic solution. Since pH = −log[H₃O⁺], a decrease in hydronium concentration produces an increase in pH, although the solution may remain acidic for some time. Repeated equal additions of water continue to dilute it, so the pH moves upward toward 7 rather than downward. Therefore option C is correct; the other choices violate the pH concept.

Bleaching powder is commonly represented as calcium oxychloride, CaOCl₂, and is prepared by passing chlorine over dry slaked lime, Ca(OH)₂. The process can be summarised as 2Cl₂ + 2Ca(OH)₂ → Ca(OCl)₂ + CaCl₂ + 2H₂O, with the product mixture called bleaching powder. The other options describe unrelated substances or processes. Therefore option A is correct.

The governing concept is the difference between concentration and acid strength. Concentration means the quantity of acid present in a given volume of solution, whereas strength describes the extent to which acid molecules ionise in water. A concentrated weak acid and a dilute strong acid are possible. Hence option B is correct; the other choices confuse these terms or make chemically false claims about pH and water.

The governing concept is water of crystallisation, which is a fixed number of water molecules incorporated into certain salt crystals. Blue hydrated copper sulphate, CuSO4·5H2O, loses this water on heating and becomes white anhydrous CuSO4. Adding water restores the hydrated blue form. Therefore option B is correct; chlorine, sodium, and carbon dioxide do not explain this reversible colour change.

The governing concept is thermal decomposition of sodium hydrogen carbonate, commonly called baking soda. On heating, the balanced change is 2NaHCO3 → Na2CO3 + CO2 + H2O. Carbon dioxide and water are released, while sodium carbonate remains as the solid product. Thus option C is correct; the calcium compounds and sodium chloride are not products of this decomposition.

The governing concept is neutralisation: an acid reacts with a suitable base to reduce acidity and form salt and water. Ant venom is commonly described at this school level as acidic, so a mild baking soda solution, which is weakly basic, can neutralise part of it and provide relief. Option C is therefore suitable. Vinegar, lemon juice, and dilute acid are acidic and would not neutralise the sting.

The governing concept is the indicator colour change during neutralisation. Phenolphthalein is pink in a sufficiently basic medium but becomes colourless when the solution is no longer sufficiently basic. Adding acid consumes hydroxide ions and lowers the solution’s basicity. Therefore option A is correct. More basic conditions preserve or intensify the pink colour, while metal formation and guaranteed carbon dioxide are irrelevant.

The governing concept is the hydrolysis and nature of salts. A salt produced from a weak acid and a strong base generally gives a basic solution because the conjugate base of the weak acid reacts with water to produce hydroxide ions. This turns red litmus blue, so option B is correct. Strong acid–weak base salts are usually acidic, strong–strong salts are approximately neutral, and acid plus water is not a salt-forming combination.

The governing concept is electrical conduction by mobile charged particles. When an acid dissolves in water, it forms ions such as hydronium and accompanying negative ions. These ions move between electrodes and carry electric charge, so measurable current or conductivity is direct evidence of their presence. Option B is correct; smelling, cooling, or inserting glass does not demonstrate mobile ions.

The governing concept is that pH measures the acidic or basic condition of water and strongly influences biological and chemical processes. Acid rain introduces acidic substances, lowering lake pH. Excess acidity can interfere with enzyme activity, reproduction, respiration, and the survival of sensitive aquatic organisms; it may also affect dissolved minerals. Thus option A is correct, while the other statements contradict the meaning of low pH.

The governing concept is acid-base neutralisation. A pH of 9 indicates that the original solution is basic. Adding acid supplies hydrogen ions, which react with hydroxide ions or other basic species and reduce the basicity. When the pH reaches 7, the solution is approximately neutral under the stated conditions. Therefore option C is correct; displacement, combination, and decomposition are not identified by this pH change.

The governing concept is controlled dehydration of a hydrated salt. Gypsum is calcium sulphate dihydrate, CaSO4·2H2O. Gentle heating removes part of its water of crystallisation and forms plaster of Paris, CaSO4·½H2O. If heating is excessive, more water may be driven off and the desired setting properties can be lost. Therefore option B is correct; calcium, chlorine, and sodium are not being removed or formed.

The governing concept is an acid–carbonate reaction. Baking soda, or sodium hydrogen carbonate, reacts with the acid and releases carbon dioxide: NaHCO₃ + H⁺ → CO₂ + H₂O. The escaping carbon dioxide gets trapped as tiny bubbles in the batter or mixture, so the food rises and becomes porous, soft, and spongy. Hydrogen does not form in this reaction, oxygen does not provide the colour, and chlorine is not involved; therefore option C is correct.