Acids, Bases, and Salts

Phenolphthalein is a base-sensitive indicator: it is pink in a sufficiently basic solution and colourless when the solution is neutral or acidic. Sodium hydroxide supplies hydroxide ions, but added acid reacts with them in a neutralisation reaction to form water. As the hydroxide concentration and pH fall, the pink colour disappears. Therefore A is correct; acid does not create the colour, and neither solid sodium hydroxide nor gaseous decomposition is involved.

Dilution increases the volume while the amount of dissolved acid remains the same. Consequently, the concentration of hydrogen ions, written as [H+], decreases. Since pH is related to concentration by pH = −log[H+], a decrease in [H+] produces an increase in pH. The solution becomes less acidic, but it does not necessarily become basic. Thus A is correct; B reverses the effect and C and D are false.

Bleaching powder is manufactured by passing chlorine gas over dry slaked lime, whose chemical formula is Ca(OH)2. The commonly represented reaction is 2Ca(OH)2 + 2Cl2 → Ca(OCl)2 + CaCl2 + 2H2O. Thus dry slaked lime is the required reactant. Sodium chloride is used to prepare brine, calcium carbonate is a limestone compound, and sodium hydroxide is not the standard material in this preparation. Therefore A is correct.

The chlor-alkali process electrolyses concentrated aqueous sodium chloride, or brine. At the cathode, water is reduced more readily than sodium ions: 2H2O + 2e− → H2 + 2OH−. Therefore hydrogen gas is released at the cathode. Chlorine gas forms at the anode, while sodium hydroxide remains in solution; sodium metal is not deposited from this aqueous solution, and oxygen is not the main cathode product. Hence A is correct.

Blue copper sulphate crystals are hydrated copper sulphate, CuSO4·5H2O. The attached water molecules are called water of crystallisation and give the crystals their blue appearance. On heating, this water is driven off, producing anhydrous copper sulphate, CuSO4, which is white. The change is not caused by release of oxygen, copper metal, or sulphur dioxide. Adding water can restore the blue hydrated form, so A is correct.

Plaster of Paris is calcium sulphate hemihydrate, CaSO4·1/2H2O. It reacts with water to form gypsum, CaSO4·2H2O: CaSO4·1/2H2O + 1 1/2H2O → CaSO4·2H2O. This hydration makes the material set and harden, so unwanted moisture can spoil its storage and handling properties. It does not form an acid, release chlorine, or produce sodium carbonate. Therefore A is correct.

Indicator colour changes reveal the solution’s acid-base nature. Methyl orange is red in an acidic medium and changes toward yellow as the medium becomes less acidic or basic. Phenolphthalein is colourless in acidic and neutral conditions, becoming pink only in a sufficiently basic solution. Both observations are therefore consistent with an acidic solution. A is correct; a basic solution would usually make phenolphthalein pink, while “metallic” is not an aqueous acid-base classification.

A very low pH indicates that the soil is strongly acidic. Slaked lime, Ca(OH)2, is a basic substance, so it reacts with and neutralises excess acidic substances in the soil. This raises the pH toward a range more suitable for crop growth. The purpose is not to increase acidity, add ordinary salt, or remove water. Because the treatment is an acid-base neutralisation, option A gives the correct reason.

The stomach normally contains hydrochloric acid, which supports digestion, but an excess can irritate the stomach lining and cause discomfort. An antacid contains a mild base, such as a hydroxide, carbonate, or bicarbonate, which reacts with excess hydrochloric acid in a neutralisation reaction. The acidity is reduced rather than strengthened. It does not form chlorine or convert water into acid. Therefore option A correctly explains the relief.

Curd and many sour foods contain acids, such as lactic acid in curd. These acids can react with copper or with metals present in brass, causing corrosion and producing metal salts. Some such salts may be harmful if they dissolve into food and are consumed. The concern is therefore a chemical reaction and possible contamination, not sweetness, conversion into gold, or transformation into water. Hence option A is correct.

The red-litmus-to-blue change shows that the solution is basic. When an acid reacts with a base, the characteristic products are salt and water; heat is often released because the reaction is exothermic. This is therefore a neutralisation reaction, so option A is correct. Decomposition involves one compound breaking down, displacement involves replacement by another element, and precipitation specifically produces an insoluble solid.

Mouth bacteria break down sugars and produce acids. As the pH decreases, the acidic environment begins to dissolve the mineral component of tooth enamel. Tooth decay becomes rapid when the pH falls below about 5.5, making option A correct. A pH of 7 is neutral, while 8.5 and 14 are alkaline values and do not represent the acidic condition that attacks enamel. Regular cleaning helps limit this process.

Acid strength describes the extent to which an acid ionises in water. A strong acid ionises almost completely, whereas a weak acid ionises only partly. Concentration is different: it describes how much acid is present per unit volume of solution. Thus a dilute strong acid and a concentrated weak acid are both possible. Option A correctly separates these ideas; the other statements wrongly treat them as identical or absolute properties.

Baking soda is sodium hydrogen carbonate. It reacts with the edible acid in baking powder to form carbon dioxide, water, and a salt. The reaction is useful because carbon dioxide bubbles become trapped in the dough and make it rise, light, and soft. Option A is correct because the acid provides a controlled release of gas. It does not convert baking soda into metal, harden the dough, or produce chlorine.

Washing soda has the formula \(\mathrm{Na_2CO_3\cdot10H_2O}\). The term \(10\mathrm{H_2O}\) shows that each formula unit of sodium carbonate is associated with 10 molecules of water of crystallisation. Therefore, option A is correct. Five water molecules occur in copper sulfate pentahydrate, \(\mathrm{CuSO_4\cdot5H_2O}\), not in washing soda. Exam tip: In a hydrated salt formula, the coefficient before \(\mathrm{H_2O}\) after the dot gives the number of water molecules of crystallisation.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating near 373 K, it loses one and a half molecules of water of crystallisation and forms calcium sulphate hemihydrate, CaSO₄·½H₂O, called plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Thus option A is correct; no calcium metal, chlorine, or sodium carbonate is produced.

A base is a substance that can neutralise an acid, but not every base dissolves appreciably in water. A base that dissolves in water produces hydroxide ions and gives an alkaline solution; such a soluble base is called an alkali. Therefore option A is the most correct. Option B confuses acids with bases, option C contradicts the existence of soluble bases, and option D ignores the colour change of indicators.

The pH scale indicates acidity or basicity: 7 is approximately neutral, values below 7 are acidic, and values above 7 are basic. A pH close to 14 indicates a very high concentration of hydroxide ions and therefore a strongly alkaline solution. Such substances may be caustic and can damage skin or eyes. Option A is correct; the other choices conflict with the meaning of the pH value.

Sodium chloride is formed by neutralisation of hydrochloric acid, HCl, a strong acid, with sodium hydroxide, NaOH, a strong base: HCl + NaOH → NaCl + H₂O. The Na⁺ and Cl⁻ ions do not hydrolyse appreciably in water, so the solution remains nearly neutral with pH close to 7. Option A is correct. Salts from weak acids or weak bases generally produce acidic or basic solutions instead.

The pH scale describes the acidic or basic nature of an aqueous solution. A higher concentration of hydroxide ions, [OH⁻], makes the solution more basic. Because pOH decreases as [OH⁻] increases and pH + pOH = 14 at 25°C, the pH rises, generally above 7. Thus option A is correct. Option B describes increasing acidity, while C and D are not general consequences of adding hydroxide ions.