Acids, Bases, and Salts

Bleaching powder, commonly represented as CaOCl2, works mainly because it releases available chlorine, which provides oxidising and disinfecting action. In a moist, open environment it may undergo decomposition and react with moisture and carbon dioxide, so the amount of available chlorine decreases over time. Consequently its bleaching and disinfecting power falls. Therefore option A is correct, and it should be stored in a dry, tightly closed container.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses only part of its water of crystallisation and changes into calcium sulphate hemihydrate, CaSO₄·½H₂O, called plaster of Paris. If heating is excessive, the material may become anhydrous calcium sulphate and lose its useful setting property. Therefore, option A is correct; the other options describe unrelated changes.

Hydrated copper sulphate, CuSO₄·5H₂O, is blue because water molecules are present in its crystal structure. Heating removes this water and produces white anhydrous CuSO₄. Adding water restores the hydrated compound and its blue colour. This reversible colour change is evidence of water of crystallisation, so option A is correct. The other choices do not explain both observations.

A pH close to 7 indicates that the sodium chloride solution is approximately neutral, not that it is free of ions. In water, NaCl dissociates into mobile Na⁺ and Cl⁻ ions. These charged particles carry electric current through the solution. Thus a neutral solution may conduct electricity, making option A correct. Options B and C confuse acidity with ionic conductivity, while D contradicts the observation.

Electrical conduction in an aqueous solution requires mobile charged particles. Sugar dissolves as neutral molecules and does not ionise appreciably, so its solution has no sufficient ions to carry current. An ionic salt such as sodium chloride dissociates into Na⁺ and Cl⁻, which move toward opposite electrodes and conduct electricity. Therefore option A is correct; B, C, and D are chemically incorrect.

The chlor-alkali process electrolyses concentrated aqueous sodium chloride, or brine. At the cathode, water is reduced and hydrogen gas is released: 2H₂O + 2e⁻ → H₂ + 2OH⁻. At the anode, chloride ions are oxidised to chlorine: 2Cl⁻ → Cl₂ + 2e⁻. Sodium hydroxide remains in solution. Hence option A gives the correct cathode–anode order; option B reverses it.

Electrolysis of brine produces hydrogen at the cathode, chlorine at the anode, and sodium hydroxide in the solution. Hydrogen and chlorine can react explosively in the presence of suitable conditions, while chlorine can also react with sodium hydroxide to form hypochlorite or chlorate products. Separating the electrode products prevents contamination and unwanted reactions, so option A is correct. The other statements do not give the chemical reason.

On heating, sodium hydrogen carbonate decomposes as 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The carbon dioxide turns limewater milky, confirming the gas. The remaining solid, sodium carbonate, is a salt of a strong base and a weak acid; it hydrolyses in water and produces OH⁻ ions. Its solution is therefore basic, so option A is correct, not acidic or neutral.

Baking powder contains sodium hydrogen carbonate and a mild edible acid, often with a dry filler. In a dry container, the reactants remain largely separate and their reaction is very slow. When water is present in moist dough, the acid and bicarbonate dissolve and react, producing carbon dioxide. The gas bubbles expand during heating and make the dough rise. Thus option A is correct.

Dental enamel contains mineral crystals mainly based on calcium hydroxyapatite. Bacteria metabolise sugars and produce acids, lowering the pH in dental plaque. When the pH falls below about 5.5, the acidic environment favours dissolution of enamel minerals faster than repair can occur. This demineralisation increases tooth-decay risk, so option A is correct. The other options do not describe the acid–mineral process.

After food is eaten, bacteria in the mouth can break down sugars and produce acids. These acids lower the local pH and may dissolve minerals from tooth enamel, increasing the risk of decay. A mildly basic toothpaste reacts with and neutralises some of these acids, helping restore a safer pH. Therefore option A is correct. The other choices would damage teeth or describe chemically impossible effects.

The governing concept is acid–base neutralisation. Acidic soil contains excess hydrogen ions and therefore has a pH below 7. Slaked lime, calcium hydroxide [Ca(OH)₂], is a base that supplies hydroxide ions. These react with hydrogen ions to form water, reducing acidity and raising the soil pH. Thus option A is correct; the other options describe chemically impossible effects.

Highly basic soil has a pH above the range preferred by many crops. Organic matter decomposes through microbial activity and may release organic acids and carbon dioxide, producing a gradual moderating effect rather than an instant chemical change. This can improve nutrient availability and soil condition. Therefore option A is the best answer; the remaining choices are chemically incorrect or exaggerated.

Marble is mainly calcium carbonate, CaCO₃. The governing reaction is acid–carbonate neutralisation: an acid reacts with carbonate to produce a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Repeated contact dissolves or erodes the marble surface. Hence option A is correct, while the other statements contradict its composition or the nature of acid rain.

Curd contains lactic acid, and pickles commonly contain acids such as acetic acid or citric acid. The governing principle is that acids can react with reactive metals, including copper, especially during prolonged contact. Such reactions may form soluble copper compounds that contaminate food and can be harmful in excess. Therefore option A is correct; the other options deny acidity or describe impossible transformations.

This question uses indicator colour changes. Phenolphthalein is colourless in acidic or neutral solution but turns pink in a basic solution, so its pink colour is decisive. Methyl orange is yellow in a neutral or basic medium and red in an acidic medium, so yellow supports the same conclusion. Thus the solution is basic and option A is correct; it is not acidic, neutral, or insoluble.

Red methyl orange indicates an acidic medium, while colourless phenolphthalein is also consistent with an acidic solution. To neutralise an acid, a suitable base is added so that hydrogen ions react with hydroxide ions to form water: H⁺ + OH⁻ → H₂O. The quantity must be controlled to avoid making the mixture strongly basic. Therefore option A is correct; salt and sugar do not provide the required neutralisation.

Reddish-brown turmeric and blue red litmus are both tests for a basic medium. In aqueous school-level chemistry, basicity is associated with an excess of hydroxide ions, OH⁻, over hydrogen ions. Therefore hydroxide ion is the best answer. Hydrogen ions would indicate acidity, whereas chloride and sulphate ions may occur in salts but do not by themselves determine the observed indicator changes. Hence option A is correct.

The pH scale classifies values above 7 as basic, so both pH 12 and pH 10 represent basic solutions. A decrease from 12 to 10 means the solution has become less basic, often because of dilution or addition of an acidic substance. Since pH 10 is still above 7, it has not become neutral or acidic. Therefore option A is correct; hydroxide concentration has decreased rather than greatly increased.

The pH scale is logarithmic: pH = −log[H⁺]. A rise of one pH unit means the hydrogen-ion concentration, and therefore acidity, decreases by a factor of 10. The change from pH 3 to pH 6 is three units, so the decrease is 10 × 10 × 10 = 1000 times. Thus option A is correct; three, one hundred, and ten times do not match the three-unit logarithmic change.