Acids, Bases, and Salts

Baking powder contains sodium hydrogencarbonate together with a suitable edible acid. In the dry mixture, the particles have limited contact and ions cannot move freely, so the acid–hydrogencarbonate reaction is slow. Moisture dissolves the ingredients and permits ion movement and effective contact; CO₂ is then released, causing dough to rise. Therefore option A gives the best explanation, while moisture does not prevent CO₂.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and produces plaster of Paris, calcium sulphate hemihydrate, CaSO₄·½H₂O. If heating is excessive, more water can be removed and anhydrous calcium sulphate may form. That material does not hydrate and set as effectively, so the plaster quality falls. Hence option A is correct.

Indicator colour ranges provide the governing evidence. Phenolphthalein is colourless in acidic or neutral conditions but becomes pink in a basic solution. Methyl orange is red in acidic medium and yellow in neutral to alkaline medium. Since both observed colours are consistent with a basic medium, the solution is classified as basic. A neutral solution would not make phenolphthalein pink, so option B is the unambiguous answer.

Neutralisation is governed by the amounts and effective concentrations of H⁺ and OH⁻, not simply by the volumes poured. Equal volumes can contain unequal mole amounts if the solutions have different concentrations, and a weak base may also provide fewer effective OH⁻ ions than a strong acid provides H⁺ ions. If H⁺ remains in excess after reaction, the final solution is acidic. Thus option A is correct.

Both observations indicate a basic medium. Turmeric changes to a reddish-brown colour in the presence of a base, and red litmus turns blue when the solution is basic. In the Arrhenius description, basicity in aqueous solution is associated with an excess of hydroxide ions, OH⁻, relative to hydrogen ions. Chloride and sulphate ions may be present in salts but do not alone explain these indicator changes. Therefore option B is correct.

Sodium hydroxide is a strong base, while carbon dioxide is an acidic oxide present in air. When solid sodium hydroxide is exposed to air, it absorbs carbon dioxide and reacts with it: 2NaOH + CO₂ → Na₂CO₃ + H₂O. Thus sodium carbonate is formed. Nitrogen and oxygen do not produce this salt, and water does not become an acid in this process, so option A is correct.

The pH scale identifies solutions with pH above 7 as basic. Both 13 and 11 are above 7, so the solution remains basic after dilution. Adding water lowers the concentration of hydroxide ions and reduces alkalinity, causing the pH to move toward 7. It does not make the solution acidic or exactly neutral, and hydroxide concentration does not increase. Therefore option A is correct.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On strong heating it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The evolved carbon dioxide turns limewater milky because calcium carbonate forms, while the solid residue is sodium carbonate. Sodium chloride, calcium sulphate, and ammonium chloride are not products of this decomposition, so option A is correct.

Acid rain introduces acidic substances into the lake and lowers its pH. Lime is a basic material, commonly represented by calcium oxide or calcium hydroxide in this context. It reacts with acidic substances and reduces their acidity, thereby raising the pH toward a less acidic value. This acid–base reaction is neutralisation, not evaporation, condensation, or decomposition. Hence option A is correct.

An acidic solution contains a relatively high concentration of hydrogen or hydronium ions. Added hydroxide ions react with them in the neutralisation step: H⁺ + OH⁻ → H₂O. This lowers the hydrogen-ion concentration. Since pH = −log[H⁺], a decrease in [H⁺] produces an increase in pH. The ions do not increase acidity or form carbon dioxide, so option A is correct.

The bleaching action of bleaching powder depends mainly on its available or active chlorine, which produces oxidising species in the presence of moisture. In a moist, open environment, the substance can slowly react with atmospheric moisture and carbon dioxide and lose available chlorine. Its oxidising capacity therefore decreases during storage. It does not turn into sodium metal, gypsum, or acid rain, so option A is correct.

A red litmus paper turns blue only when a basic medium is produced. The absence of an effect on dry paper shows that moisture is necessary for the gas to dissolve and generate the basic species or hydroxide ions responsible for the colour change. Ammonia is a familiar example: NH₃ dissolves in water and produces an alkaline solution. Thus option A is the correct conclusion.

A pH of 7 indicates a neutral solution, but neutrality does not mean that no ions are present. Sodium chloride dissociates in water into Na⁺ and Cl⁻ ions, allowing the solution to conduct electricity while remaining approximately neutral. Sugar and alcohol generally dissolve as molecules and do not provide many mobile ions. Pure distilled water has extremely weak conductivity, so option A is the best answer.

Marble consists mainly of calcium carbonate, CaCO₃. Acids in acid rain react with this carbonate to form a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Repeated reactions remove small amounts of the carbonate surface, so the statue gradually loses material and detail. Therefore option A correctly explains the corrosion.

The pH scale is logarithmic, so a rise of one pH unit means that the hydrogen-ion concentration, which represents acidity, decreases by a factor of 10. Here the pH rises from 3 to 5, a change of two units. Therefore, the hydrogen-ion concentration becomes 10 × 10 = 100 times smaller. Thus option A is correct; the other choices ignore the logarithmic nature of pH.

Bacteria in the mouth can break down sugars and produce acids. These acids lower the pH around the teeth and gradually dissolve minerals from enamel, causing decay. A mildly basic toothpaste reacts with and neutralises some of the acid, helping the mouth return toward a safer pH and protecting enamel. Therefore option A is correct; the other options would either damage enamel or describe an incorrect effect.

A metal such as magnesium reacts with a dilute acid by displacing hydrogen: Mg + 2HCl → MgCl₂ + H₂. A carbonate reacts with an acid to produce a salt, water, and carbon dioxide: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Hence magnesium gives hydrogen while calcium carbonate gives carbon dioxide, so option B is correct.

A base is a substance that reacts with an acid in a neutralisation reaction to form salt and water. Bases need not be soluble in water; for example, copper(II) oxide is a base but is insoluble in water. An alkali is specifically a base that dissolves in water. Therefore the described substance is a base, making option B correct; an indicator only changes colour and an acid does not fit the reaction described.

Methyl orange is red in a sufficiently acidic medium and yellow in a neutral or alkaline medium, with a transition range between them. Adding a base consumes some hydrogen ions and shifts the solution away from strong acidity. The yellow colour therefore indicates movement toward a less acidic, neutral, or basic condition. Option A is correct; the observation does not prove that a metal formed or that no chemical change occurred.

A pH near 7 indicates that the solution is approximately neutral, but neutrality does not mean that no ions are present. Sodium chloride dissociates in water into mobile Na⁺ and Cl⁻ ions. These charged particles carry electric current through the solution. Thus option A is correct. Options B and C wrongly restrict conductivity to acidic solutions, while option D confuses a neutral salt with an acid.