Acids, Bases, and Salts

The governing concept is neutralisation. Excess hydrochloric acid in the stomach can irritate the digestive system and contribute to indigestion. A mild base, such as the active ingredient in a suitable antacid, reacts with some of the excess acid and reduces its acidity, often producing salt and water. It does not create more acid, form metal, or convert water into gas. Therefore, the neutralisation explanation in option A is correct.

The governing concept is pH and the action of acids on tooth enamel. Bacteria use sugars and food particles to produce acids, lowering the mouth’s pH. When the pH becomes sufficiently low, the mineral-rich enamel begins to lose minerals and becomes weaker, increasing decay. Therefore, option A is correct. Bases do not immediately dissolve teeth, salt does not make teeth metallic, and ordinary water is not the main cause.

The governing concept is neutralisation. Bacteria can produce acids from food residues, and these acids attack tooth enamel. A basic toothpaste reacts with or neutralises some of this excess acid, helping the mouth move toward a less acidic pH and protecting enamel. Thus option A is correct. Toothpaste does not turn teeth into acid, remove all oral water, or fill teeth with carbon dioxide.

The governing concept is acid–base neutralisation in soil treatment. Slaked lime, Ca(OH)₂, is a basic substance. When it is mixed with highly acidic soil, it reacts with excess acidic components and reduces their effect, bringing the soil closer to a suitable pH for plant growth. Therefore option A is correct. The treatment does not increase acidity, manufacture chlorine, or convert soil into salt.

The governing concept is correction of soil pH through natural chemical changes. Organic manure decomposes through microbial activity and may produce organic acids and other mildly acidic substances. These can react with or lessen excess basicity, improving conditions for plant growth. Hence option A is correct. Manure does not produce sodium metal, remove every drop of water, or transform soil into glass; those claims are unrelated to decomposition.

The governing concept is the reaction of a strong base with an acidic oxide. Sodium hydroxide absorbs carbon dioxide from air and can form sodium carbonate, for example: 2NaOH + CO₂ → Na₂CO₃ + H₂O. This consumes some NaOH, so less active base remains and its measured strength may decrease. Option A is correct; the other choices describe impossible or irrelevant changes.

The governing concept is the colour change of acid–base indicators. Red litmus or a comparable red indicator turns blue in a basic medium. Sodium carbonate is a salt of a strong base and a weak acid; its carbonate ions hydrolyse in water and generate hydroxide ions, so the solution is basic. Therefore option A is correct. It is not strongly acidic, ion-free, or pure water.

The governing concept is the nature of salts formed by neutralisation. Sodium chloride can be obtained from hydrochloric acid and sodium hydroxide: HCl + NaOH → NaCl + H₂O. Both parent substances are strong, and Na⁺ and Cl⁻ do not significantly hydrolyse water, so the aqueous solution is generally near neutral. Thus option A is correct; the other descriptions do not explain its pH.

The governing concept is salt hydrolysis. Ammonium chloride is formed from strong hydrochloric acid and the weak base ammonium hydroxide. In water, NH₄⁺ reacts partially with water: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺. The hydronium ions make the solution acidic, so option A is correct. It is not a pure base, and the salt does ionise in water.

The governing concept is the definite crystal structure of hydrated salts. Some salts incorporate a fixed number of water molecules into their crystal lattice; this chemically associated water is called water of crystallisation. For example, CuSO₄·5H₂O contains five water molecules per formula unit. Therefore option A is correct. Surface moisture, water in every acid, and dissolved gas are not water of crystallisation.

The governing concept is dehydration of a hydrated salt. Blue copper sulphate is CuSO₄·5H₂O, whose blue colour is associated with its water of crystallisation. On heating, this water is removed and anhydrous CuSO₄ forms, which is white: CuSO₄·5H₂O → CuSO₄ + 5H₂O. Hence option A is correct; copper does not become gold and sulphate does not turn into oxygen.

The governing concept is water of crystallisation. Anhydrous copper sulphate, CuSO4, is white because this water has been removed. When water is added, hydrated copper sulphate, CuSO4·5H2O, forms and the characteristic blue colour returns. Therefore option A correctly links the observed colour change with the presence of water; the other options do not describe this reversible hydration.

Washing soda is sodium carbonate decahydrate, Na2CO3·10H2O, and its aqueous solution is alkaline. Alkaline substances help loosen acidic or greasy dirt, while carbonate ions can assist cleaning by reacting with some contaminants. Thus option A is correct. It is not a strong acid, it is definitely a salt, and fragrance is not its cleaning function.

Hard water contains dissolved calcium and magnesium ions. Washing soda supplies carbonate ions, which react with them to form sparingly soluble calcium carbonate and magnesium carbonate precipitates. Removing these ions reduces hardness and allows soap to lather more readily. Therefore option A is correct; the other choices describe unrelated changes that do not soften water.

Baking soda is sodium hydrogen carbonate, NaHCO3. On heating, it undergoes thermal decomposition: 2NaHCO3 → Na2CO3 + CO2 + H2O. The products are therefore sodium carbonate, carbon dioxide and water, exactly as stated in option A. The other options contain substances that are not formed in this decomposition and do not balance the reaction.

Baking powder contains baking soda and a mild edible acid. When the mixture becomes moist and is heated, the acid reacts with sodium hydrogen carbonate and releases carbon dioxide. The gas forms bubbles that expand during baking, making the dough or batter rise. Hence option A is correct; the reaction does not produce sodium metal or chlorine gas.

Baking powder supplies the acid–base reaction that releases carbon dioxide when the batter is heated. The gas becomes trapped in small bubbles, and their expansion gives the cake a lighter, raised structure. If too little baking powder is used, less carbon dioxide is generated and fewer bubbles expand. Therefore option A is correct; the other outcomes are unrelated.

Bleaching powder is used for its oxidising and bleaching action. The active chlorine released from it can oxidise coloured substances, converting them into less-coloured or colourless products. The same action that removes stains can also fade dyes in coloured fabric, so careful use is necessary. Thus option A is correct; the other options do not describe bleaching powder’s chemical action.

Bleaching powder is not perfectly stable during storage. Contact with atmospheric moisture and carbon dioxide can cause gradual decomposition and reduce the amount of available chlorine, which is responsible for much of its bleaching and disinfecting action. Keeping it in a tightly closed, dry container slows this deterioration. Therefore option A is correct; the other statements are chemically irrelevant.

Bleaching powder releases available chlorine when used appropriately in water. Chlorine and its active species damage essential components of many disease-causing microorganisms, thereby reducing contamination and helping disinfect the water. Thus option A correctly explains its use. It does not sweeten water, convert it into oil, or produce sodium metal; those alternatives contradict its known chemical action.