Acids, Bases, and Salts

An acid ionises in water and forms mobile ions. For example, HCl forms \(\mathrm{H_3O^+}\) and \(\mathrm{Cl^-}\) ions in water. These ions carry electric charge through the solution, so an acidic solution conducts electricity. In contrast, conduction by free electrons occurs mainly in metals, not in aqueous solutions.

Sodium chloride is formed from hydrochloric acid, a strong acid, and sodium hydroxide, a strong base. Their ions do not undergo significant hydrolysis in water, so sodium chloride produces an approximately neutral solution; option A is correct. Hydrochloric acid is acidic, sodium hydroxide is basic, and ammonium chloride generally gives an acidic solution because of the ammonium ion.

Diluting an acid with water is an exothermic process, so heat is released. Acid is therefore added to water slowly in small portions while stirring, allowing the heat to disperse safely. If water is added to concentrated acid, the small amount of water may heat suddenly and splash; therefore, option B is unsafe.

When an acid is added slowly to water, its ions become hydrated and heat is released. Therefore, dilution of an acid is an exothermic process. An endothermic process absorbs heat, whereas heat is released here. This is why acid is added to water, not water to acid, for safety.

Acid rain has a pH below 5.6. Normal rain can have a pH of about 5.6 because dissolved carbon dioxide forms a weak acid. Acids formed from sulphur dioxide and nitrogen oxides lower the pH further. A pH of 7 is neutral, so option B does not represent acid rain.

A pH below 5.5 indicates an acidic condition in which acids can dissolve or weaken tooth enamel. A mildly basic toothpaste helps neutralise some of these acids and raises the pH toward a safer value, so option A is correct. Strongly acidic toothpaste would worsen the condition, distilled water has little neutralising ability, and concentrated salt solution is not the appropriate treatment.

Hydrogen chloride shows acidic behaviour only after dissolving in water, when it ionises to form \(H_3O^+\) ions. With dry gas and dry litmus paper, water is absent; therefore, blue litmus does not turn red. Option B is incorrect because blue litmus turns red in an acidic solution.

Litmus is a natural acid–base indicator. Its governing colour rule is that an acidic solution changes blue litmus to red. Therefore, option A is correct. Blue litmus generally remains blue in a neutral or basic solution, while green and yellow are not the standard results for this indicator test. The colour change provides evidence that hydrogen ions are present in the solution.

Sodium carbonate, or washing soda
\(\mathrm{Na_2CO_3\cdot10H_2O}\), precipitates calcium and magnesium ions present in water as insoluble carbonates. Therefore, it is used to remove permanent hardness. Sodium hydrogen carbonate is not effective for this purpose.

Excess HCl in the stomach can cause acidity and irritation. An antacid contains a mild base that neutralises HCl to form salt and water: \(\text{acid} + \text{base} \rightarrow \text{salt} + \text{water}\). Therefore, an antacid is not acidic; an acidic substance could increase rather than reduce the acidity caused by HCl.

When gypsum, \(CaSO_4\cdot 2H_2O\), is heated to about 373 K, it loses part of its water of crystallisation and forms plaster of Paris, \(CaSO_4\cdot \tfrac{1}{2}H_2O\). Therefore, gypsum is correct. Heating limestone mainly produces calcium oxide (lime), not plaster of Paris.

Plaster of Paris, calcium sulfate hemihydrate \,\(\mathrm{CaSO_4\cdot\tfrac{1}{2}H_2O}\), reacts with water to form gypsum, calcium sulfate dihydrate \,\(\mathrm{CaSO_4\cdot2H_2O}\), and hardens. Common salt is sodium chloride, so it is not formed in this reaction.

Bleaching powder, commonly represented at school level as calcium oxychloride, releases chlorine or active chlorine in suitable conditions. This chlorine destroys many harmful microorganisms, so bleaching powder is commonly used to disinfect drinking water; option A is correct. Cooking food, smelting metals, and extracting edible oil require entirely different materials or processes and are not its standard uses.

In the chlor-alkali process, concentrated aqueous sodium chloride solution, called brine, is electrolysed. It produces chlorine gas, hydrogen gas, and sodium hydroxide. An aqueous calcium hydroxide solution is lime water; it is not the reactant used in the chlor-alkali process.

Dissolving concentrated sulphuric acid in water is exothermic. Adding the acid slowly to a larger volume of water while stirring lets the heat disperse, reducing sudden boiling and splashing. In contrast, adding water to acid can rapidly heat the small layer of water and cause it to splash.

NaCl is formed by neutralisation of the strong acid HCl and the strong base NaOH: \(\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}\).
The ions \(\mathrm{Na^+}\) and \(\mathrm{Cl^-}\) do not undergo significant hydrolysis in water, so an aqueous NaCl solution is generally neutral. In contrast, \(\mathrm{NH_4Cl}\) gives an acidic solution, whereas \(\mathrm{Na_2CO_3}\) and \(\mathrm{CH_3COONa}\) give basic solutions.

Acids produced by bacteria in the mouth can affect tooth enamel. When the pH falls below 5.5, demineralisation of enamel may begin, increasing the risk of tooth decay. A pH of 7 is neutral, whereas a pH below 5.5 is sufficiently acidic to harm enamel.

Toothpaste is generally mildly basic, meaning it can react with and neutralise some acids produced in the mouth by bacterial activity. This reduces acidity and helps protect tooth enamel, so option B is correct. An acidic toothpaste would add to the harmful acidity, while a neutral substance would not provide the same acid-neutralising effect. “Salty” describes taste, not the relevant chemical nature.

An antacid is a mildly basic substance or preparation that reacts with and neutralises excess hydrochloric acid in the stomach. By reducing the acid concentration, it provides relief from acidity; therefore, option A is correct. An antibiotic is used against certain bacterial infections, a vitamin supplies nutrients, and a painkiller reduces pain. None of these alternatives primarily neutralises stomach acid.

A nettle sting contains methanoic acid (formic acid), which causes irritation. Baking soda solution is mildly basic and helps neutralise the acid, so it can reduce the irritation. Vinegar and lemon juice are acidic and therefore do not neutralise the acidic effect. Common salt solution is generally neutral and is not suitable for neutralising this acid.