Acids, Bases, and Salts

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.

Direct answer: A, NaCl, is correct. Neutralisation is the reaction in which an acid reacts with a base to form salt and water. In this case, hydrochloric acid is a strong acid and sodium hydroxide is a strong base: HCl + NaOH → NaCl + H₂O. When NaCl dissolves, it produces Na⁺ and Cl⁻ ions. Neither ion undergoes significant hydrolysis in ordinary aqueous conditions, so the solution is generally neutral and has a pH close to 7. Option B, NH₄Cl, is not the expected neutral salt here because NH₄⁺ comes from the weak base ammonia and makes the solution generally acidic. Option C, Na₂CO₃, gives a basic solution because carbonate is the conjugate base of a weak acid. Option D, CH₃COONa, gives a basic solution because acetate is the conjugate base of weak acetic acid. Do not judge only by the word “salt”; identify the acid and base that produced it and consider hydrolysis. Memory cue: strong + strong → usually neutral.

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.

Vinegar is mainly a dilute aqueous solution of acetic acid, also called ethanoic acid, formed during the oxidation of ethanol by suitable microorganisms. Therefore, option A is correct. Citric acid is associated mainly with citrus fruits, lactic acid with sour milk and curd, and tartaric acid with tamarind and grapes. These distractors are common food acids but are not the principal acid in vinegar.

An ant sting contains methanoic acid (formic acid). Baking soda is a mild base, which helps neutralise this acid and can reduce the burning sensation. In contrast, vinegar, lemon juice, and orange juice are acidic, so they are not suitable choices for an acidic sting.

Sodium chloride \(\text{(NaCl)}\) is formed from the strong acid hydrochloric acid \(\text{(HCl)}\) and the strong base sodium hydroxide \(\text{(NaOH)}\). \(\text{Na}^+\) and \(\text{Cl}^-\) do not undergo significant hydrolysis in water, so the solution remains approximately neutral (\(\text{pH} \approx 7\)). In contrast, ammonium chloride is acidic because \(\text{NH}_4^+\) behaves as an acid in water.

Sodium hydroxide is a strong base. It dissolves in water to produce hydroxide ions, \(\mathrm{OH^-}\), so its aqueous solution is basic and has a pH greater than 7. Hydrochloric acid solution is acidic, whereas sodium chloride solution and distilled water are generally neutral.

Acidic soil has a pH below the desirable range for many crops. Slaked lime, calcium hydroxide, is a base that reacts with and neutralises excess acids in the soil, thereby raising its pH; option A is correct. Vinegar and lemon juice contain acids and would increase acidity. Common salt is not normally added as a neutralising agent and may harm soil quality in excess.

Highly basic soil has an excess of alkaline substances and needs an acidic or acid-forming amendment. Organic matter such as compost decomposes through microbial activity and produces organic acids, which gradually neutralise some alkalinity and improve soil condition. Sodium hydroxide and washing soda are alkaline, so they would increase alkalinity. Bleaching powder is not the suitable routine amendment for this purpose. Therefore, option A is correct.

The governing definition is that an alkali is a base that dissolves in water and produces hydroxide ions in solution. Thus, a water-soluble base is called an alkali, making option A correct. An acid produces hydrogen ions, while a salt is an ionic product that may or may not give a basic solution. A metal is an element, not the name for a soluble base. Therefore, the other options do not match the definition.

A solution made by dissolving sodium chloride
(
\mathrm{NaCl}
)
in water is called brine. Brine is used in the chlor-alkali process. Lime water is an aqueous solution of calcium hydroxide, whereas hard water contains mainly calcium and magnesium salts.

Most metal oxides are basic. They react with acids to form a salt and water. For example, magnesium oxide reacts with an acid to form a magnesium salt and water. Some oxides, such as zinc oxide and aluminium oxide, are amphoteric exceptions because they react with both acids and bases.

The pH scale shows that a value below 7 is acidic; therefore, pH 5 indicates acidic soil. To reduce acidity, a suitable base is added so that neutralisation occurs and the pH rises. Calcium hydroxide is a basic compound and can neutralise excess acids in soil, so option C is correct. Hydrochloric acid would increase acidity, while ammonium chloride is an acidic salt. Sodium chloride is essentially neutral and is not the appropriate corrective material.