Acids, Bases, and Salts

The governing concept is acid-base neutralisation and the pH scale. Acidic soil has excess acidity, so a suitable basic substance is added to react with and reduce the acid concentration. Slaked lime, Ca(OH)₂, is a base and therefore raises the soil pH; option A is correct. Hydrochloric acid, vinegar, and lemon juice are acidic and would lower the pH further.

An indicator is a substance that gives a characteristic colour change according to whether a solution is acidic or basic. Litmus is a natural indicator: blue litmus turns red in acid, and red litmus turns blue in a base. Thus option A is correct. Sodium chloride solution, glucose solution, and distilled water do not normally show this acid-base colour change.

An acid reacts with a metal carbonate to produce a salt, water and carbon dioxide. For example, \(\mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2}\). Hence, carbon dioxide is the correct answer. Hydrogen is generally evolved when an acid reacts with a metal such as zinc, not with a carbonate. Exam tip: Remember the pattern: acid + carbonate gives salt + water + \(\mathrm{CO_2}\).

\(\mathrm{NaOH}\) dissociates in water to give \(\mathrm{Na^+}\) and \(\mathrm{OH^-}\) ions, so its solution is basic. \(\mathrm{HCl}\) and \(\mathrm{CH_3COOH}\) are acids, while \(\mathrm{NaCl}\) is generally neutral. Exam tip: \(\mathrm{OH^-}\) indicates a base.

An acid-base reaction is called a neutralisation reaction. H⁺ ions from the acid combine with OH⁻ ions from the base to form water, and the remaining ions form a salt. For example, HCl + NaOH → NaCl + H₂O. Only water is not formed because a salt is also produced. Exam tip: Remember: acid + base → salt + water.

The governing rule is that neutralisation occurs when an acid reacts with a base. The hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt. For example, HCl + NaOH → NaCl + H₂O. Therefore, option A is correct; the other choices do not represent the general products of neutralisation.

The concept involves the colour behaviour of litmus during acid-base neutralisation. Initially, the acidic solution turns blue litmus red. As sodium hydroxide is added, OH⁻ ions remove H⁺ ions, so acidity decreases. Near the neutral point, both red and blue litmus colours are effectively present and the paper appears purple, making option C correct. Excess NaOH would make it blue.

Antacids are basic substances that neutralise excess hydrochloric acid in the stomach. An acid-base reaction generally produces salt and water; for example, \(Mg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O\). Hence, this is a neutralisation reaction. In a displacement reaction, one element replaces another element from a compound, which does not happen here. Exam tip: acid + base producing salt + water indicates neutralisation.

An antacid works through acid-base neutralisation, so it must be a mild base that can react with excess hydrochloric acid. Sodium hydrogen carbonate, NaHCO₃, is a commonly taught antacid and reacts with HCl to produce salt, water, and carbon dioxide. Hence option B is the intended answer. Calcium carbonate can also neutralise acid, but the question’s standard expected example is sodium hydrogen carbonate.

The governing concept is the pH scale, which is inversely related to hydrogen-ion concentration: pH = −log[H⁺]. A more acidic solution has a greater concentration of H⁺ ions and therefore a smaller pH value. Thus the most acidic solution has a very low pH, so option A is correct. A pH of seven is neutral, while a high pH indicates a basic solution.

The governing concept is the pH scale. As basicity increases, the concentration of hydrogen ions decreases and the pH value rises above seven. Therefore, the most basic solution has a very high pH, making option B correct. A very low pH indicates strong acidity, pH seven is approximately neutral, and pH one is strongly acidic rather than basic.

The governing concept is neutralisation. Lime, commonly supplied as calcium oxide or calcium hydroxide, is basic, whereas acidic soil contains excess acidic substances. The base reacts with and neutralises that acidity, so the soil pH rises towards a more suitable range for plant growth. Option A is therefore correct. Options B, C and D describe effects that are not the purpose of liming acidic soil.

The governing concept is acid–base neutralisation in the stomach. Excess hydrochloric acid can cause acidity and discomfort. An antacid is generally a mild base, such as magnesium hydroxide, so it reacts with and neutralises some of the excess acid without being strongly corrosive. Hence option A is correct. An acid would increase acidity, while an ordinary neutral salt or a metal is not the standard substance used for this relief.

Milk of magnesia is a suspension of magnesium hydroxide, Mg(OH)₂, which is a weak base. The governing concept is neutralisation: magnesium hydroxide reacts with excess hydrochloric acid in the stomach and reduces its acidity, so it works as an antacid. Therefore option A is correct. Rusting iron, acidifying water and producing litmus are unrelated to the normal function of milk of magnesia.

The governing concept is the effect of pH on tooth enamel. Bacteria in the mouth can produce acids, and when the mouth becomes sufficiently acidic, enamel begins to dissolve. Tooth decay may start when the pH falls below about 5.5. Thus option A is correct. A pH of 7 is neutral, while pH 10 and pH 14 are alkaline; these values do not represent the acidic condition associated with enamel damage in this question.

The governing concept is neutralisation of acids by bases. Acids produced by bacteria in the mouth can attack tooth enamel. Toothpaste is generally mildly basic, so it helps neutralise or reduce some of this acidity during brushing. Therefore option A is correct. Strongly acidic toothpaste would worsen the chemical attack, whereas “neutral gas” is not a suitable description of toothpaste and metallic refers to a material property unrelated to its cleaning role.

The governing concept is salt hydrolysis and the strength of the parent acid and base. NaCl is formed from the strong acid HCl and the strong base NaOH. Its Na⁺ and Cl⁻ ions undergo negligible hydrolysis in water, so the solution is approximately neutral; therefore A is correct. Sodium acetate is basic, ammonium chloride is acidic, and ammonium acetate is approximately neutral only under a more detailed comparison of weak-acid and weak-base strengths, making NaCl the clear school-level choice.

An olfactory indicator is a substance whose smell changes in an acidic or basic medium. Onion is a standard example: its characteristic smell is altered or becomes difficult to detect in a basic solution, so option A is correct. Turmeric, litmus and phenolphthalein are mainly visual indicators because they show colour changes rather than a smell-based change. The distinguishing clue is the word “olfactory,” which relates to the sense of smell.

The governing concept is ionisation in aqueous solution. Acidic properties are produced when an acid dissolves in water and forms ions, especially hydrogen ions represented as H⁺ or hydronium ions, H₃O⁺. These ions allow the solution to show acidity and conduct electricity. Hence option A is correct. Dissolving an acid is not primarily done to remove colour, form a metal or stop a gas; those choices do not explain acidic behaviour.

The governing concept is that HCl shows acidic behaviour in the presence of water. Dry hydrogen chloride molecules do not produce hydrogen or hydronium ions on dry litmus paper because water is absent. Without those ions, the acid–base indicator cannot undergo its characteristic colour change, so option A is correct. HCl is not basic; the composition of litmus and the colour of the gas do not explain the absence of the reaction.