Acids, Bases, and Salts

Vinegar is an aqueous solution containing ethanoic acid, CH₃COOH. Acids provide hydrogen ions in water and change blue litmus to red, so the observed colour change is evidence of the acidic nature of ethanoic acid. Ethanol is not a base and does not cause this result. Carbon dioxide is not the relevant explanation for vinegar’s test, and blue litmus does not require heating to change in an acid. Thus, option A is correct.

The governing concept is the indicator test for acidic substances. Acids provide hydrogen ions in aqueous solution and turn blue litmus paper red. Therefore option A is correct. Blue is the unchanged colour of the paper, while green and yellow are not the standard litmus result for an acid. This test should be performed with a suitable sample and not by tasting an unknown substance.

The relevant concept is the litmus indicator test for bases. A basic solution changes red litmus paper to blue because its alkaline nature interacts with the indicator. Thus option B is correct. Red is the original colour of the test paper, while yellow and orange are not the characteristic result for a base in the standard litmus test. Indicators should be used safely rather than identifying an unknown chemical by tasting it.

A sour taste is a common characteristic associated with acids. For example, citric acid contributes to the sour taste of lemon, and acetic acid is present in vinegar. Therefore option A is the expected classification. However, tasting should never be used to identify an unknown laboratory substance because it may be poisonous or corrosive. Bases, metals, and many salts do not generally give the characteristic sour taste of acids.

The governing concept is the usual physical behaviour of bases. Many bases, including alkaline soap solutions, feel soapy or slippery because of their interaction with oils and skin. Therefore, option B is correct. Acids generally have a sour character, salts do not normally feel slippery, and noble gases are not tested by touch. Chemicals must never be identified by touching them.

Citric acid is the main organic acid in lemon juice and causes its sour taste. Acetic acid is mainly associated with vinegar, so it is not the correct answer. Lactic acid is found in fermented milk products such as curd. Exam tip: Remember citric acid by linking it with lemon, orange, and other citrus fruits.

The governing concept is neutralisation: an acid reacts with a base to form salt and water. Lemon juice contains citric acid, so its sour taste does not make it suitable for neutralising excess stomach hydrochloric acid. Magnesium hydroxide is a basic antacid and reacts with HCl, making option A correct. Water may dilute acid, while sodium chloride is not an antacid.

Bacterial fermentation of lactose in milk produces lactic acid. This acid is mainly responsible for the sour taste of curd. Citric acid occurs mainly in sour fruits such as lemon, so option B is incorrect. Exam tip: Remember the pair: curd—lactic acid; lemon—citric acid.

Citric acid is the principal organic acid in tomato, so option B is correct. Tomato may contain a small amount of oxalic acid, but it is not its main acid. Acetic acid is the characteristic acid of vinegar, while formic acid is associated with ant stings. Exam tip: Distinguish between an acid that is present in a substance and the acid that is present in the greatest amount.

Salt nature depends on the strengths of the acid and base from which the salt is formed. Sodium chloride results from the reaction of strong hydrochloric acid with strong sodium hydroxide: HCl + NaOH → NaCl + H₂O. Its aqueous solution is approximately neutral, so option A is correct. Sodium carbonate and sodium hydrogen carbonate are basic, while ammonium chloride is acidic.

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.