Acids, Bases, and Salts

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 (NaCl) is formed from the strong acid hydrochloric acid (HCl) and the strong base sodium hydroxide (NaOH). Its ions do not undergo significant hydrolysis in water, so its aqueous solution is approximately neutral. In contrast, ammonium chloride solution is acidic because (NH₄⁺) 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.

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.