Acids, Bases, and Salts

The governing concept is the formation of acids during food spoilage or fermentation. When microorganisms act on lactose in milk, lactic acid is produced, so the pH falls and the milk develops a sour taste. Therefore option A, lactic acid, is correct. Citric acid is characteristic of citrus fruits, whereas sulphuric acid and nitric acid are strong mineral acids and are not formed during ordinary souring of milk. The change also illustrates a chemical change.

The governing concept is the occurrence of natural acids in foods. Oranges are citrus fruits, and their characteristic acidity is mainly due to citric acid. Thus option A, citric acid, is correct. Lactic acid is associated with sour milk and fermentation, tartaric acid is commonly associated with tamarind and grapes, and oxalic acid occurs in some leafy plants. The word “mainly” recognizes that fruit juice can contain several acids, but citric acid is the expected principal example.

The governing concept is the identification of acids naturally present in common foods. Tamarind has a distinctly sour taste, and tartaric acid is the characteristic acid commonly associated with it. Therefore option A is correct. Lactic acid is linked with sour milk, hydrochloric acid is present in gastric juice rather than tamarind, and carbonic acid forms when carbon dioxide dissolves in water. The question asks for the usual school-level association, not every minor component of the fruit.

The governing concept is the presence and neutralisation of acids in everyday situations. An ant sting injects an acidic secretion containing methanoic acid, also called formic acid, which causes irritation and a burning sensation. Therefore option A is correct. Citric acid is associated with citrus fruits, lactic acid with sour milk, and tartaric acid with tamarind and some fruits. A mild base can help neutralise the acid, but the sting should not be treated with unsafe chemicals.

The governing concept is acid-base neutralisation. An ant sting contains an acidic secretion, commonly associated with methanoic acid. Baking soda, or sodium hydrogen carbonate, is mildly basic; when applied appropriately, it can partially neutralise the acid and reduce irritation. Therefore option A is correct. Lemon juice, vinegar, and tamarind juice are acidic, so they would not provide the required neutralising effect. Any severe reaction requires medical attention rather than home treatment.

The governing concept is the use of indicators in acid-base testing. An indicator is a substance that produces a characteristic colour change, or another observable change, in acidic and basic media. Therefore option A, indicator, is correct. A solvent dissolves another substance, a metal is an element or alloy with metallic properties, and a fuel releases energy on burning; none of these definitions specifically describes acid-base identification. Litmus is a familiar example of an indicator.

The governing concept is the origin of natural acid-base indicators. Litmus is a dye mixture obtained from lichens, organisms formed through a close association between a fungus and a photosynthetic partner. It is processed into litmus paper, which changes colour in acidic and basic solutions. Therefore option A, lichens, is correct. Rose petals can provide other natural colour indicators, but they are not the standard source of litmus; neem and wheat are unrelated to litmus production.

The governing concept is the colour response of natural indicators. Turmeric contains curcumin, which remains yellow in neutral or acidic conditions but changes to a reddish-brown colour in a basic medium. Therefore option A is correct. Blue is associated with the response of blue or red litmus in particular conditions, while green and white are not the characteristic turmeric response to a base. This property can demonstrate the alkalinity of substances such as soap, but unknown chemicals should not be tested by direct contact.

Phenolphthalein is an acid–base indicator whose colour depends on the nature of the solution. In an acidic or neutral medium it is generally colourless, but in a basic medium it changes to pink; therefore option A is correct. Colourless is the acidic-side observation, while blue and yellow are not the characteristic phenolphthalein colour in a basic solution.

Methyl orange is an acid–base indicator. Its colour changes according to pH: it is red in an acidic solution and yellow in an alkaline solution, with an intermediate transition range. Since the question specifies an acidic medium, option A, red, is correct. Yellow describes its basic-side colour, while green and blue are not the standard observations for methyl orange.

An olfactory indicator is a substance whose smell changes or becomes difficult to detect in an acidic or basic medium. Onion is a commonly taught example because its odour can change in the presence of bases. Therefore option A is correct. Sugar, common salt, and sand do not function as standard olfactory indicators and are included as unrelated distractors.

The general reaction of a suitable metal with a dilute acid is: metal + acid → salt + hydrogen gas. For example, zinc reacts with hydrochloric acid to form zinc chloride and hydrogen. Thus option A is correct. Oxygen and nitrogen are not normally released in this reaction, while carbon dioxide is characteristic of acid–carbonate reactions rather than ordinary acid–metal reactions.

Hydrogen is tested by bringing a lighted splint near the gas, using proper laboratory safety. Hydrogen burns rapidly with oxygen and produces a characteristic ‘pop’ sound, so option A is correct. A bell or whistle is unrelated to this chemical test, and saying that no sound is produced would miss the identifying observation. The test follows hydrogen’s combustion property.

Zinc reacts with dilute hydrochloric acid according to the balanced equation: Zn + 2HCl → ZnCl₂ + H₂. The gaseous product is therefore hydrogen, making option A correct; zinc chloride remains in solution. Oxygen is not released, chlorine is not produced because hydrochloric acid supplies chloride ions, and ammonia is unrelated to this acid–metal reaction.

Acids react with metal carbonates to form a salt, water, and carbon dioxide: acid + carbonate → salt + H₂O + CO₂. For example, hydrochloric acid and calcium carbonate produce calcium chloride, water, and carbon dioxide. Therefore option A is correct. Hydrogen is associated with many acid–metal reactions, whereas oxygen and nitrogen are not products here.

Lime water is a dilute solution of calcium hydroxide. When carbon dioxide is passed through it, the reaction CO₂ + Ca(OH)₂ → CaCO₃ + H₂O forms insoluble calcium carbonate, which makes the solution appear milky. Hence option A is correct. Blue, red, and black are not the characteristic result of the standard carbon-dioxide test.

The reaction between an acid and a base is called neutralisation because the acidic and basic effects oppose and reduce each other. In a typical case, HCl + NaOH → NaCl + H₂O, producing salt and water. Therefore option A is correct. Combustion involves burning, evaporation is a physical change of state, and corrosion is the gradual deterioration of materials.

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. Thus option A is correct. Neutralisation generally gives both salt and water, not only water or the other listed combinations.

The stomach releases hydrochloric acid, or HCl, as part of gastric juice. It creates a strongly acidic medium that helps activate pepsin for protein digestion and also helps destroy many harmful microorganisms entering with food. Citric acid is found mainly in citrus fruits, lactic acid is associated with muscles and fermented foods, and tartaric acid occurs in tamarind and grapes; therefore option A is correct.

Acidity commonly results when excess hydrochloric acid is present in the stomach. An antacid is a mildly basic substance, such as a suitable hydroxide or carbonate preparation, that reacts with and neutralizes some of this excess acid. This reduces irritation and provides relief. Vinegar and lemon juice are acidic, while salt water is not a standard treatment; therefore option A is correct.