Acids, Bases, and Salts

The governing concept is the acid–base nature of oxides. Most metal oxides are basic because they react with acids to form salt and water; for example, CuO + 2HCl → CuCl₂ + H₂O. This neutralisation behaviour supports option B. Although some metal oxides may be amphoteric or neutral, the general school-level classification is basic, not acidic or sweet.

The governing concept is the classification of oxides by acid–base behaviour. Many non-metal oxides react with water to form acids; for example, CO₂ + H₂O ⇌ H₂CO₃. Their aqueous solutions can turn blue litmus red, supporting option B. The word “general” is important because a few oxides may be neutral or show special behaviour, but non-metal oxides are commonly acidic.

Turmeric is a natural acid–base indicator. Its yellow pigment changes to reddish-brown when it comes in contact with a basic or alkaline substance. Therefore, option B is correct. Acidic and neutral substances generally do not produce this characteristic reddish-brown change, while sugar is not classified as the required alkaline substance in this test.

Phenolphthalein is an acid–base indicator whose colour depends on the nature of the solution. It remains colourless in an acidic medium but changes to pink, or pinkish-red in stronger alkaline conditions, in a basic medium. Thus option C is the standard school-level answer. Red, green, and colourless do not represent its usual basic-solution colour.

Methyl orange is an acid–base indicator used to distinguish acidic and basic media. In an acidic solution, it appears red; in a basic solution, it appears yellow, with orange shades occurring during the transition range. Therefore option B is correct. Yellow describes the basic condition, while blue and green are not its characteristic colours in this test.

Methyl orange changes colour according to the acidity or alkalinity of a solution. It is red in an acidic medium and yellow in a basic medium. Consequently, option B is correct for a basic solution. Red is the plausible distractor because it belongs to the acidic condition, whereas black and white are not the standard colours shown by methyl orange in this indicator test.

Baking soda is the common name of sodium hydrogen carbonate, with the formula NaHCO₃. It is a basic salt and reacts with acids to release carbon dioxide, which explains several of its household uses. Therefore option B is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium sulphate is a different salt.

Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The term decahydrate indicates that ten water molecules are associated with each formula unit as water of crystallisation. Therefore option A is correct. Sodium hydrogen carbonate is baking soda, calcium oxychloride is bleaching powder, and sodium chloride is common salt, so each distractor names a different compound.

Common salt is the familiar crystalline ionic compound sodium chloride, with the formula NaCl. It consists of sodium ions and chloride ions in a regular lattice and is widely used in food. Sodium carbonate, calcium sulphate, and sodium hydroxide are separate compounds with different formulas and properties. Thus option A gives the correct chemical name.

Litmus is a natural acid-base indicator. In an acidic solution, the indicator changes from blue to red because the acidic medium affects the litmus dye. Therefore option A is correct. Blue litmus remains blue in a neutral solution and generally does not show this red change in a basic solution. Although other indicators may display different colours, the characteristic blue-litmus test for an acid is red.

Litmus helps distinguish acidic and basic solutions. A basic solution changes red litmus paper to blue because the alkaline medium produces the characteristic response of the litmus indicator. Hence option B is correct. Red litmus does not remain the expected test colour in a base, and the change is not to white or black. This test should be performed with a suitable indicator strip rather than by touching or tasting an unknown chemical.

A sour taste is a common property associated with acids because acidic substances release hydrogen ions in aqueous solution. For example, citric acid in lemon and tamarind acids produce a sour taste. Therefore option A is correct. Bases are often bitter and soapy, salts may have varied tastes, and pure water is generally tasteless. However, taste is only a general property and must never be used to identify an unknown laboratory chemical.

The governing concept is the characteristic nature of acids and bases. Aqueous bases generally feel slippery or soapy because they react with oils on the skin; soap solutions are commonly alkaline. Therefore, option B, base, is correct. Acids are usually associated with sourness and can be corrosive, salts do not generally have a soapy feel, and metals are solids with metallic properties. Chemicals must never be identified by touching them in a laboratory.

The governing concept is the relation between taste and chemical nature. Lemon juice has a sour taste because it contains natural acids, chiefly citric acid. Hence option A, acid, is correct. A base is commonly bitter and may feel slippery, while a salt is not identified here by the characteristic sourness of lemon juice. Lemon juice is not a metal; it is an aqueous mixture containing water, citric acid, and other dissolved substances.

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.