Acids, Bases, and Salts

Acidic behaviour is produced by ions in an aqueous medium, especially hydrogen or hydronium ions. A dry acidic gas cannot ionise sufficiently on dry litmus, so no colour change is observed. Moisture dissolves the gas and permits ion formation; these ions cause blue litmus to turn red. Hence option A is correct, while the other explanations are scientifically irrelevant or false.

Plaster of Paris is calcium sulphate hemihydrate, written as CaSO₄·½H₂O, so option B is correctly matched. Bleaching powder is not sodium chloride; baking soda is sodium hydrogen carbonate, not calcium sulphate; and washing soda is sodium carbonate decahydrate, not calcium carbonate. These comparisons establish that only option B is correct.

On the pH scale, a value of 12 is far above 7 and therefore indicates a strongly basic solution. A slippery or soapy feel is also commonly associated with bases, although touching an unknown chemical is unsafe. Such a solution may be caustic and can damage skin, so it should be handled with protective equipment. Thus option A is the safest conclusion.

An active metal reacts with an acid to form a salt and hydrogen gas: metal + acid → salt + H₂. When a burning splint is brought near hydrogen, the gas burns rapidly and produces the characteristic ‘pop’ sound. Limewater turning milky identifies carbon dioxide, while the other tests do not identify hydrogen. Therefore option A is correct.

Limewater is an aqueous solution of calcium hydroxide. When carbon dioxide is passed through it, the reaction is Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble in water and appears as a white suspension, producing the milky appearance. If excess carbon dioxide is passed, soluble calcium hydrogen carbonate may form and the milkiness can disappear. Thus option A is correct.

The governing concept is the logarithmic pH scale: pH = −log[H⁺]. A rise of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 1 to pH 4 is three units, so acidity changes by 10 × 10 × 10 = 1000 times downward. Thus option A is correct; B treats pH as a linear scale, while C and D give the wrong direction or ignore the change.

The governing concept is acid-base neutralisation. Slaked lime is calcium hydroxide, Ca(OH)₂, a basic substance. In acidic soil, its hydroxide ions react with excess hydrogen ions, reducing acidity and moving the pH upward toward a suitable range. Therefore option A is correct. B reverses the chemical effect, while C and D describe impossible changes and do not explain pH.

The governing concept is salt hydrolysis and the relative strengths of the acid and base that form a salt. A salt such as sodium chloride gives a nearly neutral solution, ammonium chloride gives an acidic solution, and sodium carbonate gives a basic solution. Hence option C is correct. Options A and B wrongly generalise one nature to every salt, and D confuses acidity with a fixed pH of zero.

The governing concept is the ecological effect of acid-base change. Acid rain contains acidic oxides and dissolved acids that can lower the pH of lake water. Aquatic organisms function within particular pH ranges; excessive acidity can disturb enzyme activity, respiration, reproduction, and survival. Therefore option A is correct. B reverses the effect, while C and D ignore the biological and chemical consequences of lower pH.

The governing concept is the basic nature of many metal oxides and their neutralisation reaction with acids. For example, copper(II) oxide reacts with hydrochloric acid to form copper(II) chloride and water. This acid-neutralising behaviour supports option A. Option B describes an acidic response and is not generally true; C is false because many metal oxides are solids, and D has no chemical basis.

The governing concept is the acidic nature of many non-metal oxides in water. Oxides such as carbon dioxide and sulfur dioxide react or dissolve in water to produce acidic solutions, which provide hydrogen ions and turn blue litmus red. Therefore option A is correct. B is chemically unrelated, C reverses the usual behaviour, and D is too absolute because several non-metal oxides dissolve or react with water.

The governing concept is the reaction of acids with reactive metals and food-safety precaution. Curd contains lactic acid and has an acidic character. During prolonged contact, the acid may react with copper and produce copper compounds that can contaminate the food and may be harmful in excess. Therefore option A is correct; B and C are impossible, while D falsely denies the acidity of curd.

The governing concept is the colour range of a universal indicator and neutralisation. Violet indicates a strongly basic medium, while blue represents a less strongly basic medium. Adding acid consumes some hydroxide ions and lowers the pH, but the remaining solution is still on the basic side of pH 7. Therefore option A is correct; it is not acidic, completely neutral, or unchanged.

The governing reaction is acid–carbonate reaction: acid + carbonate produces a salt, water, and carbon dioxide. For example, sodium carbonate with hydrochloric acid gives sodium chloride, water, and CO₂. Hydrogen is associated with the reaction of suitable active metals with acids, not carbonates. Carbon dioxide can be confirmed because it turns limewater milky, so option A is correct.

Neutralisation is governed by the ionic reaction H⁺ + OH⁻ → H₂O. An acid supplies hydrogen ions, while sodium hydroxide dissociates in water to supply hydroxide ions. These two ions combine directly to form water; sodium and the acid’s negative ion remain as spectator ions in solution. Thus option A is correct, while the other pairs do not form water in this reaction.

A base is a substance that reacts with an acid to form salt and water, usually through neutralisation. Solubility is the key distinction here: an alkali is specifically a base that dissolves in water, whereas an insoluble base is still a base but not an alkali. Since the substance is insoluble yet neutralises an acid, option A is correct; it is neither an acid nor an indicator.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, whose chemical formula is Ca(OH)₂. The process produces a chlorine-containing bleaching material commonly represented as CaOCl₂ at school level. The word “dry” matters because moisture can interfere with the preparation. Sodium chloride, gypsum, and washing soda are not the required starting substance, so option A is correct.

The governing concept is chemical identification using indicators rather than appearance. Many common acids and bases, such as dilute hydrochloric acid and sodium hydroxide solution, are colourless. Litmus, universal indicator, or pH paper undergoes a characteristic colour change that provides evidence about acidic or basic nature. Thus option A is correct; no fixed visible colour applies to every acid or base, and colour alone cannot give exact pH.

Ethanoic acid, CH₃COOH, is classified as a weak acid because it ionises only partially in water and establishes an equilibrium between molecules and ions. It still produces hydrogen ions and shows acidic properties, but less extensively than a strong acid such as hydrochloric acid. Therefore, option B is correct; the other choices describe a strong acid, a base, or a salt.

Most metal oxides are basic oxides. They commonly react with acids in a neutralisation reaction to form salt and water; for example, CuO + 2HCl → CuCl₂ + H₂O. This acid-neutralising behaviour supports option B. “Acidic” is generally associated with many non-metal oxides, while neutral and salty do not describe the usual chemical nature of metal oxides.

Non-metal oxides are generally acidic. Many dissolve or react with water to form acids, and these acidic solutions can turn blue litmus red. For example, carbon dioxide reacts with water to form carbonic acid. Thus option B is correct. Basic describes the usual behaviour of metal oxides, while metallic and salt-free are not acid–base classifications for these oxides.

Turmeric contains a natural indicator substance, mainly curcumin. In its usual yellow form it changes to reddish-brown when it contacts a basic or alkaline substance, such as a soap solution. This colour change indicates basicity, so option B is correct. Acidic and neutral substances do not normally produce this reddish-brown turmeric test, and sugar is not the relevant chemical category.

Phenolphthalein is a synthetic acid–base indicator. It is colourless in an acidic or sufficiently neutral medium, but its indicator form becomes pink to reddish-pink in a basic solution. Since the question asks specifically about a basic solution, option B, pink, is correct. Colourless belongs to the acidic range, while blue and green are not the standard phenolphthalein result.

Methyl orange is an acid–base indicator whose colour depends on the solution’s pH. In an acidic solution it appears red; as the medium becomes less acidic and then basic, it changes through orange toward yellow. Therefore option B is correct. Yellow is associated with the basic side of its transition, whereas green and blue are not the expected methyl-orange colours in this test.

A universal indicator is a mixture of indicators that displays different colours across the pH scale. A neutral aqueous solution has a pH close to 7, and the corresponding universal-indicator colour is generally green. Therefore option B is correct. Red and orange indicate acidic conditions, while violet represents a strongly basic region, not neutrality.