Acids, Bases, and Salts

Hydrogen is a combustible gas, so its confirmatory school-level test is the pop test. When a burning splint is brought near the mouth of a vessel containing hydrogen, the gas burns rapidly with a characteristic ‘pop’ sound; therefore option B is correct. Limewater turning milky indicates carbon dioxide, while turmeric and phenolphthalein colour changes indicate alkaline conditions, not hydrogen.

Acids react with carbonates to produce a salt, water, and carbon dioxide gas. The general reaction is acid + carbonate → salt + H₂O + CO₂; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Thus option B is correct. Hydrogen is released in many acid–metal reactions, whereas chlorine and ammonia require different reactants and are not general products here.

Carbon dioxide is identified by passing it through lime water, which is an aqueous solution of calcium hydroxide. The reaction is CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. The calcium carbonate produced is insoluble and forms a fine suspension, making the solution appear milky. Therefore, option A is correct; the other colours are not the characteristic result of this test.

Lime water is a clear solution of calcium hydroxide. When carbon dioxide is passed through it, calcium carbonate is formed: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble in water, so its suspended particles scatter light and produce the milky appearance. Thus option A is correct. Sodium chloride and sodium hydroxide do not cause this observation, and hydrogen is not the precipitate formed.

Non-metal oxides are generally acidic in nature. Many of them react with water to form acids; for instance, CO₂ + H₂O → H₂CO₃, and SO₂ + H₂O → H₂SO₃. They may also react with bases to produce salt and water. Therefore option A is correct. Basic, metallic and salty are not the general classifications for non-metal oxides, although a few oxides can show special or amphoteric behaviour.

Copper oxide, CuO, is a metal oxide. Its reaction with hydrochloric acid is CuO + 2HCl → CuCl₂ + H₂O. A substance that reacts with an acid to form salt and water is showing basic oxide behaviour through neutralisation. Hence option B, basic oxide, is correct. It is not an acidic oxide, a gas or a common salt; those options do not describe the stated reaction or the substance’s composition.

Carbon dioxide, CO₂, is an oxide of carbon, and carbon is a non-metal. It is classified as an acidic oxide because it reacts with water to form carbonic acid: CO₂ + H₂O ⇌ H₂CO₃. It also reacts with bases such as lime water. Therefore option B is correct. It is not a basic oxide, a metal oxide or a salt; those choices conflict with its composition and reactions.

Baking soda is the common name for sodium hydrogen carbonate, with the formula NaHCO₃. It is a mild basic salt and reacts with acids to release carbon dioxide, for example NaHCO₃ + HCl → NaCl + H₂O + CO₂. Thus option A is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium carbonate is found in substances such as limestone and chalk.

Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The ten water molecules are called water of crystallisation, so option C is correct. Sodium hydrogen carbonate, NaHCO₃, is baking soda; sodium chloride is common salt; and calcium sulphate hemihydrate is plaster of Paris. These distinctions are based on the chemical composition and names of the salts.

Common salt is sodium chloride, whose chemical formula is NaCl. It is an ionic compound formed from sodium ions and chloride ions and is widely used in food. Therefore option A is correct. Sodium carbonate is a different sodium salt, sodium hydroxide is a strong base, and calcium carbonate is a calcium salt found in materials such as limestone and chalk. Their ions and properties differ from NaCl.

Plaster of Paris is calcium sulphate hemihydrate, represented by CaSO₄·½H₂O, so option A is correct. It is prepared by carefully heating gypsum, CaSO₄·2H₂O, to remove part of its water of crystallisation. When water is added, it changes back to gypsum and sets into a hard mass. Sodium chloride, washing soda and calcium carbonate are different salts with different formulas and uses.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, which is calcium hydroxide. The reaction produces bleaching powder containing available chlorine, so chlorine is the required reactant. Hydrogen and nitrogen do not form bleaching powder, while carbon dioxide is not the bleaching or disinfecting gas involved in this preparation. Therefore, option B is correct.

The standard preparation of bleaching powder involves passing chlorine over dry slaked lime, whose chemical formula is Ca(OH)₂. Dry conditions are specified because moisture can interfere with the desired preparation. Common salt is a raw material for producing chlorine industrially, but it is not the substance over which chlorine is passed here. Thus option A is correct.

Bleaching powder is useful for water disinfection because it releases available chlorine when it reacts with water. This chlorine forms oxidising substances, including hypochlorous acid, which damage and kill many disease-causing microorganisms. It does not sweeten water, convert water into a metal, or freeze it. Therefore option A correctly states the reason.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses part of its water of crystallisation and forms calcium sulphate hemihydrate, CaSO₄·½H₂O, called plaster of Paris. Washing soda, baking soda, and common salt are different salts and are not produced by heating gypsum. Hence option A is correct.

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. When water is added, it combines with the powder and re-forms gypsum, CaSO₄·2H₂O. The new gypsum crystals interlock and produce a hard solid, which is why plaster is used for casts and moulds. No sodium chloride, gas, or sugar is formed. Option A is correct.

Hydrated copper sulphate crystals are represented as CuSO₄·5H₂O. The fixed water molecules in this crystalline structure are called water of crystallisation and are associated with the characteristic blue appearance of the hydrated salt. Sodium ions are absent, and chlorine or hydrogen gas is not responsible for the colour. Therefore option A is the intended correct answer.

Blue copper sulphate crystals contain water of crystallisation and can be written as CuSO₄·5H₂O. On heating, this water is driven out, producing anhydrous copper sulphate, which is white. Adding water restores the blue hydrated form. Sodium, chlorine, and sugar are not formed in this change, so option A gives the correct explanation.

Water of crystallisation is the definite number of water molecules chemically associated with one formula unit of certain crystalline salts. It is part of the crystal structure, not merely water absorbed on the surface. For example, CuSO₄·5H₂O contains five such water molecules per formula unit. Hence option A gives the correct definition.

Hardness of water is mainly caused by dissolved calcium and magnesium ions. Washing soda, sodium carbonate decahydrate, supplies carbonate ions that react with them to form insoluble calcium carbonate and magnesium carbonate. These precipitates can be removed, reducing hardness. Sodium and chloride ions do not cause ordinary hardness, so option A is correct.