Acids, Bases, and Salts

In an acid-base neutralisation, the essential ionic step is H⁺ + OH⁻ → H₂O. The hydrogen ion comes from the acid and the hydroxide ion comes from the base. Sodium and chloride ions, for example, remain as spectator ions and form sodium chloride rather than water. The other listed ion pairs can form salts or other substances, but they are not the direct ions that make water.

An active metal can displace hydrogen from a dilute acid. The general reaction is metal + acid → salt + hydrogen gas; for example, Zn + 2HCl → ZnCl₂ + H₂. Thus option A is correct. Carbon dioxide is usually released when an acid reacts with a carbonate, while oxygen and nitrogen are not the normal gaseous products of this metal-acid reaction.

Hydrogen is a combustible gas. When a small amount is brought into contact with a burning splint, it burns rapidly and produces a characteristic ‘pop’ sound, so option B is the identification test. Limewater turning milky indicates carbon dioxide. Turmeric and phenolphthalein are indicators of basic conditions, not specific tests for hydrogen gas. The test must be performed carefully because hydrogen-air mixtures can be explosive.

Acids react with carbonates to form three products: a salt, water, and carbon dioxide gas. The general equation is acid + carbonate → salt + water + CO₂; for example, 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂. Therefore option B is correct. Hydrogen is associated with acid-metal reactions, whereas chlorine and ammonia are not normal products of an acid-carbonate reaction.

The governing concept is the lime-water test for carbon dioxide. Lime water is an aqueous solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, the reaction Ca(OH)₂ + CO₂ → CaCO₃ + H₂O occurs. The calcium carbonate formed is insoluble and appears as a white suspension, making the solution milky. Therefore, option A is correct; the other colours are not produced in this test.

The governing concept is precipitation during the carbon-dioxide test. Lime water contains dissolved calcium hydroxide. Carbon dioxide reacts with it according to Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is a fine, insoluble white precipitate, and its suspended particles scatter light, producing the observed milkiness. Thus option A is correct. Sodium chloride and sodium hydroxide do not cause this characteristic precipitate, while hydrogen gas is not formed.

The governing concept is neutralisation between a basic metal oxide and an acid. Copper oxide reacts with hydrochloric acid as CuO + 2HCl → CuCl₂ + H₂O. Formation of salt and water from an oxide and an acid is characteristic of a basic oxide. Therefore option B is correct. Copper oxide is not an acidic oxide, it is a solid compound rather than a neutral gas, and it is not itself common salt.

The governing concept is the classification of oxides according to their acid-base behaviour. Carbon is a non-metal, and its oxide, CO₂, reacts with water to form carbonic acid: CO₂ + H₂O → H₂CO₃. It also reacts with bases such as calcium hydroxide. Thus CO₂ is classified as an acidic oxide, so option B is correct. It is not a metal oxide, a basic oxide, or a salt.

The governing concept is the identification of common salts by their chemical names and formulae. Baking soda is sodium hydrogen carbonate, NaHCO₃. When it reacts with an acid, it produces carbon dioxide, water, and another salt; for example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. Therefore option A is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium carbonate is found in limestone and chalk.

The governing concept is water of crystallisation in hydrated salts. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O; the ten water molecules are part of its crystalline structure. This makes option C correct. Sodium chloride is common salt, sodium hydrogen carbonate is baking soda, and calcium sulphate hemihydrate is plaster of Paris. Heating washing soda can remove its crystallisation water and change its properties.

The governing concept is the relationship between a common name and the chemical name of a salt. Common salt is sodium chloride, with the formula NaCl. It consists of sodium and chloride ions in an ionic crystal and is widely used in food. Therefore option A is correct. Sodium carbonate is a different salt used in washing soda, sodium hydroxide is a strong base, and calcium carbonate is present in limestone and chalk.

The governing concept is the formation and hydration of a salt. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O, obtained by carefully heating gypsum. When water is added, it changes back into gypsum, CaSO₄·2H₂O, and hardens into a solid mass. Therefore option A is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium carbonate is a different calcium salt.

The governing concept is the preparation and use of bleaching powder, also called calcium oxychloride in school-level chemistry. Chlorine gas is passed over dry slaked lime, Ca(OH)₂, to form bleaching powder. Thus option B is correct. Hydrogen and nitrogen do not produce bleaching powder, while carbon dioxide is not the reacting gas in this preparation.

Bleaching powder is prepared by passing chlorine over dry slaked lime, whose chemical formula is Ca(OH)₂. The relevant reaction is commonly represented as Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O. Therefore option A is correct. Common salt is used in the chlor-alkali process, gypsum forms plaster of Paris on heating, and washing soda is sodium carbonate; none is the direct solid used here.

The governing concept is disinfection by available chlorine. When bleaching powder is added to water, it releases chlorine-containing species that produce available chlorine, including hypochlorous acid under suitable conditions. These oxidising substances damage or kill many disease-causing microorganisms. Therefore option A is correct; sweetness, metal formation, and freezing are unrelated to the disinfecting action.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O, so it contains water of crystallisation. On controlled heating, part of this water is removed and calcium sulphate hemihydrate, CaSO₄·½H₂O, is produced. This substance is called plaster of Paris, making option A correct. Washing soda, baking soda, and common salt are different salts and are not formed by heating gypsum.

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. When water is added, it rehydrates and changes back into gypsum, calcium sulphate dihydrate, CaSO₄·2H₂O. The re-formed crystals interlock and make the material set into a hard mass, so option A is correct. It does not form sodium chloride, turn into a gas, or produce sugar.

Copper sulphate crystals are hydrated copper sulphate, CuSO₄·5H₂O, and contain a definite number of water molecules within their crystal structure. At this school level, the characteristic blue appearance is associated with the hydrated crystalline form and its water of crystallisation. Therefore option A is the expected answer. Sodium ions, chlorine gas, and hydrogen gas are not constituents responsible for these crystals’ blue colour.

Blue copper sulphate crystals are CuSO₄·5H₂O, which contain water of crystallisation. Heating drives this water out of the crystal lattice, producing anhydrous copper sulphate, CuSO₄, which is white or pale grey. Consequently the blue colour disappears, so option A is correct. No sodium is added, and chlorine or sugar is not formed in this dehydration change.

Water of crystallisation means the definite number of water molecules chemically associated with one formula unit of certain salts and incorporated into their crystal structure. For example, CuSO₄·5H₂O contains five water molecules per formula unit. This is not simply river water or any water mixed with an acid, and it is not a gas formed from a base. Therefore option A is correct.