Acids, Bases, and Salts

A base is a substance that neutralises an acid, but bases differ in their solubility. A base that dissolves in water is specifically called an alkali; examples include sodium hydroxide and potassium hydroxide. Thus option B is correct. An acid has acidic properties, a salt is generally formed in a neutralisation reaction, and an indicator changes colour to show whether a solution is acidic or basic. These terms are not synonyms for alkali.

An alkali is defined as a base that is soluble in water. Therefore, every alkali is a base, but a base that is insoluble or only sparingly soluble in water is not classified as an alkali. This makes option A correct. Bases can react with acids to form salt and water, so option C is false. Bases are not all acids, and many bases produce ions in aqueous solution, so options B and D are also incorrect.

An active metal placed in a dilute acid generally displaces hydrogen from the acid. The reaction produces a salt and hydrogen gas; for example, Zn + 2HCl → ZnCl₂ + H₂. Therefore option A is correct. Carbon dioxide is released when an acid reacts with a carbonate or hydrogencarbonate, not an ordinary active metal. Oxygen and nitrogen are not the usual gaseous products of this acid-metal reaction.

Hydrogen is a combustible gas, so it is tested by bringing a burning splint near the mouth of the test tube containing the gas. If hydrogen is present, it burns rapidly and produces a characteristic ‘pop’ sound; hence option B is correct. Lime water turning milky is the test for carbon dioxide. Turmeric and litmus colour changes indicate alkaline conditions, not specifically hydrogen gas.

When an acid reacts with a carbonate, the products are a salt, water, and carbon dioxide. The overall pattern is acid + carbonate → salt + H₂O + CO₂. For example, hydrochloric acid reacts with calcium carbonate to form calcium chloride, water, and carbon dioxide. Therefore option B is correct. Hydrogen is associated with acid-metal reactions, while chlorine and ammonia are not normal products of this reaction.

Lime water is a clear solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is bubbled through it, the reaction forms insoluble calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The suspended calcium carbonate particles make the solution appear milky, so option A is correct. Blue, black, or pink colour changes are not the standard observation for this carbon dioxide test.

Lime water contains dissolved calcium hydroxide. When carbon dioxide is passed through it, calcium carbonate is produced according to Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble in water and forms fine suspended particles, which scatter light and make the solution look milky. Thus option A is correct. Sodium chloride and sodium hydroxide do not cause this observation, and hydrogen gas is not formed in the test.

The governing concept is the classification of oxides by their reaction with water and bases. Many non-metal oxides, such as carbon dioxide and sulfur dioxide, form acids when dissolved in water; they can also react with bases to produce salt and water. Therefore, the general nature of non-metal oxides is acidic. Option B describes many metal oxides, while C is not an oxide nature and D is not the general chemical classification.

The governing concept is the acid–base behaviour of metal oxides. Copper oxide reacts with an acid, for example: CuO + 2HCl → CuCl₂ + H₂O. Formation of a salt and water in this neutralisation reaction shows that copper oxide behaves as a basic oxide. Thus option B is correct. It is not an acidic oxide, a gas, or a salt itself; those alternatives confuse the substance’s reaction or physical identity.

The governing concept is the identification of common salts by their chemical names and formulae. Baking soda is sodium hydrogen carbonate, NaHCO₃. Its hydrogen carbonate ion reacts with acids and releases carbon dioxide, which explains its use in baking and some antacid preparations. Therefore 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.

The governing concept is the relationship between a salt’s common name, chemical name, and water of crystallisation. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. The ten water molecules are part of its crystalline structure, so it is called a decahydrate. Hence option C is correct. Sodium chloride is common salt, sodium hydrogen carbonate is baking soda, and calcium sulphate hemihydrate is plaster of Paris.

The governing concept is the identification of an everyday salt by its chemical composition. Common salt consists of sodium ions and chloride ions in a 1:1 ratio, so its chemical name is sodium chloride and its formula is NaCl. 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 a separate calcium salt found in materials such as limestone.

The governing concept is the formation and identification of salts containing water of crystallisation. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. It is produced by carefully heating gypsum, CaSO₄·2H₂O, so that part of its water of crystallisation is removed. Therefore option A is correct. Sodium chloride is common salt, sodium carbonate decahydrate is washing soda, and calcium carbonate is a different calcium compound.

The governing concept is the preparation and chemical use of bleaching powder. Chlorine gas is passed over dry slaked lime, Ca(OH)₂, to produce bleaching powder, commonly represented at school level as CaOCl₂. Chlorine also explains its bleaching and disinfecting action. Thus option B is correct. Hydrogen and nitrogen do not provide the required active chlorine, while carbon dioxide is not the gas used in this preparation.

The governing preparation is the reaction of chlorine with dry slaked lime. Slaked lime is calcium hydroxide, Ca(OH)₂; when chlorine is passed over it in the dry state, bleaching powder is formed. Therefore option A is correct. Common salt is a starting material for producing chlorine industrially but is not the direct solid used in this step. Gypsum and washing soda have different compositions and roles and do not form bleaching powder in this reaction.

The governing concept is chemical disinfection by available chlorine. When bleaching powder is added to water, it releases chlorine-containing species that form hypochlorous acid in suitable conditions. These active chlorine species damage microorganisms and help kill disease-causing germs. Therefore option A correctly explains its use. Bleaching powder does not sweeten water, convert it into a metal, or freeze it; those choices are unrelated to its chemical action.

The governing concept is the effect of controlled heating on a hydrated salt. Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On careful heating, it loses part of its water of crystallisation and changes into calcium sulphate hemihydrate, CaSO₄·½H₂O, called plaster of Paris. Thus option A is correct. Washing soda, baking soda, and common salt are different sodium or calcium salts and are not the product of heating gypsum.

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. When water is added, it combines with water and changes into gypsum, CaSO₄·2H₂O. This hydration reaction produces a solid mass, so the material sets and becomes hard. Sodium chloride, gas, and sugar are not products of this setting reaction; therefore, option A is correct.

Blue copper sulphate crystals are hydrated copper sulphate, CuSO₄·5H₂O. The definite water molecules held within the crystal lattice are called water of crystallisation, and their presence gives the crystals their characteristic blue appearance. Sodium ions, chlorine gas, and hydrogen gas are not constituents responsible for this colour. Hence, option A is the best answer.

Blue copper sulphate crystals contain water of crystallisation and can be represented as CuSO₄·5H₂O. On strong heating, this water is driven out of the crystal structure. The hydrated salt therefore changes to anhydrous copper sulphate, which is white. Reheating is not an addition of sodium, formation of chlorine, or production of sugar, so option A is correct.