Acids, Bases, and Salts

Washing soda is sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The word decahydrate means that each formula unit of sodium carbonate is associated with ten molecules of crystallisation water. Therefore option A is correct. Sodium hydrogen carbonate is baking soda, calcium oxychloride is bleaching powder, and calcium sulphate hemihydrate is plaster of Paris.

The governing concept is the removal of hardness-causing ions. Hard water contains dissolved calcium and magnesium salts. Washing soda, sodium carbonate (Na2CO3), reacts with these ions and forms insoluble carbonates such as calcium carbonate and magnesium carbonate. These precipitates can be separated, leaving softer water. Therefore, option A is correct; the other options do not describe the chemical process of softening.

Water of crystallisation is the definite number of water molecules chemically associated with one formula unit of some hydrated salts. It is part of the crystal structure, not merely moisture or dissolved water. For example, CuSO4·5H2O contains five water molecules per formula unit, while washing soda is Na2CO3·10H2O. Thus option A gives the correct meaning; B, C and D describe unrelated substances or situations.

Blue copper sulphate crystals are hydrated copper sulphate, CuSO4·5H2O. The governing concept is dehydration: on heating, the water of crystallisation is driven out, producing white anhydrous copper sulphate, CuSO4. If water is added again, the blue hydrated form can return. Therefore option A is correct. The other choices incorrectly suggest a change into gold, oxygen, or additional water absorption.

White anhydrous copper sulphate is CuSO4 and lacks water of crystallisation. When water is added, it becomes hydrated copper sulphate, commonly represented as CuSO4·5H2O, whose crystals are blue. The governing concept is hydration and restoration of the crystal structure. Hence option A is correct. Copper does not burn, sulphur gas is not produced, and the salt does not become an acid.

Plaster of Paris is calcium sulphate hemihydrate, CaSO4·1/2H2O. Its governing property is that it reacts with water and changes into gypsum, CaSO4·2H2O. This hydration causes the powder to set into a hard mass, so moisture during storage can spoil its usefulness. Therefore option A is correct; the other options describe chemically impossible effects.

Plaster of Paris is used because it undergoes a controlled setting reaction with water. Calcium sulphate hemihydrate changes into gypsum, CaSO4·2H2O, forming a hard, mouldable support around the injured limb. The hardened cast restricts movement and helps protect the fracture while healing occurs. Thus option A is correct; it neither dissolves bone nor remains liquid or disappears.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, calcium hydroxide, Ca(OH)2. In the school-level representation, the product is written as CaOCl2, commonly called bleaching powder, and its bleaching action is associated with chlorine. Therefore chlorine is the correct gas in option A. Hydrogen, nitrogen and helium do not produce bleaching powder in this preparation.

Bleaching powder is a reactive salt whose available chlorine can decrease during prolonged exposure to air. Moisture and carbon dioxide promote decomposition or conversion of active constituents into less effective substances. Consequently, an open sample gradually loses bleaching and disinfecting power. Option A is correct. The alternatives involving gold, pure water or iron have no chemical basis and do not explain loss of activity.

The chlor-alkali process is the electrolysis of concentrated sodium chloride solution, or brine. At the cathode, water produces hydrogen gas and hydroxide ions; at the anode, chloride ions are oxidised to chlorine gas. The remaining sodium ions combine with hydroxide ions to form sodium hydroxide. Thus option A is correct, while the other gas pairs are not products of brine electrolysis.

Chlorine is one of the products of brine electrolysis in the chlor-alkali process. Because chlorine is a strong oxidising and disinfecting agent, it is used in controlled amounts to kill many disease-causing microorganisms in drinking water. It is also used for bleaching paper pulp and textiles. Therefore option A is correct; the other options are not recognised applications of chlorine.

Baking soda is the common name of sodium hydrogen carbonate, whose formula is NaHCO₃. It is a mild base and is used in baking because it can release carbon dioxide under suitable conditions, helping dough rise. Sodium chloride is common salt, sodium hydroxide is a strong base, and calcium carbonate is found in materials such as limestone and chalk; therefore, option A is correct.

Baking powder contains baking soda and a mild edible acid. When the mixture becomes wet and is heated, the acid reacts with sodium hydrogen carbonate and produces carbon dioxide. The gas forms bubbles that make baked food soft and spongy; the acid also helps prevent an unpleasant alkaline taste. Thus option A is correct. The other choices contradict the reaction because gas is formed and baking soda does not become a metal.

Crystals of sodium carbonate commonly contain water of crystallisation; washing soda is represented as Na₂CO₃·10H₂O. On heating, some or all of this water is driven out of the crystal lattice as water vapour, so the measured mass decreases. Sodium atoms do not disappear, carbon does not turn into gold, and the crystal does not consume air. Therefore, option A correctly explains the mass loss.

Acids react with metal carbonates to form a salt, water, and carbon dioxide: acid + carbonate → salt + H₂O + CO₂. This reaction makes option A correct. Acids generally turn blue litmus red, so option B reverses the indicator change. In aqueous solution acids produce ions, especially hydrogen or hydronium ions, which rejects option C. They do not produce oxygen with all metals, so D is also false.

The reaction between an acid and a base is called neutralisation. In the usual neutralisation reaction, the hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining ions form a salt; for example, HCl + NaOH → NaCl + H₂O. Metal oxidation usually forms an oxide, hydrogen combustion forms water alone, and dissolving salt is not a new formation reaction. Hence A is correct.

An acidic solution has a pH below 7. When a base is added, hydroxide ions react with hydrogen or hydronium ions, reducing the effective acidity. Consequently, the pH rises toward 7 at the neutralisation point and may become greater than 7 if excess base is added. It does not always remain zero or continually decrease. Thus option A is the correct general trend during gradual neutralisation.

The strength of an acid describes its degree of ionisation in water, not simply the amount of acid present. A strong acid, such as hydrochloric acid in an aqueous solution, ionises almost completely, whereas a weak acid ionises only partially and establishes an equilibrium. Therefore option A is correct. A weak acid remains an acid, strong acids can dissolve in water, and both types produce ions to some extent, so B, C, and D are incorrect.

Concentration refers to the amount of solute present in a given amount of solution. A concentrated acid solution contains a relatively large amount of dissolved acid, while a dilute solution contains a smaller amount because more solvent is present. This idea is different from acid strength, which depends on ionisation. Colour and name do not define concentration, and dilution does not turn an acid into a base. Therefore option A is correct.

Vinegar contains acetic acid, so its aqueous solution is acidic. Acids turn blue litmus paper red because the indicator changes colour in an acidic environment; therefore option A is correct. Sodium hydroxide, soap solution, and lime water are basic substances or solutions and generally turn red litmus blue rather than changing blue litmus to red. Identifying the nature of each solution before applying the indicator rule avoids confusion.