Acids, Bases, and Salts

Lime water contains dissolved calcium hydroxide. When carbon dioxide is passed through it, calcium carbonate forms according to Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble in water and produces a fine white suspension, which causes the observed milkiness. Therefore option A is correct. Calcium hydrogen carbonate is soluble in water, while sodium chloride and sodium hydroxide do not form the white precipitate responsible for this observation.

When carbon dioxide is first passed through lime water, insoluble calcium carbonate forms and makes the solution milky: Ca(OH)2 + CO2 → CaCO3 + H2O. If excess CO2 is passed, the calcium carbonate reacts further with carbon dioxide and water to form soluble calcium hydrogen carbonate: CaCO3 + CO2 + H2O → Ca(HCO3)2. Because the precipitate dissolves, the milkiness decreases. The other options do not explain this observation.

Acid strength is related to the extent of ionisation in water, not simply to how concentrated the acid is. Hydrochloric acid ionises almost completely: HCl + H2O → H3O+ + Cl−. This produces a high concentration of hydronium ions for a given concentration of acid, so HCl is classified as a strong acid. It is soluble in water and is neither a base nor a salt, which eliminates options B, C, and D.

Ethanoic acid is a weak acid because only a fraction of its molecules ionise in water and an equilibrium is established: CH3COOH + H2O ⇌ CH3COO− + H3O+. Consequently, its hydronium-ion concentration is lower than that of a strong acid of the same concentration. Weak does not mean insoluble or harmless; ethanoic acid dissolves in water and remains an acid. Therefore option A is correct.

Electrical conductivity in an aqueous solution depends mainly on the concentration and mobility of charged ions. At the same concentration, HCl ionises almost completely and produces many H3O+ and Cl− ions. Ethanoic acid ionises only partially, so it produces fewer ions, even though it dissolves in water. The larger number of charge carriers makes the HCl solution more conducting. Options B, C, and D contradict the properties of HCl.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Sodium carbonate is the solid residue, while carbon dioxide and water are gaseous or vapour products under the usual conditions. Hence option A is correct; the other choices are unrelated salts and are not formed in this decomposition.

Washing soda is the hydrated salt sodium carbonate decahydrate, written as Na₂CO₃·10H₂O. The ten water molecules are called water of crystallisation and are part of the crystal structure. Therefore option A is correct. Sodium hydrogen carbonate is baking soda, sodium chloride is common salt, and calcium sulphate hemihydrate is plaster of Paris, so these alternatives describe different substances.

Hard water contains dissolved calcium and magnesium ions, which react with soap to form scum and reduce lather. Washing soda supplies carbonate ions that precipitate these ions as insoluble carbonates, such as calcium carbonate. Removing the ions reduces hardness, so option A is correct. It does not soften water by forming hydrogen or chlorine gas, and making water acidic would not be the governing process.

The governing concept is the removal of water of crystallisation by controlled heating. Gypsum, CaSO4·2H2O, loses part of its combined water when heated moderately and forms calcium sulphate hemihydrate, CaSO4·½H2O, which is plaster of Paris. Common salt, baking soda and washing soda do not give plaster of Paris in this preparation.

Plaster of Paris is the partially dehydrated form of gypsum. Gypsum has the formula CaSO4·2H2O, whereas controlled heating produces CaSO4·½H2O. This compound is called calcium sulphate hemihydrate, so option A is correct. Calcium carbonate is limestone or chalk, sodium carbonate decahydrate is washing soda, and sodium chloride is common salt.

The governing concept is rehydration followed by setting. Plaster of Paris is calcium sulphate hemihydrate, CaSO4·½H2O. When water is added, it combines with the compound and reforms gypsum, CaSO4·2H2O. This hydration reaction produces a hard solid mass. It does not form sodium chloride, gas or sugar, so A is the only suitable answer.

The governing concept is water of crystallisation in hydrated salts. Blue copper sulphate crystals are CuSO4·5H2O, so each formula unit contains a fixed number of water molecules within the crystal structure. This hydrated form has the characteristic blue appearance. Sodium ions, chlorine gas and carbon dioxide are not responsible for the colour of copper sulphate crystals.

Blue copper sulphate crystals are hydrated CuSO4·5H2O. On heating, they lose their water of crystallisation and become anhydrous copper sulphate, which is white. Thus the colour disappears because the hydrated crystal structure changes, not because sodium or chlorine is added or carbon dioxide is formed. Adding water can restore the blue hydrated form.

The governing preparation is the reaction of chlorine with dry slaked lime. Dry slaked lime is calcium hydroxide, Ca(OH)2, and passing chlorine over it produces bleaching powder, commonly represented as CaOCl2, along with water under the usual school-level description. Moisture is avoided because the lime must remain dry for proper preparation. Therefore A is correct.

The governing concept is chemical disinfection by available chlorine. When bleaching powder comes into contact with water, it releases chlorine or chlorine-containing species that form hypochlorous acid. These reactive species damage and kill many disease-causing microorganisms. The effect is not related to sweetness, conversion into a metal, or freezing, so option A correctly explains its use.

Brine is a concentrated aqueous solution of sodium chloride. During electrolysis, chloride ions produce chlorine at the anode, water produces hydrogen at the cathode, and sodium hydroxide remains in the solution. Because the process yields chlorine and an alkali, sodium hydroxide, it is called the chlor-alkali process. Neutralisation, evaporation and filtration are different processes.

The governing reaction is electrolysis of aqueous sodium chloride, or brine. Chloride ions are oxidised at the anode to form chlorine gas, while water is reduced at the cathode to form hydrogen gas and hydroxide ions. Sodium ions combine with hydroxide ions in solution to give sodium hydroxide. Therefore A lists all three main products; the other sets belong to unrelated processes.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions lose electrons: 2Cl⁻ → Cl₂ + 2e⁻, so chlorine gas is released there. At the cathode, water produces hydrogen gas, while sodium hydroxide remains in solution. Therefore option B is correct; hydrogen belongs at the cathode, not the anode.

In the chlor-alkali process, brine is electrolysed. At the cathode, water molecules gain electrons and form hydrogen gas: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Chloride ions form chlorine at the anode, not at the cathode. The hydrogen can be tested with a lighted splint, producing a characteristic pop. Therefore option A is correct.

Tooth enamel is mainly a mineral that can be attacked when the mouth becomes sufficiently acidic. Bacteria metabolise sugars and produce acids, lowering the pH around the teeth. When the pH falls below about 5.5, enamel demineralisation becomes more likely and the risk of tooth decay rises. A pH of 7 is neutral, while 10 and 14 are alkaline. Hence option A is correct.

After food is eaten, oral bacteria can produce acids from fermentable sugars. These acids lower the pH and may attack tooth enamel, increasing the risk of decay. A basic toothpaste supplies alkaline substances that neutralise some of the excess acid and help move the mouth toward a safer pH. It does not dissolve enamel or make pH zero. Therefore option A is correct.

Red litmus turning blue indicates that the solution is basic. Turmeric also changes from yellow to reddish-brown in the presence of a base, confirming the same conclusion. In aqueous school-level acid-base theory, basicity is associated with an excess concentration of hydroxide ions, OH⁻, rather than hydrogen ions. Chloride and sulphate ions alone do not explain these indicator changes. Hence option B is correct.

Limewater is a clear solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, the reaction Ca(OH)₂ + CO₂ → CaCO₃ + H₂O occurs. Calcium carbonate is an insoluble white precipitate, so it makes the solution appear milky. Therefore, option A is correct; the other choices name substances that are not formed in this test.

The governing concept is solubility. A base is a substance that can neutralise an acid, whereas an alkali is specifically a base that dissolves in water and produces hydroxide ions in solution. Some bases, such as copper(II) oxide, are insoluble or only sparingly soluble in water. Therefore, option B is correct; options A, C and D contradict the definitions and properties of bases.

The governing concept is acid strength, which depends on the extent of ionisation in water, not simply on concentration. Hydrochloric acid ionises almost completely, producing a high concentration of H₃O⁺ and Cl⁻ ions. Therefore option B is correct. Partial ionisation describes a weak acid, insolubility is false for its aqueous solutions, and HCl is an acid rather than a base.