Acids, Bases, and Salts

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions lose electrons and form chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻. At the cathode, water gains electrons and produces hydrogen gas: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Therefore the anode–cathode order is chlorine and hydrogen, making option A correct; option B reverses the electrodes, while C and D name gases not produced in this process.

In the chlor-alkali process, an aqueous solution of sodium chloride is electrolysed. Chlorine is released at the anode, while water is reduced at the cathode to form hydrogen and hydroxide ions. The sodium ions remaining in solution combine with hydroxide ions, producing sodium hydroxide: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Thus option A is correct. The other hydroxides are not formed from brine because calcium, magnesium, and ammonium ions are absent.

Bleaching powder, commonly represented as calcium oxychloride, is useful in water treatment because it releases chlorine or hypochlorous substances in the presence of moisture. These powerful oxidising species damage and kill many disease-causing microorganisms, reducing contamination. Therefore option A correctly states its disinfecting action. It does not sweeten water, convert water into a metal, or transform it into oil; options B, C, and D have no chemical basis.

Bleaching powder is prepared by passing chlorine gas over dry slaked lime, calcium hydroxide. The reaction is commonly represented as Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O, although the product may contain related calcium compounds depending on conditions. Thus chlorine is the required gas and option A is correct. Hydrogen, oxygen, and nitrogen do not provide the characteristic chlorinating and disinfecting product, so options B, C, and D are unsuitable.

Baking soda is sodium hydrogen carbonate, NaHCO₃. On heating, it undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The released carbon dioxide forms bubbles in the dough or batter, and these expanding bubbles make the food rise. Hence option A is correct. The process is not caused by stopping hydrogen, absorbing oxygen, or solidifying water, so B, C, and D are incorrect.

Baking powder contains baking soda, sodium hydrogen carbonate, together with a mild edible acid such as tartaric acid. When moistened and heated, the acid reacts with NaHCO₃ and produces carbon dioxide. The gas bubbles expand through the mixture and make cakes or bread light and spongy. Therefore option A is correct. The acid is not added to form a metal, darken food, or prevent gas formation; B, C, and D contradict the purpose of baking powder.

Baking soda is sodium hydrogen carbonate, NaHCO₃, whereas washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. The different names reflect different compositions, formulas, and uses: baking soda is used in baking and as a mild antacid, while washing soda is used for cleaning and softening hard water. Thus option A is correct. The other pairs name unrelated salts or acids and do not identify these two substances.

Hard water contains dissolved calcium and magnesium ions. Washing soda, sodium carbonate decahydrate, dissolves to provide carbonate ions. These react with Ca²⁺ and Mg²⁺ to form insoluble carbonates, such as CaCO₃, which can be removed: Ca²⁺ + CO₃²⁻ → CaCO₃(s). Consequently, option A is correct. Washing soda does not soften water by making it acidic, gaseous, or black, so B, C, and D are implausible.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Careful heating removes part of its water of crystallisation and produces Plaster of Paris, calcium sulphate hemihydrate: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Limited heating is important; excessive heating can remove more water and reduce the desired setting property. Therefore A is correct, while calcium does not burn and gypsum does not become sulphur or common salt.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. In water, it supplies carbonate ions that react with dissolved calcium and magnesium ions responsible for permanent hardness, forming insoluble carbonates that can be separated. For example, Ca²⁺ + CO₃²⁻ → CaCO₃(s). Hence option A is correct. Baking soda does not provide enough carbonate for this standard treatment, while Plaster of Paris and bleaching powder have different compositions and uses.

The governing concept is water of crystallisation: some salts contain a fixed number of water molecules within their crystal structure. Blue copper sulphate is CuSO₄·5H₂O. On heating, these water molecules are driven off, producing anhydrous CuSO₄, which is white. Thus option A is correct. The copper does not become a gas, sulphate does not turn into water, and oxygen is not added.

The governing concept is hydration and water of crystallisation. Heating removes water from blue CuSO₄·5H₂O and forms white anhydrous CuSO₄. When water is added again, the salt incorporates the water molecules and the hydrated crystal form is restored, giving the characteristic blue colour. Therefore option A is correct; the other options describe unrelated substances or products.

The governing concept is that water of crystallisation can influence the structure and appearance of a salt. When this water is removed by heating, the hydrated salt may become anhydrous and its colour can change; for example, blue hydrated copper sulphate becomes white. Therefore option A is correct. Removal does not always convert a salt into a gas or metal, and a visible change is possible.

The governing concept is that combustion requires a continuous supply of oxygen. In a soda-acid extinguisher, a reaction produces carbon dioxide, which is non-combustible and denser than air. It spreads around the flame and displaces or cuts off atmospheric oxygen, so combustion cannot continue. Option A is correct. CO₂ does not supply oxygen, form hydrogen, or extinguish fire by making it acidic.

The governing concept is the pH scale: a lower pH indicates a higher concentration of hydrogen ions and therefore greater acidity. Acid rain forms when pollutants such as sulphur and nitrogen oxides produce acids in atmospheric moisture, making its pH lower than ordinary rain. Hence option A is correct. A pH of 14 is strongly basic, and acid rain is not always basic or free from dissolved substances.

The governing concept is the reaction of acids with carbonates. Marble is mainly calcium carbonate, CaCO₃. Acids present in acid rain react with it, gradually dissolving or altering the stone and sometimes producing salts, water, and carbon dioxide. Therefore option A is correct. Acid rain cannot turn marble into gold; it is not neutral, and marble is a solid rock rather than a gas.

The governing concept is the chlor-alkali process. Common salt, sodium chloride, is dissolved in water to make brine. Electrolysis of brine produces sodium hydroxide in solution, chlorine gas at the anode, and hydrogen gas at the cathode: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Thus option A is correct; salt has many industrial uses and does not form only acids.

The governing concept is the strength of a base, which depends on the extent of ionisation in water and the concentration of hydroxide ions produced. Sodium hydroxide dissociates almost completely: NaOH(aq) → Na⁺(aq) + OH⁻(aq). Therefore it is a strong base, so option A is correct. Hydrogen ions indicate acidity, neutrality means pH 7, and sodium hydroxide is not a gas under ordinary conditions.

The governing concept is acid strength, which is related to the extent to which an acid ionises in water. Hydrochloric acid ionises essentially completely: HCl(aq) → H⁺(aq) + Cl⁻(aq), so it produces a high concentration of hydrogen or hydronium ions and shows strong acidity. Option A is correct. Hydroxide ions characterise bases, while a neutral substance would not show this acidic ionisation.

The governing concept is the identification of acids and bases using indicators and pH. A basic solution changes red litmus to blue, while a pH greater than 7 indicates alkalinity; pH 11 is therefore basic. Both independent observations support option A. An acidic solution has pH below 7 and generally turns blue litmus red, a neutral solution has pH about 7, and an ion-containing solution is not ion-free.