Acids, Bases, and Salts

Baking soda is a basic salt. When a suitable edible acid is added and the mixture becomes moist or warm, an acid–carbonate reaction produces carbon dioxide, which raises the batter. The acid also neutralises excess sodium hydrogen carbonate, reducing the unpleasant alkaline or bitter taste and limiting unwanted residue. Oxygen, sodium chloride, and burning are not the intended results.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. Chloride ions are oxidised at the anode to chlorine gas, while water is reduced at the cathode to hydrogen gas and hydroxide ions. Sodium ions combine with hydroxide ions in solution to form sodium hydroxide. Thus chlorine and sodium hydroxide are in option A; the other pairs contain incorrect elements or products.

In the chlor-alkali process, brine is electrolysed. At the cathode, water gains electrons and is reduced: 2H2O + 2e− → H2 + 2OH−. Therefore hydrogen gas is evolved at the cathode. Chlorine is produced at the anode by oxidation of chloride ions, while oxygen and carbon dioxide are not the expected cathode products under these conditions.

Chlorine is a strong disinfectant because it reacts with water to form substances such as hypochlorous acid, which can destroy many microorganisms. Controlled chlorination is therefore used to make drinking water safer. Antacids reduce stomach acidity, carbon dioxide makes cakes rise, and chlorine is not normally selected simply to acidify soil. Hence option A correctly matches the property with its use.

Hydrochloric acid reacts with sodium hydroxide in a neutralisation reaction: HCl + NaOH → NaCl + H2O. A measurable temperature rise means that the reaction releases heat to the surroundings, so it is exothermic. The formation of water and salt confirms a chemical change. Therefore option A is correct; an endothermic process would absorb heat and usually lower the temperature without external heating.

A base is a substance that can neutralise an acid, whereas an alkali is specifically a base that dissolves in water to give hydroxide ions. Turning red litmus blue indicates basic character, but the statement says the substance is only slightly soluble. Therefore the broader and safer term is base. An alkali would require clear water solubility; a strong acid would show the opposite litmus change.

Ammonium chloride is formed from hydrochloric acid, a strong acid, and ammonium hydroxide, a weak base. In water, the ammonium ion undergoes hydrolysis and produces hydronium ions, making the solution acidic. Sodium carbonate gives a basic solution, while sodium chloride and potassium nitrate are generally neutral because they come from strong acid–strong base combinations. Thus A is the only acidic salt.

Carbon dioxide is a non-metal oxide and reacts with water to form carbonic acid, so it is acidic. Calcium oxide is a metal oxide and reacts with water to form calcium hydroxide, a base, so it is basic. Option B reverses the required order. Sodium oxide and magnesium oxide are basic metal oxides, while sulfur dioxide and carbon dioxide are both acidic non-metal oxides. Therefore A is correct.

The governing concept is neutralisation between an acidic oxide and a base. Carbon dioxide, CO₂, is an acidic oxide, while calcium hydroxide, Ca(OH)₂, is a base. They react as CO₂ + Ca(OH)₂ → CaCO₃ + H₂O, producing calcium carbonate and water. Thus option A directly represents the required reaction. The other choices show metal–water, metal–acid, and combination reactions, not acidic oxide–base neutralisation.

Water of crystallisation is the definite number of water molecules chemically associated with one formula unit of a hydrated salt. Washing soda is hydrated sodium carbonate, Na₂CO₃·10H₂O, so it contains ten water molecules of crystallisation. Baking soda is NaHCO₃ and does not contain such water in its formula; sodium chloride and calcium oxide are also represented here as anhydrous substances. Therefore option A is correct.

Washing soda is the common name for hydrated sodium carbonate. The sodium carbonate part is Na₂CO₃, and the crystal contains ten molecules of water of crystallisation, so its complete formula is Na₂CO₃·10H₂O. Sodium bicarbonate, NaHCO₃, is baking soda. CaSO₄·½H₂O is plaster of Paris, whereas CaSO₄·2H₂O is gypsum. Hence option A is unambiguously correct.

Plaster of Paris is calcium sulphate hemihydrate. During its preparation, gypsum, CaSO₄·2H₂O, is heated carefully so that part of its water of crystallisation is removed, giving CaSO₄·½H₂O. Therefore option A is correct. Option B is gypsum, the starting material; option C is washing soda; and option D is baking soda. The half-water notation represents the average water content per calcium sulphate unit.

Gypsum is CaSO₄·2H₂O, and controlled heating removes only part of its water of crystallisation to produce plaster of Paris, CaSO₄·½H₂O. If heating is excessive, dehydration can proceed too far and the desired hemihydrate may not be obtained, reducing the material’s useful setting behaviour. Thus option A is correct. The other choices introduce unrelated sodium or chlorine compounds and do not describe gypsum dehydration.

Hard water contains dissolved calcium and magnesium salts. Washing soda, Na₂CO₃·10H₂O, supplies carbonate ions that react with soluble calcium or magnesium ions to form insoluble carbonates, which can be removed. This decreases the ions responsible for hardness, so option A is correct. Vinegar and lemon juice are acids, while common salt does not generally precipitate the hardness-causing ions and therefore is not the standard softening substance.

Sodium hydroxide, NaOH, is a strong alkali. In concentrated form or even at significant laboratory exposure, it is highly corrosive and can damage living tissue, causing severe irritation or chemical burns. Therefore option A correctly explains the safety risk. It does not freeze skin, is not a neutral salt, and its effect is not limited to colour change. Gloves, eye protection, and proper handling are essential.

Using the stated classroom model, the bee sting is treated as acidic. Baking soda, sodium hydrogen carbonate (NaHCO₃), is a mild base, so a properly prepared mild paste may help neutralise some of the acidic effect. Therefore option A is the best answer among those given. Lemon juice, vinegar, and hydrochloric acid are acidic and would not provide the intended neutralisation. In real cases, medical guidance should be followed if symptoms are severe.

Under the stated school-level assumption, a wasp sting is basic in nature. Vinegar contains acetic acid and is a mild acid, so it can theoretically neutralise part of the basic effect; hence option A is the intended answer. Baking soda, slaked lime, and sodium hydroxide are basic or alkaline substances and would not provide the required acid–base neutralisation. Actual stings should be managed carefully, especially if allergic symptoms appear.

The governing concept is acid–base neutralisation. Excess hydrochloric acid in the stomach can cause acidity and irritation. Antacids contain mild bases, such as magnesium hydroxide or suitable carbonate compounds, which react with excess acid and reduce its acidity. Therefore option A is correct. An antacid is not intended to increase acid, convert food into acid, or fill the stomach with hydrogen gas; those statements do not describe its function.

The governing concept is the behaviour of acids in aqueous solution. Turning blue litmus red is the characteristic test for an acidic solution, which contains hydrogen ions, represented as H+ or hydronium ions in water. These mobile ions also carry electric charge, explaining the good conductivity. Hydroxide ions indicate a base, while chloride and sodium ions alone do not establish acidity.

The governing concept is ionisation in water. Hydrochloric acid dissociates almost completely into H+ (more precisely H3O+) and Cl− ions, and these mobile charged particles conduct electricity. Sugar and alcohol may dissolve, but their molecules remain largely uncharged, so their solutions conduct very poorly. Distilled water contains only a tiny concentration of ions and is therefore a very poor conductor compared with hydrochloric acid.