Acids, Bases, and Salts

In the chlor-alkali process, concentrated aqueous sodium chloride is electrolysed. Water is reduced at the cathode, producing hydrogen gas: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Hydrogen is combustible and, when a small sample is brought near a burning splint, it burns with a characteristic ‘pop’ sound. Therefore option B identifies the cathode gas. Limewater tests carbon dioxide, while the litmus and turmeric changes are not the hydrogen test.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and produces plaster of Paris, calcium sulphate hemihydrate, CaSO₄·½H₂O. If heating is excessive, still more water is driven off and anhydrous calcium sulphate may form. This material does not rehydrate and set in the desired way, so the useful plaster quality is reduced. Hence option A is correct; the other products are chemically unrelated.

Indicator colours reveal the approximate nature of a solution. Phenolphthalein is colourless in acidic and neutral media and turns pink in a sufficiently basic medium. Methyl orange is red in an acidic medium, yellow in a basic medium, and orange near its transition range. The simultaneous observations—colourless phenolphthalein and red methyl orange—therefore indicate an acidic solution. Option B is correct; a basic solution would make methyl orange yellow and usually phenolphthalein pink.

Sodium hydroxide is deliquescent, so its pellets absorb water vapour from humid air and may become wet or dissolve. It also reacts with carbon dioxide in the atmosphere: 2NaOH + CO₂ → Na₂CO₃ + H₂O. The formation of sodium carbonate changes the composition of the material and lowers its purity for uses requiring NaOH. Therefore option A correctly includes both moisture absorption and chemical reaction with CO₂; the other statements have no chemical basis.

Evolution of carbon dioxide when an acid is added is a characteristic test for a carbonate or hydrogencarbonate. Sodium carbonate reacts with an acid, for example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. In water, Na₂CO₃ undergoes hydrolysis and produces a basic solution because carbonate ions react with water to form hydroxide ions. Ammonium chloride is acidic, sodium chloride is nearly neutral, and copper sulphate is not the required carbonate. Thus option C is correct.

Tooth enamel is mainly a mineral containing calcium compounds, and acids can react with and gradually dissolve this mineral. When the pH in the mouth falls below about 5.5, the environment becomes sufficiently acidic for demineralisation of enamel to become significant, especially with repeated or prolonged exposure. Therefore option C is the most suitable range. A pH near 7 is approximately neutral, while values above 7 or near 14 are alkaline rather than acidic causes of typical enamel erosion.

The governing concept is the difference between electrical neutrality and the absence of ions. Sodium chloride dissolves in water and dissociates into Na⁺ and Cl⁻ ions. Their positive and negative charges are equal, so the solution is electrically neutral, but the mobile ions can still conduct electricity. Therefore option A is correct; neutrality does not mean that ions are absent, while the other options contradict the solubility and composition of sodium chloride.

The governing reaction is the reaction of a carbonate with an acid. Marble is composed mainly of calcium carbonate, CaCO₃. Acidic rain supplies hydrogen ions, and carbonate reacts to produce a calcium salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. This gradually erodes the surface, so option A is correct. The other choices misidentify marble or reverse the effect of acids.

The governing concept is thermal decomposition of sodium hydrogen carbonate, commonly called baking soda. On heating, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The released carbon dioxide accounts for the gas, while the remaining sodium carbonate dissolves to give a basic solution because carbonate ions hydrolyse water and produce hydroxide ions. Thus option B is correct; the other substances are not the solid product of this decomposition.

The governing concept is that the pH scale is logarithmic: pH = −log[H⁺]. The difference between pH 2 and pH 5 is 3 units. Therefore the hydrogen-ion concentration ratio is 10³ = 1000. The pH 2 solution has one thousand times more H⁺ ions and is consequently one thousand times more acidic, so option C is correct. A difference of three is not a threefold acidity ratio.

The governing concept is the removal of hardness-producing ions by precipitation. Hard water mainly contains dissolved Ca²⁺ and Mg²⁺ ions. Washing soda, Na₂CO₃, supplies CO₃²⁻ ions, which form sparingly soluble calcium carbonate and magnesium carbonate: Ca²⁺ + CO₃²⁻ → CaCO₃ and Mg²⁺ + CO₃²⁻ → MgCO₃. These precipitates can be separated, so option B is correct; sodium and chloride ions do not cause hardness.

The governing concept is neutralisation and correction of soil pH. Highly acidic soil has an excess of acidic substances and a low pH. Slaked lime, Ca(OH)₂, is a base; when applied in a carefully measured amount, it reacts with acidic components and raises the soil pH toward a more suitable range. Therefore option C is correct. Lemon juice, vinegar, and dilute acid are acidic and would generally intensify the problem rather than correct it.

The governing concept is the stability of bleaching powder and its available chlorine. Bleaching powder is used because it can release chlorine for bleaching and disinfection. Exposure to moisture and air promotes slow decomposition and loss of available chlorine, reducing its effectiveness. Thus option A is correct, and it should be stored in a dry, tightly closed container. The other options describe impossible or chemically unrelated changes, such as formation of gold or gypsum.

The governing concept is acid–base indicators and neutralisation. A red litmus paper turning blue initially shows that the solution is basic. As acid is added, it consumes hydroxide ions and neutralises the base. When neither red nor blue litmus changes colour, the solution is at the neutral point under the stated school-level conditions. Therefore option C is correct; highly acidic or basic solutions would affect at least one indicator.

The governing concepts are the pH scale and partial neutralisation. Both pH 3 and pH 4 are below 7, so the solution remains acidic. However, increasing pH by one unit means the hydrogen-ion concentration has fallen by a factor of ten: [H⁺] at pH 4 is one-tenth of that at pH 3. Therefore acidity decreased but did not disappear, making option A correct. It did not become a strong base.

The governing concept is the basic nature of most metal oxides. A basic metal oxide reacts with an acid in a neutralisation reaction, producing a salt and water; for example, CuO + 2HCl → CuCl₂ + H₂O. Thus option A correctly describes their usual behaviour. Option B is wrong because metal oxides are not always acidic, while C and D have no chemical basis. Some metal oxides can be amphoteric, but that does not change the general rule.

The governing indicator principle is that an acidic solution changes blue litmus to red. Many non-metal oxides, such as carbon dioxide and sulphur dioxide, dissolve or react with water to form acidic solutions. Therefore the oxide described is acidic, so option B is correct. Option A would be supported by a red-to-blue change, whereas metallic and neutral-solid descriptions do not explain the observed litmus result. The test concerns the solution formed in water.

The governing concept is an acid–base reaction that releases a gas. Baking soda, sodium hydrogen carbonate (NaHCO₃), reacts with the edible acid when moisture is present, and heating helps the reaction proceed: acid + NaHCO₃ → salt + water + CO₂. The carbon dioxide bubbles expand in the batter and make baked food rise and become porous. Therefore A is correct; chlorine, metal, and permanent acidity are not products or purposes of this mixture.

The governing concept is water of crystallisation. Hydrated copper sulphate, CuSO₄·5H₂O, is blue because water molecules are incorporated in its crystal structure. On heating, it loses this water and becomes white anhydrous CuSO₄. Adding water restores the hydrated form and the blue colour. Therefore option B is correct. Sodium chloride and sodium carbonate are not identified by this reversible blue–white change, while calcium carbonate is not a blue hydrated salt.

The governing safety principle is immediate dilution and removal of the corrosive substance. The affected skin should be washed with a large amount of running water as soon as possible, which reduces the acid concentration and carries it away. Hence option A is correct. Applying a strong base is unsafe because neutralisation may release heat and cause another chemical injury. Rubbing can spread or damage the skin, and waiting allows the acid to penetrate further; medical help should be sought when needed.