Acids, Bases, and Salts

The governing test for carbonate ions uses acidification followed by limewater confirmation. A carbonate reacts with a dilute acid to release carbon dioxide: CO₃²⁻ + 2H⁺ → CO₂ + H₂O. Passing this gas into limewater produces calcium carbonate, making it milky. Smelling, magnet testing, or dry litmus cannot confirm carbonate ions, so A is correct.

Permanent hardness is caused mainly by dissolved calcium and magnesium sulphates or chlorides, which do not disappear on boiling. Washing soda, Na₂CO₃, supplies carbonate ions that precipitate these ions as insoluble calcium or magnesium carbonates. Removing them softens the water. Vinegar, lemon juice, and hydrochloric acid add acidity rather than removing hardness, so option A is correct.

The governing concept is the behaviour of acid–base indicators. An acid changes blue litmus to red, while a base changes red litmus to blue. If neither paper changes colour, the solution is consistent with a neutral solution, although this observation alone may not establish exact pH. It cannot prove a strong acid or base, and a solution is not necessarily a gas. Thus A is the most appropriate conclusion.

Moisture is essential because the gas must dissolve in the thin water layer before acidic ions can affect the indicator. Turning blue litmus red shows that the resulting aqueous solution is acidic. This observation does not identify one particular gas and does not imply hydrogen; nor does it indicate a base. Therefore option A correctly describes the evidence without making an unjustified identification.

A base is a substance that reacts with an acid in a neutralisation reaction to form salt and water, for example CuO + 2HCl → CuCl₂ + H₂O. Bases need not dissolve in water. An alkali is specifically a water-soluble base, so the stated insolubility rules out that term. The substance is neither an acid nor an indicator; therefore A is correct.

The governing principle is laboratory safety and objective testing. Some acids are corrosive, poisonous, or harmful to body tissues, so tasting an unknown substance can cause injury and is never an acceptable identification method. Indicators, pH paper, and controlled chemical tests provide safer evidence of acidic behaviour. Acids can have tastes, so B is false; taste cannot measure pH reliably, and indicators are not limited to metals. Hence A is correct.

A universal indicator gives a colour related to pH. Deep red corresponds to a strongly acidic solution with a low pH, whereas violet or purple corresponds to a strongly basic solution with a high pH. Thus the first solution has greater acidic character and the second has greater basic character. Neutral solutions would be near green, and real solutions contain ions; therefore A is correct.

Electrical conduction in an aqueous acid solution depends on the number and mobility of charged particles, not on colour. At equal concentration, the acid that ionises more completely produces more mobile H₃O⁺ and counter-ions, so it conducts better; this is the behaviour of a stronger acid relative to the other. A weak acid ionises only partially. Thus A gives the correct reason, while B, C, and D are irrelevant or false.

The governing relation is pH = −log₁₀[H⁺]. A rise of one pH unit means a tenfold decrease in hydrogen-ion concentration. The change from pH 2 to pH 5 is three units, so [H⁺] changes by 10³ = 1000 in the decreasing direction. For example, 10⁻² mol L⁻¹ becomes 10⁻⁵ mol L⁻¹. Therefore option A is correct; it is not merely a threefold change.

The governing test is the carbonate test. Carbon dioxide turns limewater milky by forming insoluble calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. When excess CO₂ is passed, soluble calcium hydrogen carbonate forms, so the milkiness disappears. Carbonates and hydrogen carbonates release CO₂ with dilute acids; ordinary chlorides, sulphates and nitrates do not give this characteristic result. Therefore option A is correct.

The governing concept is salt hydrolysis and the pH of salt solutions. A pH of 5 is below 7, so the solution is acidic. Salts produced from a strong acid and a weak base generally contain a cation that reacts with water to generate hydronium ions, making the solution acidic; ammonium chloride is a familiar example. A weak-acid/strong-base salt is usually basic, while a strong-acid/strong-base salt is approximately neutral. Thus A is correct.

Phenolphthalein is pink in a sufficiently basic medium and becomes colourless when the pH falls below its transition range, roughly pH 8.2–10. As acid is added to the base, hydroxide ions are consumed in the neutralisation reaction H⁺ + OH⁻ → H₂O. Therefore disappearance of the pink colour indicates that the mixture has approached the indicator endpoint, usually near neutralisation. It does not prove metal formation, compulsory CO₂ production or salt vapour. Option A is correct.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions are oxidised to chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻. At the cathode, water is reduced to hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Sodium ions remain in solution and combine with hydroxide ions to give aqueous NaOH. Hence option A is correct; sodium metal is not deposited in this aqueous process.

The governing pattern is neutralisation: an acid reacting with a base produces salt and water. Metal oxides such as copper(II) oxide or magnesium oxide consume acid and form corresponding salts, for example CuO + 2HCl → CuCl₂ + H₂O. Since the oxide neutralises H⁺ ions, it behaves as a basic oxide. An acidic oxide would generally react with a base instead, and the other choices do not describe this chemical behaviour. Therefore option A is correct.

Blue litmus turning red is the characteristic test for an acidic medium. If a non-metal oxide dissolves in water and produces such a solution, the oxide has formed an acid, for example SO₂ + H₂O → H₂SO₃ or CO₂ + H₂O ⇌ H₂CO₃. This is why many non-metal oxides are classified as acidic oxides. Basic oxides would turn red litmus blue, while a neutral salt or metallic base does not fit the observation. Thus A is correct.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O, so it contains water of crystallisation. Controlled heating removes part of this water and produces plaster of Paris, CaSO₄·½H₂O: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. If heating is too strong or prolonged, still more water is removed and anhydrous calcium sulphate, CaSO₄, can form. The other options introduce substances or products not involved. Therefore A is correct.

Indicator colour ranges provide the governing evidence. Methyl orange is yellow in a neutral or alkaline medium and red in a sufficiently acidic medium. Phenolphthalein is pink in an alkaline medium and colourless in acidic or near-neutral conditions. Seeing both yellow methyl orange and pink phenolphthalein therefore indicates that the solution is basic, sufficiently above the phenolphthalein transition range. An acidic solution would make methyl orange red and phenolphthalein colourless. Hence option A is correct.

Acidic properties in aqueous solution arise from hydronium ions, H₃O⁺, formed when an acid transfers a proton to water. Hydrogen chloride ionises in water: HCl + H₂O → H₃O⁺ + Cl⁻. These mobile ions account for acidic indicators, electrical conduction and reactions with bases. Dry HCl gas does not show the same aqueous acidic behaviour because water is needed for ion formation. Therefore option A expresses the correct reason; the other choices are chemically incorrect.

Electrical conduction in an acid solution depends on mobile ions, not simply on the presence of acid molecules. Water provides the medium in which many acids ionise and in which ions can move through the solution. With very little water, ionisation and ionic mobility may be reduced, so the measured conductance can be low; a very viscous or poorly hydrated medium also limits movement. The acid is not inherently always non-conducting, and colour or container material is not the governing reason. Thus A is correct.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, so its crystals contain water of crystallisation. When exposed to dry air, some of this water may be lost through efflorescence. The hydrated crystals then lose their clear crystalline appearance and may become opaque, crumbly, or powder-like. Therefore, option A is correct; no hydrogen gas, copper sulphate, or acid is formed.