Acids, Bases, and Salts

The governing concept is qualitative gas testing. Carbon dioxide reacts with limewater, calcium hydroxide, to form insoluble calcium carbonate, making the liquid milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Hydrogen produces a characteristic pop when tested with a burning splint, but that result is absent here. Oxygen supports burning rather than turning limewater milky, and chlorine has different bleaching and reactivity tests. Therefore the gas is carbon dioxide, option A.

The governing concept is hydrolysis of salts and the resulting solution character. Sodium carbonate is formed from a strong base, sodium hydroxide, and a weak acid, carbonic acid. Its carbonate ions hydrolyse in water and generate hydroxide ions, so the solution is basic and can turn red litmus blue. Ammonium chloride and copper sulphate generally give acidic solutions, while sodium chloride is approximately neutral. Hence sodium carbonate, option A, is the suitable example.

The governing concept is the logarithmic definition of pH: pH = −log₁₀[H⁺]. If the hydrogen-ion concentration becomes 10 times larger, pH decreases by 1 unit. A hundredfold increase is 10², so the decrease is log₁₀(100) = 2 units. The change is therefore not 100 units and cannot be zero. Assuming comparable conditions and activities, option A gives the correct approximate difference.

The governing concept is the characteristic reaction of acids with metals and carbonates. Zinc, a sufficiently reactive metal, displaces hydrogen from a dilute acid: Zn + 2HCl → ZnCl₂ + H₂. Sodium carbonate reacts with an acid to produce a salt, water, and carbon dioxide: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Therefore the gases are hydrogen and carbon dioxide respectively. Option A is correct; the reverse pairing in B is wrong.

The governing concept is the nature of salts formed from strong or weak acids and bases. Sodium chloride is produced from hydrochloric acid, a strong acid, and sodium hydroxide, a strong base. Its ions do not significantly hydrolyse water, so its aqueous solution is approximately neutral with pH 7. Sodium carbonate is basic, ammonium chloride is acidic, and sodium hydroxide is a base rather than a salt. Therefore, option A is correct.

The governing concept is the bleaching action of chlorine-releasing compounds. Bleaching powder releases chlorine or active chlorine in suitable conditions, and this oxidising action destroys or removes coloured substances from fibres. Thus it is used to whiten textiles and paper. Gypsum is mainly used in plaster and cement, sodium chloride is common salt, and copper sulphate is not the standard textile bleaching agent. Therefore option A is correct.

The governing concept is water of crystallisation, the definite number of water molecules chemically associated with some salt crystals. Washing soda is commonly represented as Na2CO3·10H2O. On heating, some or all of this crystallisation water can be removed, changing the hydrated salt into a less hydrated or anhydrous form. Heat of neutralisation, litmus change, and metal displacement describe different processes. Hence option A is correct.

The governing concept is that pH reflects the relative concentration of hydrogen ions and hydroxide ions in an aqueous solution. Adding an acid generally increases hydrogen-ion concentration, so pH decreases. Adding a base increases hydroxide-ion concentration and usually lowers the relative hydrogen-ion concentration, so pH increases. Colour and the container do not determine pH, and pH can change readily. Therefore option A is correct.

The governing concept is the exothermic nature of dissolving many strong bases. When a strong base such as sodium hydroxide dissolves, hydration of its ions releases heat. If the solid is added rapidly or water is added carelessly, the solution may heat suddenly and splash corrosive material. Therefore the substance should be added slowly with suitable protection. It does not normally freeze, become an acid, or convert water into oxygen. Option A is correct.

The governing concept is the definition of a base in aqueous solution: it produces hydroxide ions or increases their concentration. Basic solutions turn red litmus paper blue. They generally turn phenolphthalein pink and methyl orange yellow, so the statements saying these indicators are always colourless or red are incorrect. Blue litmus is associated with acids turning it red, not with bases. Therefore option A is correct.

The governing safety principle is immediate dilution and removal of a corrosive chemical. The affected skin should be washed at once with a large amount of running water, while contaminated clothing is removed if appropriate and laboratory supervision or medical help is sought. Water reduces the acid concentration and carries it away. Adding a strong base can cause another corrosive reaction and heat, rubbing spreads damage, and doing nothing is unsafe. Hence option A is correct.

The governing concept is the pH scale and its relation to hydrogen-ion concentration. A pH below 7 indicates an acidic solution, which means the effective concentration of hydrogen ions, often represented as H+ or hydronium ions H3O+, has increased relative to hydroxide ions. Adding acid therefore shifts the ion balance toward hydrogen ions. Calcium and sodium ions are not the defining cause of acidity, while increased hydroxide ions would indicate a basic tendency. Option A is correct.

The governing concept is salt hydrolysis and the strength of the acid and base from which a salt is formed. A salt made from a strong acid and strong base, such as sodium chloride, is approximately neutral. A salt from a strong acid and weak base, such as ammonium chloride, is acidic, while one from a weak acid and strong base, such as sodium carbonate, is basic. Consequently, salts do not always have pH 7, making option A the incorrect statement.

The governing concept is thermal decomposition of a hydrogen carbonate. On heating, sodium hydrogen carbonate decomposes according to 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Carbon dioxide and water leave as gases, so sodium carbonate remains as the main solid residue. Sodium chloride and calcium salts are not products of this reaction; therefore option A is correct.

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.

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.

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.

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.