Acids, Bases, and Salts

The governing concept is the pH scale and the indicator behaviour of bases. A soapy feel suggests that the solution is alkaline, while turning red litmus blue is a direct test for a base. Basic solutions have a hydrogen-ion concentration lower than that of neutral water, so their pH is greater than 7. Option B describes an acidic solution, and C and D are fixed values that do not follow from the observations. Therefore, option A is correct.

The governing concept is acid-base neutralisation. Blue litmus turning red first shows that the original solution is acidic. Sodium hydroxide is a soluble base, so its hydroxide ions react with hydrogen ions from the acid to form water: H⁺ + OH⁻ → H₂O. As sodium hydroxide is added, acidity decreases and the pH moves toward 7 near the equivalence point. The other choices contradict this reaction. Thus option A is correct.

The governing concept is exothermic dilution. When many concentrated, especially strong, acids dissolve and ionise in water, hydration of the ions releases energy as heat. This increases the temperature of the solution. The warming is not caused by freezing, conversion of acid into a metal, or burning litmus; none of those processes explains ordinary acid dilution. Because the released heat can cause splashing, acid should be added slowly to water with stirring, never water rapidly into concentrated acid. Option A is correct.

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 colour change of a natural acid-base indicator. Turmeric contains curcumin, which remains yellow in its ordinary form but changes to a reddish-brown colour in a basic medium. Therefore turmeric paper is especially useful for detecting alkaline solutions. Neutral water generally causes no characteristic change, and the test is not a universal test for gases or metals. The observation identifies basicity rather than a particular substance, so option A is correct.

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 indicator colour change with pH. Methyl orange is a synthetic acid-base indicator that appears red in an acidic medium, changes through orange in its transition range, and appears yellow in a sufficiently basic medium. Because the question specifies an acidic solution, the correct observation is red. Yellow corresponds to the basic side, while blue and pink are not the standard methyl-orange colours for this test. Therefore option A is correct.

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 concept is neutralisation, in which an acid reacts with a base to form salt and water, often with heat release. Excess hydrochloric acid in the stomach causes acidity, while a mild basic antacid reacts with and reduces that acid. Dissolving sugar is not an acid-base reaction, and drying cloth or melting ice are physical changes. Therefore using an antacid for acidity is the valid daily-life example, so 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.