Acids, Bases, and Salts

The governing concept is the indicator test for bases. A substance that turns red litmus blue produces a basic aqueous medium, so the metal oxide behaves as a basic oxide under the stated conditions. Many metal oxides form hydroxides with water, for example sodium oxide forms sodium hydroxide. An acidic oxide would turn blue litmus red; colour alone cannot determine chemical nature, and the substance is not a gas.

The governing concept is the colour response of litmus indicators. Acids change blue litmus to red because they provide H+ ions in water, whereas bases change red litmus to blue because they provide OH− ions or accept protons. Therefore option A states both changes in the correct direction. Option B reverses them, while C and D incorrectly deny indicator effects and the differing pH ranges.

A pH below 7 identifies the solution as acidic. Acids react with metal carbonates according to the pattern acid + carbonate → salt + water + carbon dioxide. Carbon dioxide is confirmed by passing it through limewater; insoluble calcium carbonate forms and makes the limewater milky. A glowing splint is associated with oxygen, while the litmus and white-fume choices do not identify carbon dioxide.

The governing concept is the pH scale: lower pH means greater acidity, pH 7 is neutral at ordinary school-level conditions, and values above 7 indicate increasing basicity. Thus moving from pH 2 to pH 7 reduces acidity to neutrality, and continuing to pH 10 increases basicity. Option B reverses the direction; C and D do not show a continuous decrease in acidity with increasing basicity.

The governing concept is the identification of salts from their reactions and hydrolysis. Carbon dioxide released with an acid indicates a carbonate or hydrogencarbonate ion. Sodium carbonate is a white solid and its solution is basic because CO3^2− hydrolyses water to produce OH− ions. Sodium chloride is nearly neutral, copper sulphate is not the expected carbonate test substance, and ammonium chloride gives an acidic solution.

For a sufficiently strong acidic solution, pH = −log[H+]. Tenfold dilution reduces the hydrogen-ion concentration by a factor of 10, so the logarithm changes by one unit and the pH increases by approximately 1. Starting from pH 3, the new value is about pH 4. It does not fall to 2, jump to neutral 7, or become the basic value 12.

The governing relation is pH = −log10[H+]. A decrease of one pH unit means a tenfold increase in hydrogen-ion concentration. Here the pH decreases by two units, from 4 to 2, so [H+] increases by 10^2 = 100 times. Therefore option A is correct. The decrease option reverses the relationship, and a twofold increase ignores the logarithmic scale.

The governing concept is salt hydrolysis. A pH of 3 is distinctly acidic, so the salt produces excess H3O+ ions in water. Salts formed from a strong acid and a weak base, such as ammonium chloride from HCl and NH4OH, commonly give acidic solutions because the conjugate acid of the weak base hydrolyses. A weak-acid/strong-base salt is basic, a strong/strong salt is approximately neutral, and weak/weak salts are not always neutral.

Since pH 9 is greater than 7, the salt solution is basic. A salt formed from a weak acid and a strong base has an anion that reacts with water to produce
OH^-
ions, making the solution basic. A salt of a strong acid and weak base is generally acidic, while one from a strong acid and strong base is approximately neutral. Exam tip: For pH > 7, usually identify a salt of a weak acid and a strong base.

Electrical conductivity in an aqueous acid solution is caused by the movement of ions. When concentration and other conditions are the same, greater conductivity indicates a greater concentration of mobile ions, usually because the acid ionises more extensively. Therefore, the first acid forms more ions in solution, so option A is correct. Option B is opposite to the evidence, while C and D contradict the observed conductivity.

The acidic effect of an aqueous acid depends mainly on the concentration of hydrogen ions, represented as H₃O⁺ in water. Hydrochloric acid is a strong acid and ionises almost completely, whereas ethanoic acid is weak and ionises only partially at the same concentration. Thus hydrochloric acid produces more hydrogen ions and has the stronger effect, making option A correct. The other statements are chemically false.

Hydrogen chloride shows its acidic behaviour only when water allows it to ionise. In water, HCl produces hydronium ions, H₃O⁺, which cause blue litmus to turn red. Dry HCl gas and dry litmus do not provide the aqueous medium needed for this ion formation, so no colour change occurs. Therefore option A explains both observations; the other choices are incorrect.

Ammonia itself is a covalent gas, but in the presence of water it dissolves and reacts to produce hydroxide ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The hydroxide ions make the solution basic and turn red litmus blue. Dry litmus lacks water, so this ion formation and colour change cannot occur. Hence option A is correct; the remaining choices misidentify the substance or indicator.

Carbonates react with acids to release carbon dioxide, for example: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Sodium carbonate is also a salt of a strong base and a weak acid, so its aqueous solution is alkaline because carbonate ions hydrolyse water and produce OH⁻ ions. Sodium chloride is neutral, copper sulphate is not the expected carbonate, and ammonium chloride is acidic. Thus option A is correct.

An acid reacts with a hydrogen carbonate to form a salt, water, and carbon dioxide: acid + hydrogen carbonate → salt + H₂O + CO₂. Passing the gas through limewater produces insoluble calcium carbonate, making the limewater milky. A pop sound is the test for hydrogen, while litmus and phenolphthalein indicate acidity or basicity rather than confirming this gas. Therefore option A is correct.

Initially, carbon dioxide reacts with limewater, calcium hydroxide, to form insoluble calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The suspended CaCO₃ causes milkiness. When excess CO₂ is passed, the precipitate reacts further with water and carbon dioxide to form soluble calcium hydrogen carbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. The solid dissolves, so the milkiness disappears and option A is correct.

Diluting a concentrated acid is highly exothermic: hydration of the acid particles releases heat. Adding acid slowly to a larger volume of water allows the released heat to spread through the water and be dissipated more safely, reducing the chance of sudden boiling and dangerous splashing. Water should not be poured into concentrated acid, because a small amount of water can heat rapidly and splatter acid. Thus option A is correct.

The governing concept is that dilution of a concentrated acid is highly exothermic, meaning it releases considerable heat. If a small amount of water is poured suddenly into acid, the water may heat and boil locally, causing the acid to splash violently. Therefore option A is correct. The solution does not freeze, acid cannot change into sugar, and option D is wrong because heat is released. For safety, acid should be added slowly to water with stirring.

The governing concept is the deliquescent and reactive nature of sodium hydroxide. Solid NaOH absorbs moisture from air, so it becomes wet and its composition is altered. It also reacts with atmospheric carbon dioxide to form sodium carbonate: 2NaOH + CO2 → Na2CO3 + H2O. Thus option A is correct. Sodium hydroxide does not form gold with oxygen, does not make salt with nitrogen, and clearly does change in moist air.

The governing concept is the limited stability of bleaching powder, whose disinfecting and bleaching action depends on available chlorine. Moisture and prolonged exposure to air promote decomposition and reduce the amount of effective chlorine. A dry, tightly closed container slows this deterioration, so option A is correct. The purpose is not to convert bleaching powder into gypsum or sodium chloride, and taste has no relation to its storage or chemical activity.