Acids, Bases, and Salts

Dilution increases the total volume while the original amount of dissolved acid is distributed through that larger volume. Consequently, the hydrogen-ion concentration, [H⁺] or more precisely [H₃O⁺] in water, decreases. Since pH = −log[H⁺], the pH rises and the solution becomes less acidic, although it does not necessarily become basic. Thus option A is correct; dilution does not destroy all ions or automatically produce a basic solution.

Sodium hydroxide is a strong, hygroscopic base that can absorb moisture and react with carbon dioxide from air. The acidic oxide CO₂ reacts with NaOH to form sodium carbonate, or sodium hydrogen carbonate under some conditions; for example, 2NaOH + CO₂ → Na₂CO₃ + H₂O. This converts part of the original substance into another compound and lowers its purity. Therefore option A is correct.

The governing concept is neutralisation. In an aqueous reaction, an acid supplies H⁺ ions, represented more accurately as H₃O⁺, and a base supplies OH⁻ ions. These ions combine to form water: H⁺ + OH⁻ → H₂O. The remaining positive and negative ions combine to form the salt, so the overall products are salt and water. Hence option A is correct; the other statements are not general ionic reactions.

Bleaching powder is commonly represented at school level as calcium oxychloride, CaOCl₂, and is a chlorine-releasing disinfectant. In contact with water or dilute acids, it can produce hypochlorous acid or chlorine-containing species that destroy many microorganisms. Therefore option A is correct for drinking-water disinfection. Gypsum is a calcium sulphate material, washing soda is sodium carbonate, and copper sulphate has different uses and is not the intended chlorine-releasing disinfectant here.

Hard water contains dissolved calcium and magnesium ions, which react with soap and form scum. Sodium carbonate supplies carbonate ions that can precipitate these ions as insoluble carbonates, such as calcium carbonate. Removing or reducing these ions decreases hardness, so option A is correct. Sodium carbonate does not convert water into an acid, generate chlorine, or vaporise it; those statements do not describe water softening.

The pH scale expresses hydrogen-ion concentration: pH 7 is neutral, and values below 7 indicate acidity. A value below 7 but far from zero shows an acidic solution, yet pH alone does not prove that it is highly corrosive; concentration and the substance’s properties also matter. Therefore option A is the most careful conclusion. Options B, C, and D contradict the meaning of an acidic pH.

The governing qualitative test is that carbon dioxide turns limewater milky by forming insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Acids react with carbonates to release carbon dioxide, so white calcium carbonate can produce this observation. Option A is therefore correct. Sodium chloride and copper sulphate do not normally release CO₂ with dilute acid, while sodium hydroxide neutralises acid instead of giving this gas.

Acid rain contains acidic substances formed when pollutants such as sulfur and nitrogen oxides react in the atmosphere. When this rain enters lakes, rivers, or ponds, it can lower their pH. Many aquatic organisms and their eggs require a limited pH range, so excessive acidity can disturb survival, reproduction, and mineral balance. Thus option A gives the correct general reason; the other claims are absolute or chemically incorrect.

The nature of a salt depends on the acid and base from which it is formed and on the ions’ reaction with water. Sodium chloride, from a strong acid and strong base, gives an approximately neutral solution. Sodium carbonate is basic, whereas ammonium chloride is acidic because of hydrolysis. Therefore option A is correct. The other choices wrongly claim that every salt has pH 7, never dissolves, or never forms ions.

For equal amounts and comparable concentrations, pH depends on the hydrogen-ion concentration, according to pH = −log[H⁺]. Complete ionisation releases a greater proportion of hydrogen ions than partial ionisation, so the first acid is more likely to have the larger [H⁺] and therefore the lower pH. Option A is correct. The second acid would generally have a higher pH, while neither acid is necessarily neutral.

Sodium hydrogen carbonate, NaHCO₃, is a mild base. In excess stomach acidity, it reacts with hydrochloric acid and reduces the concentration of excess acid: NaHCO₃ + HCl → NaCl + H₂O + CO₂. Therefore, option A is correct because neutralisation can provide relief. Option B is opposite to its action, while C and D do not describe an acid–base reaction.

Dilution adds water and increases the solution’s volume, while the amount of dissolved acid is not instantly removed. Consequently, the number of hydrogen ions per unit volume, or [H⁺], decreases. Since pH = −log[H⁺], a decrease in hydrogen-ion concentration causes pH to rise, although the solution may remain acidic. Therefore option A is correct; dilution does not make it a strong base.

Baking soda is sodium hydrogen carbonate, a basic salt. In baking powder, an acidic component reacts with it and produces carbon dioxide, which helps the dough rise, while also preventing an excess of unreacted alkaline material. If the acid is absent, some baking soda can remain after heating and impart a bitter or soapy taste. Hence option A is correct; the other claims do not describe this reaction.

A strong acid supplies H⁺ ions and a strong base supplies OH⁻ ions. During complete neutralisation, they combine in equal effective amounts: H⁺ + OH⁻ → H₂O. The remaining ions form a neutral salt when the acid and base are suitably matched, so at about 25°C the final solution has pH close to 7. Thus option A is correct; pH 3 or 11 would indicate excess acid or base.

Both sodium carbonate, Na₂CO₃, and sodium hydrogen carbonate, NaHCO₃, react with a dilute acid to release carbon dioxide. The reactions differ in their salt and water products, but the gas test is the same: CO₂ turns limewater milky because insoluble calcium carbonate forms. Thus option A is correct. A pop sound identifies hydrogen, not carbon dioxide, and chlorine is not produced here.

Ammonia gas itself needs moisture to show its basic behaviour. On the wet litmus, NH₃ dissolves in water and reacts: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The hydroxide ions make the moist paper basic, so red litmus turns blue. Therefore option A is correct. Dry ammonia cannot ionise effectively without water, and the other options contradict the indicator test.

Blue copper sulphate crystals are hydrated copper sulphate, commonly represented as CuSO₄·5H₂O. Heating removes their water of crystallisation and produces white anhydrous CuSO₄. When water is added again, hydration occurs and the characteristic blue hydrated form is restored. Hence option A is correct. No copper metal or sulphur gas is formed, and the colour change is not caused by the solution becoming basic.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, whereas baking soda is sodium hydrogen carbonate, NaHCO₃. Thus they differ in both composition and properties: washing soda is used in cleaning and water softening, while baking soda is used in baking and some neutralisation applications. Option A is correct; the other choices assign incorrect names or formulas to these salts.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions lose electrons and form chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻. At the cathode, water gains electrons and produces hydrogen gas and hydroxide ions. Sodium hydroxide remains in the solution. Therefore, chlorine is formed at the anode and hydrogen at the cathode, making option A correct; the other choices reverse or misidentify the products.

Bleaching powder is useful because it contains available chlorine, which is released during suitable reactions and produces the bleaching and disinfecting effect. On prolonged exposure to air, moisture and carbon dioxide, some of this available chlorine may be lost and the material gradually becomes less effective. Thus option A gives the relevant storage-related reason. It does not completely become sodium chloride, gypsum, or washing soda, so options B, C and D are unsuitable.

Indicator colours must be interpreted together with their transition ranges. Phenolphthalein is colourless in acidic, neutral and mildly basic solutions, becoming pink only in a more strongly basic range. Methyl orange is yellow when the solution is not sufficiently acidic, including neutral and basic conditions. Hence the observations are consistent with a neutral or slightly basic solution, so A is the best answer. A strong base would generally turn phenolphthalein pink.

A pH of 8.5 indicates that the soil is alkaline because it is above 7. To lower basicity, the soil should receive a suitable mildly acidic amendment rather than another strong base. Organic matter can decompose and produce acidic substances gradually, helping improve alkaline soil when used appropriately. Slaked lime, sodium hydroxide and soap are basic materials and would tend to maintain or increase alkalinity. Therefore option A is the most suitable choice.

Electrical conduction through a solution requires mobile charged particles. An acid ionises or reacts with water to produce hydronium ions and corresponding negative ions; these ions move toward opposite electrodes and carry electric current. Solid sodium chloride is not necessary, because dissolved ions from the acid are sufficient for conduction. Therefore option A is correct. Colour, the glass container and low temperature do not provide the mobile charge carriers required for an electric current.

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.