Acids, Bases, and Salts

Electric current in a liquid requires mobile charged particles. When an acid dissolves in water, it ionises or dissociates to produce ions such as H₃O⁺ and a corresponding negative ion. These ions move between the electrodes and carry charge, so conductivity is evidence for their presence. Therefore option A is correct. Colour change, loss of mass, or freezing does not by itself prove that ions are present.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, commonly called brine. At the anode, chloride ions form chlorine gas; at the cathode, water produces hydrogen gas and hydroxide ions. The sodium ions combine with hydroxide ions to form sodium hydroxide in solution. Hence chlorine and sodium hydroxide are valid products, making option A correct. The other pairs belong to unrelated reactions or processes.

Limewater is a dilute solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is bubbled through it, the reaction Ca(OH)₂ + CO₂ → CaCO₃ + H₂O occurs. Calcium carbonate is insoluble and forms a white suspension, making the limewater milky. This visible, characteristic change makes option A correct. The other options give products or colour changes that do not describe this identification test.

A pH of 9 indicates that the salt solution is mildly basic. A salt made from a weak acid and a strong base, such as sodium acetate in the usual school-level example, undergoes hydrolysis: the anion reacts with water and produces OH⁻ ions. Therefore option A is the most probable condition. Strong acid–weak base salts are generally acidic, strong acid–strong base salts are approximately neutral, and acid alone does not define a salt.

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.

The acidic effect of an ant sting can be reduced by neutralisation with a mild base. Baking soda, or sodium hydrogen carbonate (NaHCO₃), is mildly basic, so a paste of it can help counter the acid. Therefore, option A is correct. Lemon juice and vinegar are acidic, and dilute hydrochloric acid is also an acid, so none of them would neutralise the acidic effect. Exam tip: for an acidic sting or substance, choose a suitable mild base.

Acidic behaviour in the school-level aqueous model depends on the formation and movement of ions, especially H₃O⁺. Many substances that are acids do not release mobile hydrogen-containing ions in the dry state, so dry hydrogen chloride, for example, does not change dry litmus. In water, ionisation occurs and the solution conducts electricity and shows acidic properties. Thus option A is necessary; darkness, low temperature, or a metal container is not the governing condition.

The governing concept is that acids commonly occur in several natural food substances. Lemon juice contains citric acid, vinegar contains ethanoic acid, and tamarind contains organic acids such as tartaric acid. Therefore, option A is the only group in which all listed substances are acidic. The other groups contain soaps, hydroxides, carbonates, or antacids, which are basic or alkaline rather than acidic.

The governing concept is the identification of bases by their alkaline nature. Soap solution is generally basic, lime water contains calcium hydroxide and is basic, and sodium hydroxide is a strong base. Thus option A contains only basic substances. The substances in options B and C are acidic foods or liquids, while option D contains strong mineral acids and lemon juice, so those groups cannot be correct.

The governing reaction is: acid + reactive metal → salt + hydrogen gas. For example, zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen. Hydrogen is combustible and produces the characteristic ‘pop’ when tested with a burning splint, so option A is correct. Carbon dioxide generally extinguishes a flame, while chlorine and ammonia do not give this standard pop test.

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.

Red litmus turning blue indicates that the solution is basic. Lime water is an aqueous solution of calcium hydroxide, \(\mathrm{Ca(OH)_2}\). When carbon dioxide is passed through it, a white precipitate of calcium carbonate forms: \(\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O}\). Therefore, option A is correct. Sodium chloride solution is generally neutral, whereas vinegar and dilute sulphuric acid are acidic, so they do not turn red litmus blue. Exam tip: The milky appearance of lime water on passing \(\mathrm{CO_2}\) is a key test for calcium carbonate formation.

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.