Acids, Bases, and Salts

For an aqueous solution, pH 7 represents neutrality under ordinary school-level conditions. At this point, the acidic effect associated with hydrogen ions and the basic effect associated with hydroxide ions are considered balanced. Hence option C is correct. Option A would describe an acidic solution with pH below 7, and option B would describe a basic solution with pH above 7. A pH value does not mean that water is absent, so option D is incorrect.

A universal indicator is a mixture of indicators that displays different colours over a range of pH values. Green is conventionally associated with a pH near 7, the neutral point of the scale. Thus, in the usual school interpretation, green indicates a neutral solution, making option B correct. Acidic solutions generally produce yellow, orange, or red shades, whereas strongly basic solutions tend toward blue or violet shades.

The colour of a universal indicator changes according to the pH of a solution. Red is found at the low-pH end of the scale, where the concentration of hydrogen ions is relatively high and the solution is strongly acidic. Therefore option A is correct. A neutral solution near pH 7 appears green, a mildly basic solution may appear blue-green, and a strongly basic solution generally gives a violet or purple shade.

Universal-indicator colours are linked to the pH scale. Violet or purple occurs near the high-pH end, where hydroxide-ion concentration is relatively high and the solution shows strong basicity. Consequently, option C is correct. Acidic solutions have pH values below 7 and usually give yellow-to-red shades, while a neutral solution near pH 7 is green. A slightly acidic solution cannot normally produce the violet colour associated with strong alkalinity.

An active metal displaces hydrogen from a dilute acid. The general reaction is: metal + acid → salt + hydrogen gas. For example, zinc reacts with dilute hydrochloric acid as Zn + 2HCl → ZnCl₂ + H₂. Thus option A is correct. Carbon dioxide is typically released when an acid reacts with a carbonate, while oxygen and chlorine are not the general products of an acid-active-metal reaction.

Hydrogen is a combustible gas, so it can burn rapidly when mixed with air. In the standard laboratory test, a burning splint is brought near the mouth of the container; hydrogen ignites with a characteristic ‘pop’ sound. Hence option A is correct. A whistling, rattling, or ordinary blast sound is not the accepted identification test. The test should be performed carefully because hydrogen-air mixtures may ignite suddenly.

Acids react with carbonates to produce three products: a salt, water, and carbon dioxide. The general equation is acid + carbonate → salt + water + CO₂. For example, hydrochloric acid reacts with calcium carbonate to form calcium chloride, water, and carbon dioxide. Therefore option B is correct. Hydrogen is associated with acid-metal reactions, whereas nitrogen and oxygen are not the characteristic gases released by acid-carbonate reactions.

Lime water is a clear solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, the reaction Ca(OH)₂ + CO₂ → CaCO₃ + H₂O occurs. The calcium carbonate formed is insoluble and appears as a white suspension, making the lime water milky. Therefore option A is correct. The red, blue, and black changes are not the characteristic result of this carbon-dioxide test.

Lime water contains dissolved calcium hydroxide. When carbon dioxide is passed through it, calcium carbonate forms according to Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble in water and produces a fine white suspension, which causes the observed milkiness. Therefore option A is correct. Calcium hydrogen carbonate is soluble in water, while sodium chloride and sodium hydroxide do not form the white precipitate responsible for this observation.

When carbon dioxide is first passed through lime water, insoluble calcium carbonate forms and makes the solution milky: Ca(OH)2 + CO2 → CaCO3 + H2O. If excess CO2 is passed, the calcium carbonate reacts further with carbon dioxide and water to form soluble calcium hydrogen carbonate: CaCO3 + CO2 + H2O → Ca(HCO3)2. Because the precipitate dissolves, the milkiness decreases. The other options do not explain this observation.

The reaction in which an acid reacts with a base is called neutralisation. The acidic effect of hydrogen ions and the basic effect of hydroxide ions are reduced because H+ and OH− combine to form water; the remaining ions generally form a salt. For example, HCl + NaOH → NaCl + H2O. Evaporation, filtration, and melting are physical processes, not acid-base reactions, so option A is correct.

Neutralisation is an acid-base reaction. The hydrogen ions supplied by the acid combine with hydroxide ions supplied by the base to produce water: H+ + OH− → H2O. The positive and negative ions left from the acid and base combine to form a salt. Thus, the principal products are salt and water, as in HCl + NaOH → NaCl + H2O. The other choices are unrelated to ordinary neutralisation.

The essential ionic step in neutralisation is H+ + OH− → H2O. Acids furnish hydrogen ions in aqueous solution, while soluble bases furnish hydroxide ions. These ions combine to form water; spectator ions such as Na+ and Cl− remain in solution and can form sodium chloride. Calcium-carbonate and copper-sulphate pairs do not represent the general water-forming step, so option A is correct.

Sodium hydroxide is a soluble base and dissociates in water: NaOH(aq) → Na+(aq) + OH−(aq). The hydroxide ions increase the concentration of OH− in the solution and are responsible for its basic properties, such as changing indicators and neutralising acids. Sodium ions do not cause basicity, while hydrogen, carbonate, and chloride ions are not the characteristic ions released by NaOH in this process. Therefore option B is correct.

Hydrochloric acid ionises almost completely in water: HCl(aq) → H+(aq) + Cl−(aq), with H+ more accurately represented in water as H3O+. The hydronium or hydrogen-ion concentration gives the solution its acidic properties and allows it to react with bases. Hydroxide ions indicate basicity, whereas sodium and carbonate ions are not produced by hydrochloric acid. Hence option A is correct.

Acid strength is related to the extent of ionisation in water, not simply to how concentrated the acid is. Hydrochloric acid ionises almost completely: HCl + H2O → H3O+ + Cl−. This produces a high concentration of hydronium ions for a given concentration of acid, so HCl is classified as a strong acid. It is soluble in water and is neither a base nor a salt, which eliminates options B, C, and D.

Ethanoic acid is a weak acid because only a fraction of its molecules ionise in water and an equilibrium is established: CH3COOH + H2O ⇌ CH3COO− + H3O+. Consequently, its hydronium-ion concentration is lower than that of a strong acid of the same concentration. Weak does not mean insoluble or harmless; ethanoic acid dissolves in water and remains an acid. Therefore option A is correct.

Electrical conductivity in an aqueous solution depends mainly on the concentration and mobility of charged ions. At the same concentration, HCl ionises almost completely and produces many H3O+ and Cl− ions. Ethanoic acid ionises only partially, so it produces fewer ions, even though it dissolves in water. The larger number of charge carriers makes the HCl solution more conducting. Options B, C, and D contradict the properties of HCl.

The governing concept is the acid–base nature of oxides. Many non-metal oxides, such as carbon dioxide and sulfur dioxide, react with water to form acids or acidic solutions. These solutions can turn blue litmus red. Therefore, the general nature of non-metal oxides is acidic, making option A correct. Basic and metallic describe other substances, while “salty” is not the usual classification of an oxide.

An acidic oxide is identified by its reaction with bases and, in many cases, by its ability to form an acid with water. Carbon dioxide is a non-metal oxide; it reacts with sodium hydroxide to produce sodium carbonate and water: CO₂ + 2NaOH → Na₂CO₃ + H₂O. Hence option A is correct. Turning red litmus blue indicates a base, and the other choices misidentify carbon dioxide.