Acids, Bases, and Salts

The governing concepts are salt hydrolysis and ionic conduction. A salt formed from a strong acid and a strong base, such as sodium chloride, generally gives a neutral solution with pH close to 7 because its ions do not significantly hydrolyse water. It conducts electricity because the dissolved salt separates into mobile Na⁺ and Cl⁻ ions. Therefore A is correct. Strong-acid/weak-base salts are acidic, weak-acid/strong-base salts are basic, and an ion-free salt could not conduct.

The governing process is the chlor-alkali process, in which concentrated aqueous sodium chloride (brine) is electrolysed. Chloride ions move to the anode and are oxidised: 2Cl⁻ → Cl₂ + 2e⁻. Thus chlorine gas is formed at the anode and is used for disinfecting or purifying water, making option B correct. Hydrogen is produced at the cathode, not the anode; sodium metal is not deposited from aqueous brine, and carbon dioxide is unrelated.

The governing concept is neutralisation: an acid reacts with a base to reduce its acidic properties, usually forming salt and water. If the irritating material from an ant sting is acidic, a suitable mild base can react with some of it and reduce the acidity at the skin surface, giving relief. Therefore option A is correct. Options B and D contradict the acid–base reaction, while C is scientifically impossible. Only a safe, mild substance should be used, not a concentrated chemical.

The governing concept is the colour response of acid–base indicators. Turmeric changes to reddish-brown in a basic medium, and phenolphthalein is colourless in acidic or neutral conditions but turns pink in a basic solution. Since both independent observations indicate alkalinity, option B is correct. An acidic solution would not produce this pair of changes, neutrality would keep phenolphthalein colourless, and “gaseous” describes a state of matter rather than the chemical nature shown by the indicators.

The governing concept is stoichiometric neutralisation, which depends on the number of reactive acid and base equivalents, not merely on equal volumes. For example, equal volumes of 0.1 M HCl and 0.1 M NaOH can approach pH 7, but unequal concentrations or a weak acid/base can leave excess H⁺ or OH⁻ and produce an acidic or basic pH. Therefore A is correct. Acids and bases do react, pH is not determined only by colour, and salts are not always acidic.

The governing concept is the reactivity series of metals. Metals above hydrogen can generally displace hydrogen from dilute acids. Magnesium is above hydrogen and reacts to form a magnesium salt and hydrogen gas, producing brisk bubbles: Mg + 2HCl → MgCl₂ + H₂. Copper is below hydrogen, so it does not normally liberate hydrogen from dilute non-oxidising acids. Therefore option A is correct; the other statements are chemically false.

The relevant concept is the preparation and uses of bleaching powder. Bleaching powder, commonly represented as CaOCl₂ at school level, releases available chlorine. This chlorine oxidises many coloured substances, so the material is used for bleaching textiles and paper-related materials. It also kills many disease-causing microorganisms in water, making it a disinfectant. Washing soda, gypsum, and baking soda do not provide this combined bleaching and disinfecting action. Hence B is correct.

The governing concept is pH and dilution. A basic solution has a hydroxide-ion concentration greater than that of neutral water. Adding water increases volume without adding an equivalent amount of base, so the hydroxide-ion concentration decreases. Consequently, pOH decreases and pH moves downward toward 7, although the solution remains basic if it is not diluted infinitely. It does not become acidic immediately, so option A is correct.

The governing process is the chlor-alkali process. When an aqueous solution of common salt, sodium chloride, is electrolysed, sodium hydroxide is formed along with chlorine and hydrogen: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Sodium hydroxide is a strong alkali used to make soap through saponification and is also important in paper manufacture. The other listed substances are not the relevant product of this process, so A is correct.

This is the lime-water test for carbon dioxide. Initially, carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate, which appears as a white precipitate and makes the solution milky: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. If excess carbon dioxide is passed, the precipitate reacts further to form soluble calcium hydrogen carbonate, Ca(HCO₃)₂, so the milkiness disappears. Thus option A explains the initial change.

The governing concept is that the pH of a salt solution depends on the acid and base from which the salt is formed. Sodium chloride, from a strong acid and strong base, gives an approximately neutral solution. Ammonium chloride is formed from a strong acid, hydrochloric acid, and a weak base, ammonia; its aqueous solution is acidic because ammonium ions hydrolyse and produce hydronium ions. Therefore B disproves the claim, while the other choices do not provide an acidic salt solution.

The governing concept is neutralisation and the definition of pH. An acidic solution contains an excess of hydrogen or hydronium ions. Added hydroxide ions react with them to form water: H₃O⁺ + OH⁻ → 2H₂O, or equivalently H⁺ + OH⁻ → H₂O. This lowers hydrogen-ion concentration, and because pH = −log[H⁺], the pH rises. The other options do not describe neutralisation, so A is correct.

The relevant concept is the reaction of food acids with reactive metals. Curd contains lactic acid and is therefore acidic. During prolonged contact, acids can react with copper, allowing copper compounds or salts to form and possibly contaminate the food. Excess copper intake can be harmful, so acidic foods are not safely stored in copper or brass vessels unless properly lined. The claims that curd becomes sweet, becomes water, or cannot react are false; hence A is correct.

The governing concept is the logarithmic pH scale: pH = −log₁₀[H⁺]. A rise from pH 2 to pH 3 is an increase of one pH unit, so the hydrogen-ion concentration becomes 10 times smaller. For example, it changes from about 10⁻² mol/L to 10⁻³ mol/L. The solution is still acidic because its pH is below 7, but its acidity, measured through hydrogen-ion concentration, decreases tenfold. Therefore A is correct.

The governing concept is the identification and composition of common salts. Baking soda is sodium hydrogen carbonate, NaHCO₃. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. Bleaching powder is a chlorine-containing bleaching compound commonly represented at school level as CaOCl₂; it is not sodium chloride, NaCl, which is common salt. Therefore option D is the incorrect pair.

The governing concept is the characteristic reaction of acids. An acid turns blue litmus red, showing that the solution is acidic. Acids also react with carbonates to form a salt, water, and carbon dioxide; the carbon dioxide escapes as visible bubbles. Therefore option A is correct. A basic or neutral solution would not give both of these observations under the stated conditions.

The pH scale expresses hydrogen-ion activity: values well above 7 indicate a strongly basic solution. A pH of 13 is therefore consistent with a highly alkaline, potentially caustic substance that can damage skin and eyes. Option A is correct, so the solution should be handled with gloves and eye protection. Pure water is near pH 7, while acids have pH below 7.

Salt hydrolysis explains the observation. A salt produced from a strong acid and a weak base commonly gives an acidic aqueous solution because its positive ion hydrolyses water and increases hydrogen-ion concentration. Thus option A is the best conclusion, although the wording “may be” is important because solution behaviour also depends on relative strengths and concentration. Strong acid–strong base salts are generally neutral.

Universal indicator gives a range of colours linked to pH. Blue generally indicates an alkaline solution, whereas green represents a pH close to neutral. Adding acid consumes some hydroxide ions and lowers the pH, moving the colour from blue toward green. Therefore option B is correct. The observation does not by itself prove that the solution became strongly acidic or that only a gas formed.

The relevant concept is the chemical nature of oxides, not their physical state. Carbon dioxide is a non-metal oxide; it dissolves partly in water to form carbonic acid and reacts with bases such as limewater. For example, CO2 + Ca(OH)2 produces CaCO3 precipitate and water. Hence option A is correct. It is not a metal oxide or a strong base, and the claim that it never dissolves is false.