Acids, Bases, and Salts

Sodium hydrogen carbonate, NaHCO₃, is a mild base. In the stomach it reacts with excess hydrochloric acid in a neutralisation reaction: NaHCO₃ + HCl → NaCl + H₂O + CO₂. This reduces excessive acidity, so option A is correct. It does not increase stomach acid, make water acidic, or act by dissolving tooth enamel. Its use should still be controlled because the reaction releases carbon dioxide and adds sodium.

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, whereas baking soda is sodium hydrogen carbonate, NaHCO₃. The hydrate contains ten molecules of crystallisation water, while the hydrogen carbonate is a different salt with different uses. Therefore, option B is correct. Option A reverses their identities, and options C and D name unrelated salts: sodium chloride and calcium sulphate.

Hard water contains dissolved calcium and magnesium ions, commonly supplied by their salts. Soap contains fatty-acid ions that react with Ca²⁺ and Mg²⁺ to form insoluble scum. These precipitates consume soap before it can form lather, so option A identifies the cause. Hard water does contain ions, is not an acidic gas, and soap is not completely unable to dissolve. Washing soda can soften water by removing these ions.

Anhydrous copper sulphate, CuSO₄, is white because it contains no water of crystallisation. When water is added, hydrated copper sulphate, commonly represented as CuSO₄·5H₂O, forms and the solid becomes blue. This characteristic colour change is therefore used as a simple test for water, making option A correct. Chlorine, hydrogen, and nitrogen do not cause this specific white-to-blue change in anhydrous copper sulphate.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses part of its water of crystallisation and forms plaster of Paris, calcium sulphate hemihydrate, CaSO₄·½H₂O. Thus their principal difference is the amount of chemically combined water, so option A is correct. Metallic shine, chlorine smell, and litmus colour are not the defining chemical differences between these two calcium sulphate forms.

Ammonium chloride is formed from a strong acid, HCl, and a weak base, NH₄OH. In water, NH₄⁺ undergoes hydrolysis and produces H₃O⁺ ions, making the solution acidic; consequently, blue litmus turns red. Therefore, option A is correct. Sodium chloride from a strong acid and strong base is nearly neutral, while sodium carbonate and sodium acetate generally produce alkaline solutions and turn red litmus blue.

The governing concept is the hydrolysis of salts in water. Sodium carbonate is formed from a weak acid, carbonic acid, and a strong base, sodium hydroxide. Its carbonate ions react with water and generate hydroxide ions, so the solution becomes basic. Sodium chloride and potassium nitrate come from strong acid–strong base combinations and are nearly neutral, whereas ammonium chloride, from a weak base and strong acid, is acidic. Therefore, option A is correct.

Salt behaviour depends on the strengths of the acid and base from which the salt is formed. Ammonium chloride comes from hydrochloric acid, a strong acid, and ammonia or ammonium hydroxide, a weak base. The ammonium ion reacts with water and produces hydronium ions, making the solution acidic. Sodium carbonate is basic, while sodium chloride and potassium chloride are nearly neutral. Hence option B is the unambiguous answer.

The relevant principle is the relation between salt hydrolysis and the strengths of the parent acid and base. Sodium chloride is produced by the reaction of strong hydrochloric acid with strong sodium hydroxide: HCl + NaOH → NaCl + H₂O. Neither ion undergoes significant hydrolysis, so its aqueous solution is approximately neutral. Ammonium chloride is acidic, while carbonate and hydrogen carbonate salts are basic. Therefore, option C is correct.

An oxide is classified partly by the element combined with oxygen and partly by its reaction with water or bases. Carbon is a non-metal, so CO₂ is a non-metal oxide. In water it forms carbonic acid: CO₂ + H₂O ⇌ H₂CO₃, and it reacts with bases to form salts and water. It therefore shows acidic oxide behaviour. The litmus and metal-based alternatives contradict these facts, so A is correct.

The defining chemical behaviour of a basic oxide is its reaction with an acid to produce salt and water. Calcium oxide is a metal oxide and reacts with hydrochloric acid, for example: CaO + 2HCl → CaCl₂ + H₂O. It can also react with water to form calcium hydroxide, an alkali. Acid rain and red litmus indicate acidic behaviour, and calcium is a metal, so those distractors are incorrect. Thus A is correct.

The Arrhenius concept explains the observation: an acid produces hydrogen-related ions in aqueous solution. Dry HCl gas has no water medium in which HCl can ionise, so it does not supply hydrogen ions to the litmus and does not show the colour change. In water, HCl ionises essentially completely, producing H₃O⁺ ions, which turn blue litmus red. Therefore, A is correct; the other choices reverse or deny the observation.

The governing relationship is pH = −log₁₀[H⁺]. If the hydrogen-ion concentration changes from [H⁺] to 10[H⁺], the new pH is −log₁₀(10[H⁺]) = −1 − log₁₀[H⁺], which is one unit lower. The solution therefore becomes more acidic. A numerical value cannot be fixed without the starting pH, but the general change is clear. Hence option A is correct.

pH is defined as −log₁₀[H⁺], so a lower pH means a greater hydrogen-ion concentration and stronger acidity. The difference between pH 3 and pH 5 is two units; therefore the pH 3 solution has 10², or about 100 times, the hydrogen-ion concentration of the pH 5 solution, assuming comparable conditions. It is consequently more acidic, making option C correct.

On the pH scale, values above 7 indicate basic solutions, and a higher pH generally means a lower hydrogen-ion concentration and greater basicity. Since 13 is greater than 10, the pH 13 solution is more basic. The three-unit difference also corresponds to a thousand-fold difference in hydrogen-ion concentration under comparable conditions. Options C and D incorrectly call the solutions acidic or neutral, so option B is correct.

Acidic soil contains excess acidic substances that can harm plant growth. Lime, commonly calcium oxide, calcium hydroxide, or related carbonate material, is basic and reacts with the acids in the soil. This acid–base reaction reduces acidity and produces salt and water or related neutral products; the process is called neutralisation. Evaporation, condensation, and magnetisation do not involve an acid reacting with a base. Therefore, option A is correct.

The governing concept is acid–base balance in soil. When organic matter decomposes, microorganisms may produce humus and weak organic acids. These substances can gradually lower excessive alkalinity and make the soil more suitable for plant growth. Therefore, option A is correct. Organic matter does not turn soil into metal, remove all water, or always increase basicity, so options B, C, and D are incorrect.

The governing concept is the common property of acids: many naturally occurring acids have a sour taste, although substances must never be tasted in a laboratory. Lemon juice contains citric acid, so option B is the suitable answer. Soap solution, lime water, and sodium hydroxide solution are basic rather than acidic; their identification should be made with indicators and safe laboratory procedures.

The governing concept is the characteristic behaviour of bases. Aqueous bases may feel slippery or soapy because they interact with oils on the skin, but direct contact is unsafe. Sodium hydroxide solution is a strong base, so option C is correct. Vinegar, tamarind juice, and lemon juice contain acids and are therefore plausible acidic distractors, not the expected basic substance.

The governing concept is the use of olfactory indicators. Such indicators provide evidence about a solution through a change in smell rather than a colour change. Vanilla’s odour becomes weaker in a basic medium, so this observation indicates a basic solution and makes option A correct. Metallic shine, boiling point, and water of crystallisation are unrelated properties and cannot be identified by this observation.