Acids, Bases, and Salts

Diluting a concentrated acid with water is generally exothermic, meaning that heat is released. If water is poured rapidly into a small amount of concentrated acid, the water can heat suddenly and boil locally, causing dangerous acid droplets to splash. Therefore acid should be added slowly to a larger volume of water with stirring and protection. Option A is correct.

When water is added directly to concentrated acid, the small amount of water may absorb heat very rapidly. It can become hot or boil suddenly, forcing concentrated acid out of the container as droplets. This can cause severe burns, so the safer rule is to add acid slowly to water while stirring. Option A correctly explains the accident; the other statements contradict the exothermic process.

Neutralisation is the reaction between an acid and a base, commonly producing salt and water. In many such reactions, energy is released as heat, so the reaction mixture or its container becomes warmer. This observation supports option A. Freezing, formation of no substance, and production of only metal are not the expected results of acid–base neutralisation, so options B, C and D are incorrect.

On the pH scale, 7 represents neutrality, values below 7 indicate acidity, and values above 7 indicate basicity. A fall from pH 7 to pH 3 therefore changes the solution from neutral to acidic. Because pH is logarithmic, pH 3 also represents a much greater hydrogen-ion concentration than pH 7. Hence option A is correct; the other choices reverse or deny the pH relationship.

The governing relation is pH = −log₁₀[H⁺], so [H⁺] = 10⁻ᵖᴴ. For pH 2, the hydrogen-ion concentration is 10⁻² mol/L; for pH 4, it is 10⁻⁴ mol/L. The ratio is 10⁻² ÷ 10⁻⁴ = 10² = 100. Thus, the pH 2 solution has 100 times the hydrogen-ion concentration of the pH 4 solution, making option A correct.

The governing concept is dilution. A basic solution has pH above 7 because its hydroxide-ion concentration exceeds its hydrogen-ion concentration. Adding water lowers the concentration of hydroxide ions, so the solution becomes less basic and its pH moves downward toward 7. It does not instantly become acidic; therefore, option B is correct.

The governing concept is the characteristic colour range of acid-base indicators. Phenolphthalein is colourless in acidic or neutral solution but turns pink in a sufficiently basic solution. Methyl orange is yellow in neutral and basic conditions, so its yellow colour supports, but alone would not prove, basicity. Together the observations identify option A, a basic solution.

The governing reactions involve carbon dioxide and limewater. Initially, CO₂ reacts with calcium hydroxide to form insoluble calcium carbonate, producing a milky appearance: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. When excess CO₂ is passed, the precipitate reacts further to form soluble calcium hydrogen carbonate, Ca(HCO₃)₂. The precipitate dissolves, so option A is correct.

The governing pattern is metal plus acid producing a salt and hydrogen. Sulphuric acid supplies the sulphate ion, SO₄²⁻, while magnesium forms Mg²⁺ ions. Combining these ions in a 1:1 ratio gives MgSO₄, magnesium sulphate. The balanced reaction is Mg + H₂SO₄ → MgSO₄ + H₂, so option B is correct; chloride, sodium, and carbonate ions are not supplied by the reactants.

The governing distinction is between metal-acid and carbonate-acid reactions. Reactive metals such as zinc, magnesium, and iron generally react with dilute acids to release hydrogen gas. A carbonate reacts with an acid to form a salt, water, and carbon dioxide; for example, Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Therefore, sodium carbonate in option C is correct.

Acidic soil contains an excess of acidic substances and therefore needs a basic material for correction. Lime, commonly containing calcium compounds such as calcium oxide or calcium hydroxide, reacts with and neutralises the excess acidity. Vinegar, lemon juice, and tamarind water are acidic, so they would increase rather than reduce soil acidity. Hence, option C is the most suitable treatment.

Basic soil has an excess of alkaline substances, so adding another strong or moderately basic material would not correct it. Organic matter can decompose through microbial activity and produce humus and various mildly acidic substances. These products may gradually lower the soil’s alkalinity. Lime, sodium hydroxide, and washing soda are basic materials and would generally maintain or increase basicity. Therefore, option C is correct.

Tooth enamel is mainly mineral in nature and begins to dissolve when acids lower the pH in the mouth below about 5.5. Bacteria can act on food residues and produce acids, creating this condition. A pH of 7 is neutral and is not the critical acidic condition stated here. Water alone and the absence of bacteria do not explain enamel damage. Thus, option A is correct.

Excess hydrochloric acid in the stomach can cause irritation and a burning sensation. An antacid contains a mild base, such as a suitable hydroxide, carbonate, or bicarbonate, which reacts with the excess acid. The acid-base reaction reduces acidity and forms salt, water, and sometimes carbon dioxide. This process is called neutralisation, so option C is correct; the other choices are physical processes or unrelated effects.

Sodium hydrogen carbonate, NaHCO₃, decomposes on heating according to the reaction 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The released carbon dioxide does not support combustion and forms a layer that reduces the contact between the fire and atmospheric oxygen. Therefore it can help extinguish flames. It does not burn hydrogen, increase fire, or form only water, so option A is correct.

Baking soda is sodium hydrogen carbonate, a mild base. If used alone, it may leave an unpleasant alkaline or bitter taste. The edible acid in baking powder reacts with it and produces carbon dioxide, for example: acid + NaHCO₃ → salt + water + CO₂. The gas bubbles expand during heating and make the batter rise, while the acid reduces the alkaline taste. Hence, option B is correct.

Hard water contains dissolved calcium and magnesium salts. Washing soda, sodium carbonate (Na₂CO₃), reacts with these ions and can precipitate them as insoluble carbonates, such as calcium carbonate and magnesium carbonate. Removing these ions reduces hardness, so soap can form lather more easily. It does not acidify water, convert water into hydrogen gas, or destroy soap. Therefore, option A is correct.

Bleaching powder is used in water treatment mainly because it releases chlorine or chlorine-based disinfecting species in suitable conditions. These substances kill or inactivate many harmful microorganisms, making the water safer. The bleaching action and disinfecting action are related, but sweetness and metal formation are irrelevant. Its purpose is not simply to make water acidic. Thus, the disinfecting property described in option B is the correct reason.

Common salt, sodium chloride, is dissolved in water to prepare brine. During electrolysis of brine, chloride ions form chlorine at the anode, hydrogen is produced at the cathode, and sodium hydroxide remains in solution. Because the process produces chlorine and an alkali, it is called the chlor-alkali process. Crystallisation and evaporation only separate substances physically, while acid rain is unrelated. Hence, option B is correct.

The chlor-alkali process is the electrolysis of concentrated aqueous sodium chloride, called brine. Chloride ions are oxidised at the anode to form chlorine gas, water is reduced at the cathode to form hydrogen gas, and sodium hydroxide remains in the solution. Therefore, the three principal products are chlorine, hydrogen, and sodium hydroxide. The other groups contain substances not produced by this process, so option A is correct.

The governing concept is the pH scale and the behaviour of indicators. A solution that changes neither blue litmus nor red litmus is approximately neutral, so its pH is close to 7 at room temperature. Therefore, option B is correct. A pH of 2 is acidic, while pH values 12 and 14 are strongly basic and would affect red litmus.

Ammonium chloride, NH4Cl, is formed from hydrochloric acid, a strong acid, and ammonium hydroxide, a weak base. In water, the ammonium ion hydrolyses and produces hydronium ions, making the solution acidic. Hence option B is correct and blue litmus turns red. Option A would generally describe a basic salt; options C and D are chemically false.

Sodium carbonate is produced from carbonic acid, a weak acid, and sodium hydroxide, a strong base. The carbonate ion reacts with water and generates hydroxide ions, so the solution is basic and turns red litmus blue. Therefore, option A is correct. A strong-acid/weak-base salt would usually be acidic, while a strong-acid/strong-base salt is approximately neutral.

Sodium chloride is formed by the neutralisation of hydrochloric acid, a strong acid, and sodium hydroxide, a strong base: HCl + NaOH → NaCl + H2O. Na+ and Cl− ions do not significantly hydrolyse water, so the solution is approximately neutral, near pH 7. Thus option B is correct; options C and D contradict its ionic, soluble nature.

A base reacts with an acid in a neutralisation reaction to produce salt and water. Many metal oxides show this same behaviour; for example, CuO + 2HCl → CuCl2 + H2O. Therefore, the oxide displays basic nature and option B is correct. Acidic nature is associated with reactions characteristic of non-metal oxides, while radioactivity and volatility are unrelated properties.