Acids, Bases, and Salts

The hardness of water is mainly caused by dissolved calcium and magnesium ions, often present as bicarbonates, chlorides, or sulphates. Washing soda, Na₂CO₃·10H₂O, supplies carbonate ions that precipitate these ions as insoluble calcium carbonate and magnesium carbonate. Removing them softens the water, so option A is correct. Sodium and chloride ions generally do not cause ordinary hardness.

Baking soda is sodium hydrogen carbonate, NaHCO₃. Carbonates and hydrogen carbonates react with acids to form a salt, water, and carbon dioxide; for example, NaHCO₃ + HCl → NaCl + H₂O + CO₂. Effervescence is caused by the released carbon dioxide gas, so option B is correct. Hydrogen, chlorine, and oxygen are not products of this ordinary acid–bicarbonate reaction.

Baking powder contains sodium hydrogen carbonate along with a mild edible acid. When the mixture becomes moist and is heated, they react and release carbon dioxide gas. The gas expands and gets trapped as bubbles in the batter or dough, increasing its volume and making the food light and spongy. Hydrogen, chlorine, and nitrogen are not the gases responsible for this rising effect, so option A is correct.

On heating, baking soda, or sodium hydrogen carbonate, undergoes thermal decomposition: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. Sodium carbonate remains as the principal solid product, while carbon dioxide escapes as a gas and water is formed as vapour. Sodium chloride, calcium carbonate, and ammonium chloride are not products of this decomposition, so option A is correct.

Brine is a concentrated aqueous solution of sodium chloride. During its electrolysis, chloride ions are discharged at the anode to produce chlorine, while water is reduced at the cathode to produce hydrogen and hydroxide ions. The remaining sodium ions combine with hydroxide ions to form sodium hydroxide. This complete process is therefore called chlor-alkali electrolysis, making option A correct.

The chlor-alkali process electrolyses aqueous sodium chloride, or brine. Chloride ions form chlorine gas at the anode, and water forms hydrogen gas at the cathode. Hydroxide ions remain in solution and combine with sodium ions to produce sodium hydroxide. Thus the three important products are chlorine, hydrogen, and sodium hydroxide; the other combinations do not describe brine electrolysis, so A is correct.

In chlor-alkali electrolysis, brine supplies chloride ions and water. At the anode, oxidation occurs: chloride ions lose electrons and form chlorine gas, represented by 2Cl⁻ → Cl₂ + 2e⁻. Hydrogen is produced at the cathode, while carbon dioxide is unrelated to this process. Under the standard school-level treatment of concentrated brine, chlorine is therefore the correct answer, option B.

At the cathode, reduction takes place. In the aqueous brine solution, water molecules gain electrons and produce hydrogen gas and hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Chlorine forms at the anode, whereas nitrogen and sulphur dioxide are not products of this electrolysis. Hence hydrogen is the cathode gas and option A is correct.

Laboratory chemicals may be corrosive, poisonous, or irritating even when only a small quantity is present. Therefore tasting an unknown acid or base can injure the mouth, throat, or digestive system and is never an acceptable identification method. Indicators, universal indicator, or pH paper provide safer evidence of acidic or basic character. Since acids and bases can be harmful, option A is correct.

Hydrogen chloride shows acidic behaviour only after dissolving in water and ionising to produce hydronium ions. Dry HCl gas passing over dry blue litmus has no available water, so the ions needed for the colour change are not formed and the paper remains blue. In moist conditions HCl would turn blue litmus red. Therefore option B is the precise conclusion.

Hydrochloric acid reacts with sodium carbonate and releases carbon dioxide: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Initially, CO₂ reacts with limewater to form insoluble calcium carbonate, CaCO₃, which makes it milky. With excess CO₂, this precipitate reacts with water to form soluble calcium hydrogen carbonate, Ca(HCO₃)₂, so the solution clears. Thus A is correct.

The pH scale expresses hydrogen-ion concentration: values below 7 indicate acidic solutions, 7 is neutral, and values above 7 indicate basic solutions. Therefore X, with pH 3, is acidic, while Y, with pH 11, is basic. The numerical separation also shows that they do not have the same acidic character. Hence the correct statement is option B; option A reverses the meanings.

The governing concept is the reaction of a metal with a dilute acid. Magnesium reacts with dilute sulphuric acid to form magnesium sulphate and hydrogen: Mg + H₂SO₄ → MgSO₄ + H₂. Hydrogen is identified by bringing a burning splint near the gas; it burns rapidly with a characteristic pop sound. Option A is a test for a basic gas, B is not specific, and D indicates carbon dioxide, so option C is correct.

The governing concept is neutralisation. In a typical acid-base reaction, hydrogen ions from the acid combine with hydroxide ions from the base to form water, while the remaining positive and negative ions combine to form a salt. For example, HCl + NaOH → NaCl + H₂O. Thus water is only one product; salt is also formed. The other options contradict the reaction, so B is correct.

The governing concept is neutralisation and the logarithmic pH scale. Sodium hydroxide initially makes the solution basic, so its pH is above 7. As hydrochloric acid is added, hydrogen ions reduce the hydroxide concentration and the pH falls. At the equivalence point, equal reacting amounts of strong acid and strong base produce nearly neutral sodium chloride solution, with pH about 7. Therefore C is correct; A and D show the wrong trend.

The governing concept is the basic character of many metal oxides. A basic oxide reacts with an acid in a neutralisation reaction to produce salt and water; for example, CuO + 2HCl → CuCl₂ + H₂O. Some metal oxides also dissolve in water to form bases, although not every metal oxide is soluble. Colour, insolubility, and carbon dioxide formation are not defining criteria. Hence option A is correct.

The governing concept is the acidic behaviour of many non-metal oxides. Such oxides react with bases to form salt and water, just as acids do. For example, carbon dioxide reacts with calcium hydroxide: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Some non-metal oxides may also form acids with water, but insolubility and hydrogen release are not general defining properties. Therefore option A is correct.

The governing concept is safe dilution of a concentrated acid. Mixing acid and water releases considerable heat, so the process is exothermic. Adding acid slowly to a larger volume of water allows the heat to disperse and reduces the chance of sudden boiling and splashing. The reverse procedure can eject hot corrosive droplets and cause serious burns. Water does not solidify the acid, and density or colour is irrelevant. Thus A is correct.

The governing relationship is pH = −log₁₀[H⁺]. Therefore, a lower pH corresponds to a greater hydrogen-ion concentration. A solution of pH 2 has [H⁺] = 10⁻² mol L⁻¹, while pH 5 has 10⁻⁵ mol L⁻¹; the former is 1000 times greater. pH 7 is neutral and pH 9 is basic with a much lower hydrogen-ion concentration. Hence A is correct.

The governing concept is the effect of dilution on hydrogen-ion concentration. Diluting an acidic solution tenfold reduces [H⁺] to one-tenth. Since pH = −log[H⁺], the pH rises by about one unit for an ideal strong-acid solution, but it generally remains below 7 unless the dilution is sufficient or other effects are considered. Thus acidity decreases without automatically becoming basic. Option B is correct; A, C, and D contradict this relationship.