Acids, Bases, and Salts

The governing concept is the fixed composition of hydrated salts. Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O, so ten water molecules are incorporated in the crystal structure in a definite ratio. This chemically associated water is called water of crystallisation, making option A correct. The other choices are not recognised chemical terms for water present in a salt crystal.

The governing concept is the test for carbon dioxide. On heating, sodium hydrogen carbonate decomposes: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The released CO₂ reacts with calcium hydroxide in lime water to form insoluble calcium carbonate, CaCO₃, which makes the solution milky. Thus option A is correct; blue colour and a hydrogen pop test are unrelated.

The governing concept is the acid–carbonate reaction used in baking. Baking powder contains baking soda and a mild edible acid. When moistened and heated, the acid reacts with sodium hydrogen carbonate and releases CO₂, whose bubbles expand and make the batter rise. Without enough acid, less CO₂ is produced, so option A is correct; chlorine formation, salt conversion, and metallic water are irrelevant.

The governing concept is the fire triangle: combustion needs fuel, heat, and oxygen. Carbon dioxide is denser than air and does not support combustion, so it spreads over the burning material and displaces or limits the oxygen reaching the flame. Removing oxygen interrupts combustion, making option A correct. It does not strengthen fire, convert fuel into acid, or burn water.

The governing concept is the stability and storage of bleaching powder. Moisture and carbon dioxide from air can gradually decompose or deactivate its available chlorine, reducing its bleaching and disinfecting action. Consequently, bleaching powder should be kept in a dry, tightly closed container. Option A correctly states the practical effect; the other options describe impossible transformations and do not explain why dry storage is required.

The governing concept is electrolysis of concentrated aqueous sodium chloride, or brine. At the anode, chloride ions are oxidised to chlorine; at the cathode, water is reduced to hydrogen and hydroxide ions. The remaining sodium ions combine with hydroxide ions to form sodium hydroxide. Therefore option A lists all three useful products, while the other choices omit or misidentify them.

The governing electrochemical rule is oxidation at the anode, meaning loss of electrons. In brine, chloride ions migrate toward the anode and are oxidised according to 2Cl⁻ → Cl₂ + 2e⁻, releasing chlorine gas. Hence option A is correct. Hydrogen is produced at the cathode, sodium metal is not deposited from aqueous brine, and sodium hydroxide remains in solution rather than precipitating as stated.

The governing concept is the cathode reaction in aqueous brine. Water gains electrons at the cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Sodium ions remain in the solution and combine with hydroxide ions to form NaOH, so the region becomes basic. Thus option A is correct. Chlorine forms at the anode, salt does dissolve, and sodium metal is not deposited in water.

The governing concept is the setting reaction of plaster of Paris. Calcium sulphate hemihydrate, CaSO₄·½H₂O, combines with water and changes into hard gypsum, CaSO₄·2H₂O. Because this hydration and setting begin soon after mixing, the paste must be shaped and applied promptly. Option A is correct; it does not escape as a gas, acidify water, or become a metal.

Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and produces plaster of Paris, CaSO₄·½H₂O, which sets after mixing with water. Excessive heating can remove nearly all the water and form over-burnt anhydrous calcium sulphate, which hydrates very slowly and therefore loses useful setting ability. Thus option A is correct; the other options describe impossible products.

Indigestion associated with excess acidity involves an unnecessarily high concentration of hydrochloric acid in the stomach. A mild basic antacid reacts with and neutralises some of this acid, raising the pH toward a less irritating value. It must be mild because a strong alkali could damage tissues. More hydrochloric acid would worsen the condition, while sugar and carbon dioxide do not neutralise stomach acid. Therefore A is correct.

The pH scale shows acidity inversely: a lower pH means a greater concentration of hydrogen ions and a more acidic environment. Acids produced by bacteria can react with and dissolve minerals in tooth enamel, especially when the mouth remains acidic for a long time. This softens the enamel and promotes cavities. Hence A is correct; low pH is not basic, does not make teeth metallic, and is not harmless.

When the irritating substance from an ant sting is treated as acidic, a mild base is the appropriate chemical counteragent. Baking soda, sodium hydrogen carbonate (NaHCO₃), provides mild alkalinity and can neutralise part of the acid, reducing its irritating effect. Lemon juice and vinegar are acidic, so they would not neutralise the acid, and adding a stronger acid would worsen irritation. Therefore option A is the best answer.

A soil pH of 5 indicates that the soil is acidic. Slaked lime, Ca(OH)₂, is a basic substance; when applied in a suitable quantity, it neutralises some excess acidity and moves the soil pH toward a range more favourable for many crops. Adding acid or vinegar would increase acidity. Large additions of ammonium chloride are not a suitable correction because such fertiliser can contribute to soil acidification. Thus A is correct.

A pH of 9 indicates that the soil is alkaline. During decomposition, organic manure can produce humus and various weakly acidic substances. These may react with or buffer some alkaline components, helping to reduce excessive basicity and improve soil conditions. The change is gradual and does not make the pH zero. Organic manure does not turn soil into metal and does not only increase alkalinity. Therefore A is correct.

Marble is composed mainly of calcium carbonate, CaCO₃. Acid rain contains dissolved acidic substances, commonly including sulfuric or nitric acid formed from atmospheric pollutants. An acid reacts with calcium carbonate, producing a salt, water, and carbon dioxide; for example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Repeated reaction dissolves and roughens the marble surface, so A is correct.

Methyl orange is an acid-base indicator. Its molecular form and ionic form have different colours, so the observed colour changes with the acidity of the solution. Red indicates an acidic medium, while yellow indicates a basic or less acidic medium in the simplified school-level description. This makes it useful for identifying the nature of a solution and for titration observations. Therefore A is correct; the other choices are unrelated.

Phenolphthalein is colourless in an acidic or neutral medium and pink in a sufficiently basic medium. The initial pink colour therefore shows that the original solution was basic. Adding acid consumes hydroxide ions through neutralisation, lowering the pH; when the solution leaves the indicator’s pink transition range, the colour disappears. Thus A is correct. The observation does not show that the original solution was strongly acidic.

Turning red indicator paper blue shows that the solution is basic rather than acidic or neutral. Phenolphthalein becoming pink supports the same conclusion because it is pink in a sufficiently alkaline medium. Of the listed values, pH 11 is clearly basic, whereas pH 7 is neutral and pH 3 or 1 are acidic. Therefore option A is the only value consistent with both indicator observations.

Hardness of water is mainly caused by dissolved calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). Washing soda, sodium carbonate (Na₂CO₃), reacts with these ions and forms insoluble carbonates such as calcium carbonate and magnesium carbonate. These precipitates can be separated, so the hardness-producing ions are removed. The other options do not represent the ions responsible for ordinary hard water.