Ans: pH of a neutral solution is 7.

Ans: Hydrogen gas (H₂) is evolved.

Ans: Neutralisation is the reaction between an acid and a base to form salt and water. Example: HCl + NaOH → NaCl + H₂O. Heat is released in this reaction (exothermic).

Ans: Preparation: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. Properties: mild non-corrosive base, reacts with acid to release CO₂. Uses: cooking (baking), antacid, in fire extinguishers.

Ans: 1. Digestive system: HCl in stomach aids digestion; antacids neutralise excess acid. 2. Tooth decay: bacteria produce acid (pH < 5.5) that damages enamel; toothpaste (basic) neutralises it. 3. Soil: pH affects crop growth; lime is added to acidic soil. 4. Self-defence in animals: bee sting is acidic (baking soda relieves it); wasp sting is alkaline (vinegar relieves it).

Ans: Dry HCl gas does not produce H⁺ ions because ionisation requires water (H₂O). The reaction H⁺ ions form only in aqueous solution: HCl(g) + H₂O → H₃O⁺ + Cl⁻. Without water, no H⁺ ions are produced, and since litmus colour change depends on H⁺ (or OH⁻) ions, dry HCl does not change litmus colour. This shows that the presence of water is necessary for acids/bases to show acidic/basic properties.

Ans: Plaster of Paris (CaSO₄·½H₂O): Prepared by heating gypsum (CaSO₄·2H₂O) at 373 K (100°C): CaSO₄·2H₂O → CaSO₄·½H₂O + 3/2 H₂O. It is a white powder that sets hard on adding water (re-absorbs water to form gypsum). Uses: Making casts for fractured bones, moulds, chalk, fire-proofing. Baking Soda (NaHCO₃): Prepared by Solvay process: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl. Decomposes on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑. Uses: Baking (CO₂ makes dough rise/leavening), antacid (neutralises excess stomach acid), in fire extinguishers (releases CO₂ to smother fire).

Ans: Washing soda (Na₂CO₃·10H₂O) is prepared by heating baking soda and then dissolving in water: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O (washing soda). Uses: (1) As a cleansing agent in laundry. (2) Softening hard water (removes Ca²⁺ and Mg²⁺ as their carbonates). (3) Manufacturing glass, soap, paper, borax. (4) Removing permanent hardness of water.

Ans: pH is important in many everyday contexts: (1) Digestion: Stomach secretes HCl (pH 1–2) for enzyme pepsin to work. Antacids (Mg(OH)₂) neutralise excess acid and relieve indigestion. (2) Tooth decay: Bacteria in the mouth produce acid (from sugar) — when pH drops below 5.5, tooth enamel (calcium phosphate) dissolves. Toothpaste (basic, pH~8) neutralises the acid and prevents decay. (3) Bee sting and wasp sting: Bee sting is acidic (formic acid, pH~3.5) — treat with baking soda (base). Wasp sting is basic — treat with vinegar (acid). (4) Acid rain: Rain with pH<5.6 (due to SO₂ and NOₓ) damages buildings, statues, crops, aquatic life. (5) Soil pH: Farmers add lime (CaO) or slaked lime (Ca(OH)₂) to acidic soil; add fertilizers with acidic pH to basic soil.

Ans: The chlor-alkali process is the electrolysis of brine (concentrated NaCl solution). Reaction: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). Products and uses: (1) Sodium hydroxide (NaOH): Making soap, detergents, paper, artificial fibres (rayon), degreasing metals. (2) Chlorine (Cl₂): Making bleaching powder (Ca(ClO)Cl), disinfecting drinking water (water treatment), manufacturing PVC, solvents, pesticides. (3) Hydrogen (H₂): As fuel (hydrogen fuel cells), making ammonia (fertilisers), margarine production (hydrogenation of oils).

Ans: Ant sting contains methanoic (formic) acid. Tamarind contains tartaric acid.

Ans: On dilution the concentration of H₃O⁺ ions per unit volume decreases, so the pH increases and moves towards 7.

Ans: The formula of bleaching powder is CaOCl₂ (calcium oxychloride).

Ans: Curd and other sour foods contain acids, which react with the metals of brass and copper vessels and form poisonous salts. These can spoil the food and cause food poisoning, so such foods should not be stored in those vessels.

Ans: Acids release hydrogen ions H⁺ (as H₃O⁺) in water. These ions, along with the negative ions, are free to move and carry electric current, so the solution conducts electricity.

Ans: On heating, baking soda (NaHCO₃) decomposes to give sodium carbonate, water and carbon dioxide, which turns lime water milky. Washing soda crystals only lose their water of crystallisation (they appear to melt in their own water) and do not release CO₂ at that stage.

Ans: Water of crystallisation is the fixed number of water molecules chemically bonded in one formula unit of a salt. Examples: copper sulphate pentahydrate CuSO₄·5H₂O (5 molecules; blue crystals turn white on heating and blue again on adding water) and washing soda Na₂CO₃·10H₂O (10 molecules). Gypsum CaSO₄·2H₂O also has it.

Ans: Tooth enamel is calcium phosphate, the hardest substance in the body. It does not dissolve in water but corrodes when the pH in the mouth falls below 5.5. Bacteria produce acids by degrading sugar and food particles left in the mouth after eating, which lowers the pH. It can be prevented by cleaning the mouth after eating, brushing with a basic toothpaste that neutralises the acid, and avoiding too many sugary foods.

Ans: (a) Acid + metal → salt + hydrogen: Zn + 2HCl → ZnCl₂ + H₂; the gas burns with a pop sound. (b) Acid + metal carbonate → salt + water + CO₂: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂; the gas turns lime water milky. (c) Acid + base → salt + water (neutralisation): NaOH + HCl → NaCl + H₂O. (d) Acid + metal oxide → salt + water: CuO + 2HCl → CuCl₂ + H₂O; the black copper oxide dissolves to give a blue-green solution.

Ans: The pH scale, from 0 to 14, measures the concentration of hydrogen ions in a solution. pH 7 is neutral, below 7 is acidic and above 7 is basic. The lower the pH, the stronger the acid and the higher the H₃O⁺ concentration; the higher the pH, the stronger the base. It is measured by universal indicator or pH paper. Approximate values: gastric juice 1.2, lemon juice 2.2, pure water 7, blood 7.4, sodium hydroxide 14. Each unit change in pH means a tenfold change in H⁺ concentration.