Chapter 2: Acids, Bases and Salts

Answer: Red litmus paper turns blue in a basic solution. So, dip red litmus paper into each test tube. The one that turns blue contains the basic solution. Now take this blue litmus paper (obtained after dipping in the base) and dip it into the remaining two test tubes. The solution that turns the blue litmus paper red is the acidic solution. The one that shows no colour change is distilled water.

Answer: Curd and sour substances contain acids (e.g., lactic acid, citric acid). These acids react with the metal (brass or copper) to form toxic metal salts, which can spoil the food and cause food poisoning. Hence they should not be kept in such vessels.

Answer: Hydrogen gas (H₂) is liberated. Example: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g). Test: Bring a burning splinter near the gas; it burns with a 'pop' sound, confirming hydrogen.

Answer: The gas is carbon dioxide (CO₂). The metal compound A is calcium carbonate (CaCO₃).
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Answer: In aqueous solutions, HCl and HNO₃ dissociate to give H⁺ ions (protons), which are responsible for acidic properties. Alcohol and glucose do not dissociate to produce H⁺ ions; they remain as molecules, hence they do not show acidic character.

Answer: An aqueous solution of an acid contains H⁺ ions and anions. These ions are free to move and carry electric charge, thus the solution conducts electricity.

Answer: Dry HCl gas does not dissociate into H⁺ ions because no water is present. H⁺ ions are responsible for the acidic character and colour change of litmus. Hence, dry HCl gas does not affect dry litmus paper.

Answer: Dilution of acid is a highly exothermic process. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out, causing burns. Adding acid to water slowly with constant stirring dissipates the heat safely.

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

Answer: Adding excess base increases the concentration of hydroxide ions (OH⁻) in the solution, making it more basic (higher pH).

Answer: (d) 10 – basic solution turns red litmus blue; pH > 7.

Answer: (b) HCl – egg-shells contain CaCO₃, which reacts with HCl to produce CO₂ that turns lime water milky.

Answer: (d) 16 mL – Since the amount of NaOH is doubled, the volume of HCl required will also double.

Answer: (c) Antacid – they neutralise excess stomach acid.

Answer:
(a) Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
(b) Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
(c) Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen
2Al(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂(g)
(d) Iron + Hydrochloric acid → Iron(II) chloride + Hydrogen
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Answer: Take a beaker and dissolve glucose or alcohol in water. Set up an electric circuit with a bulb, carbon rods (electrodes) and a battery. Dip the electrodes into the solution. The bulb does not glow, indicating no electricity conduction. This shows that they do not dissociate into H⁺ ions; hence they are not acids.

Answer: Distilled water is pure water with no ions. Rain water contains dissolved carbon dioxide and other gases, which form carbonic acid and other ions, making it a conductor of electricity.

Answer: Acids dissociate to give H⁺ ions only in the presence of water. In the absence of water, they remain as covalent molecules and cannot release H⁺ ions, thus no acidic behaviour.

Answer:
(a) D (pH 7) neutral
(b) C (pH 11) strongly alkaline
(c) B (pH 1) strongly acidic
(d) A (pH 4) weakly acidic
(e) E (pH 9) weakly alkaline
Increasing H⁺ concentration: C (pH 1) > B (pH 4) > D (pH 7) > E (pH 9) > A (pH 11).

Answer: Fizzing will occur more vigorously in test tube A (HCl). This is because HCl is a strong acid and dissociates completely to give a high concentration of H⁺ ions, whereas acetic acid is a weak acid and dissociates partially, releasing fewer H⁺ ions.

Answer: Curd is formed by the fermentation of milk by lactic acid bacteria. Lactic acid is produced, which increases the acidity, so the pH will decrease (become less than 6).

Answer:
(a) Baking soda (sodium hydrogencarbonate) is basic; it neutralises some of the lactic acid formed, thus raising the pH.
(b) Curd formation requires lactic acid production. The alkaline condition from baking soda delays the drop in pH, so the milk takes longer to curdle.

Answer: Plaster of Paris (CaSO₄·½H₂O) absorbs moisture from the air and converts into gypsum (CaSO₄·2H₂O), a hard solid. This would make it useless for casting and setting purposes. Hence, it must be stored in a moisture-proof container.

Answer: A neutralisation reaction is a reaction between an acid and a base to form salt and water.
Examples:
(i) HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
(ii) H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

Answer: Washing soda (Na₂CO₃·10H₂O): (i) Used in glass, soap, and paper industries. (ii) Used for removing permanent hardness of water.
Baking soda (NaHCO₃): (i) Used as an antacid to neutralize stomach acidity. (ii) Used in baking powder to produce CO₂ for making cakes and bread fluffy.

Answer: Litmus – red litmus turns blue in base, blue litmus turns red in acid.

Answer: Bleaching powder; formula: CaOCl₂ (or Ca(OCl)Cl).

Answer: Sodium hydroxide, NaOH.

Answer: Sodium (Na) loses one electron to become Na⁺, and chlorine (Cl) gains that electron to become Cl⁻. The electrostatic attraction between Na⁺ and Cl⁻ forms an ionic bond, giving NaCl (sodium chloride).

Answer: Acidic: 0–6; Neutral: 7; Basic: 8–14.