Acids,Bases and Salts — NCERT Solutions

Concept: Hard water contains dissolved salts of calcium and magnesium. Certain sodium compounds precipitate these ions, thereby softening the water.

Final Answer: Washing soda (Sodium carbonate, Na₂CO₃·10H₂O) is used for softening hard water. It reacts with the soluble calcium and magnesium salts present in hard water to form insoluble carbonates, thus removing hardness.

Given: Solution of sodium hydrogencarbonate (NaHCO₃) is heated.

What happens: When sodium hydrogencarbonate solution is heated, it decomposes to form sodium carbonate, water, and carbon dioxide gas is evolved.

Equation:
$$2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2\uparrow$$

Final Answer: On heating, sodium hydrogencarbonate decomposes to give sodium carbonate, water and carbon dioxide gas.

Given: Plaster of Paris reacts with water.

Concept: Plaster of Paris (CaSO₄·½H₂O) absorbs water and sets into a hard mass called Gypsum (CaSO₄·2H₂O).

Equation:
$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$
$$\text{(Plaster of Paris)} \hspace{60pt} \text{(Gypsum)}$$

Final Answer: Plaster of Paris reacts with water to form gypsum, which is a hard solid mass.

Correct Option: (d) 10

Justification: A solution that turns red litmus blue is basic (alkaline) in nature. Basic solutions have a pH greater than 7. Among the given options, only 10 is greater than 7, so the pH is likely to be 10.

Correct Option: (b) HCl

Justification: Crushed egg-shells are made of calcium carbonate (CaCO₃). When an acid reacts with CaCO₃, CO₂ gas is produced, which turns lime-water milky. Only HCl (hydrochloric acid) among the options is an acid:
$$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2\uparrow$$
NaCl, LiCl, and KCl are neutral salts and do not react with CaCO₃ to produce CO₂.

Correct Option: (d) 16 mL

Justification:

• 10 mL of NaOH requires 8 mL of HCl for complete neutralisation.
• If the volume of NaOH is doubled (20 mL), the volume of HCl required will also double (since concentration of both solutions remains the same).

$$\text{Volume of HCl required} = 8 \times \frac{20}{10} = 16 \text{ mL}$$

Final Answer: 16 mL of HCl solution will be required.

Correct Option: (c) Antacid

Justification: Indigestion is caused by excess acid produced in the stomach. Antacids are mild bases (e.g., Milk of Magnesia — Mg(OH)₂, baking soda — NaHCO₃) that neutralise the excess acid and provide relief from indigestion.

Concept: When a dilute acid reacts with a metal, a salt and hydrogen gas are produced.

(a) Dilute sulphuric acid + Zinc granules:

Word equation:
Zinc + Dilute sulphuric acid → Zinc sulphate + Hydrogen gas

Balanced chemical equation:
$$\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$$

(b) Dilute hydrochloric acid + Magnesium ribbon:

Word equation:
Magnesium + Dilute hydrochloric acid → Magnesium chloride + Hydrogen gas

Balanced chemical equation:
$$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\uparrow$$

(c) Dilute sulphuric acid + Aluminium powder:

Word equation:
Aluminium + Dilute sulphuric acid → Aluminium sulphate + Hydrogen gas

Balanced chemical equation:
$$2\text{Al} + 3\text{H}_2\text{SO}_4 \rightarrow \text{Al}_2(\text{SO}_4)_3 + 3\text{H}_2\uparrow$$

(d) Dilute hydrochloric acid + Iron filings:

Word equation:
Iron + Dilute hydrochloric acid → Ferrous chloride + Hydrogen gas

Balanced chemical equation:
$$\text{Fe} + 2\text{HCl} \rightarrow \text{FeCl}_2 + \text{H}_2\uparrow$$

Concept: Acids produce H⁺(aq) ions in aqueous solution, which make the solution conduct electricity. Alcohols and glucose, though they contain hydrogen, do not ionise in water to produce H⁺ ions, so they do not conduct electricity.

Activity:

Materials required: Two carbon electrodes, a battery, a bulb, connecting wires, solutions of HCl, glucose, and alcohol.

Procedure:

1. Set up an electrical circuit with a battery, a bulb, and two carbon electrodes dipped in a beaker.
2. Pour dilute HCl solution into the beaker. Observe whether the bulb glows.
3. Replace HCl with glucose solution. Observe whether the bulb glows.
4. Replace glucose solution with alcohol solution. Observe whether the bulb glows.

Observation:

• The bulb glows when HCl solution is used — indicating it conducts electricity due to the presence of H⁺ and Cl⁻ ions.
• The bulb does NOT glow when glucose or alcohol solution is used — indicating they do not produce ions in solution.

Conclusion: Glucose and alcohol contain hydrogen atoms but do not ionise in water to release H⁺ ions. Therefore, they are not acids. Only those substances that produce H⁺(aq) ions in aqueous solution are classified as acids.

Given: Distilled water vs. rain water — their electrical conductivity.

Concept: Electrical conductivity in a liquid requires the presence of free ions.

Explanation:

• Distilled water is pure water (H₂O). It does not contain any dissolved salts, minerals, or gases. It does not ionise appreciably to produce free ions. Hence, it is a poor conductor of electricity.

• Rain water, as it falls through the atmosphere, dissolves gases like CO₂, SO₂, and NO₂. These gases react with water to form acids:

$$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3$$
These acids ionise to produce H⁺ and other ions. The presence of these ions makes rain water a good conductor of electricity.

Final Answer: Distilled water lacks free ions, so it does not conduct electricity. Rain water contains dissolved acidic gases that produce ions, enabling it to conduct electricity.

Concept: Acidic behaviour is due to the production of H⁺(aq) ions in solution, which requires water.

Explanation:

Acidic behaviour of HCl (or any acid) is due to the ionisation of HCl molecules to produce hydrogen ions (H⁺):
$$\text{HCl} \xrightarrow{\text{water}} \text{H}^+(aq) + \text{Cl}^-(aq)$$

In the absence of water, dry HCl gas cannot ionise because there are no water molecules to facilitate the dissociation. Without H⁺ ions, HCl cannot show acidic properties such as turning blue litmus red, reacting with metals, etc.

Final Answer: Dry HCl gas does not ionise in the absence of water and therefore cannot produce H⁺ ions. Since acidic behaviour depends on the presence of H⁺(aq) ions, dry HCl does not show acidic behaviour.

Given:

Concept:

• pH = 7 → Neutral
• pH < 7 → Acidic (lower pH = stronger acid)
• pH > 7 → Basic/Alkaline (higher pH = stronger base)

(a) Neutral solution:
Solution D (pH = 7) is neutral.

(b) Strongly alkaline solution:
Solution C (pH = 11) is strongly alkaline (highest pH among basic solutions).

(c) Strongly acidic solution:
Solution B (pH = 1) is strongly acidic (lowest pH).

(d) Weakly acidic solution:
Solution A (pH = 4) is weakly acidic (acidic but pH closer to 7).

(e) Weakly alkaline solution:
Solution E (pH = 9) is weakly alkaline (basic but pH closer to 7).

Increasing order of hydrogen-ion concentration:

Higher pH means lower H⁺ concentration. So increasing H⁺ concentration means decreasing pH order:

$$\text{C (pH 11)} < \text{E (pH 9)} < \text{D (pH 7)} < \text{A (pH 4)} < \text{B (pH 1)}$$

Final Answer: C < E < D < A < B (in increasing order of hydrogen-ion concentration).

Given:

• Test tube A: Magnesium ribbon + HCl
• Test tube B: Magnesium ribbon + CH₃COOH (acetic acid)
• Same amount and concentration of both acids.

Concept: The rate of reaction of a metal with an acid depends on the concentration of H⁺ ions produced by the acid. Stronger acids produce more H⁺ ions.

Explanation:

• HCl is a strong acid — it completely ionises in water to produce a large number of H⁺ ions:

$$\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$$

• CH₃COOH is a weak acid — it partially ionises in water, producing fewer H⁺ ions:

$$\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+$$

Since HCl produces more H⁺ ions at the same concentration, it reacts more vigorously with magnesium:
$$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\uparrow$$
$$\text{Mg} + 2\text{CH}_3\text{COOH} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg} + \text{H}_2\uparrow$$

Final Answer: Fizzing (due to evolution of H₂ gas) will occur more vigorously in test tube A (HCl), because HCl is a strong acid and produces a higher concentration of H⁺ ions compared to acetic acid (a weak acid) at the same concentration.

Given: Fresh milk has pH = 6.

Concept: Curd formation involves the production of lactic acid by bacteria acting on lactose (milk sugar).

Explanation:
When milk turns into curd, bacteria (Lactobacillus) convert lactose present in milk into lactic acid. As lactic acid is produced, the concentration of H⁺ ions in the milk increases.

An increase in H⁺ ion concentration means the solution becomes more acidic, and therefore the pH decreases (falls below 6).

Final Answer: The pH of milk will decrease (become less than 6) as it turns into curd, because lactic acid is produced during the process, making the curd more acidic than fresh milk.

Given: Baking soda (NaHCO₃) is added to fresh milk (pH = 6).

(a) Why does he shift the pH to slightly alkaline?

Baking soda (NaHCO₃) is a mild base. When added to milk, it neutralises some of the existing acid and makes the milk slightly alkaline (pH slightly above 7).

Reason: Milk in slightly alkaline condition does not turn sour (acidic) quickly. Since bacteria need an acidic environment to grow and produce lactic acid, making the milk alkaline prevents it from turning sour quickly, thereby increasing its shelf life.

(b) Why does this milk take a long time to set as curd?

Curd is formed when lactic acid (produced by bacteria) lowers the pH of milk sufficiently. Since baking soda has made the milk slightly alkaline, the bacteria first need to neutralise this alkalinity before the pH can drop enough to set the curd.

Reason: The added baking soda acts as a buffer — the lactic acid produced by bacteria first reacts with the baking soda (neutralisation), so it takes longer for the pH to drop to the level required for curd formation. Hence, the milk takes a longer time to set as curd.

Concept: Plaster of Paris reacts with water to form gypsum.

Explanation:
Plaster of Paris is calcium sulphate hemihydrate (CaSO₄·½H₂O). When it comes in contact with moisture (water), it reacts and sets into a hard mass called gypsum (CaSO₄·2H₂O):

$$\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$

If Plaster of Paris absorbs moisture from the air, it will set into gypsum and become useless for further applications (e.g., making casts, toys, or supporting fractured bones).

Final Answer: Plaster of Paris should be stored in a moisture-proof container to prevent it from absorbing moisture from the atmosphere and setting into gypsum prematurely, which would make it unfit for use.

Definition:
A reaction in which an acid and a base react with each other to form a salt and water is called a neutralisation reaction. In this reaction, the acidic and basic properties of the reactants are cancelled (neutralised).

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

Example 1: Reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl):
$$\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$$
(Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water)

Example 2: Reaction between sodium hydroxide (NaOH) and sulphuric acid (H₂SO₄):
$$2\text{NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$$
(Sodium hydroxide + Sulphuric acid → Sodium sulphate + Water)

Uses of Washing Soda (Na₂CO₃·10H₂O — Sodium carbonate):

1. Softening of hard water: Washing soda is used to remove the hardness of water by precipitating the soluble calcium and magnesium salts as insoluble carbonates.

2. Cleaning agent: It is used as a cleaning agent in homes and laundries for washing clothes. It is also used in the manufacture of glass, soap, and paper.

Uses of Baking Soda (NaHCO₃ — Sodium hydrogencarbonate):

1. Baking/cooking: It is used as a leavening agent in baking. When heated, it releases CO₂ gas which makes the dough soft and fluffy:

$$2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2\uparrow$$

2. Antacid: It is used as a mild antacid to neutralise excess acid in the stomach and provide relief from acidity and indigestion:

$$\text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow$$