Class 10 Science Notes Chapter 2 (Chapter 2) – Examplar Problems (English) Book

Alright class, let's dive deep into Chapter 2: Acids, Bases and Salts from your NCERT Science Exemplar. This is a crucial chapter, not just for your board exams, but also forms the basis for many concepts tested in various government competitive exams. Pay close attention to the reactions and applications.

Chapter 2: Acids, Bases and Salts - Detailed Notes

1. Introduction

• Acids: Substances that produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water. They are generally sour in taste and turn blue litmus red.
  • Examples: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃), Acetic acid (CH₃COOH), Citric acid (found in citrus fruits), Lactic acid (in sour milk/curd).
• Bases: Substances that produce hydroxide ions (OH⁻) when dissolved in water. They are generally bitter in taste, soapy to touch, and turn red litmus blue.
  • Examples: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium hydroxide (Ca(OH)₂), Magnesium hydroxide (Mg(OH)₂), Ammonium hydroxide (NH₄OH).
  • Alkalis: Bases that are soluble in water are called alkalis (e.g., NaOH, KOH). All alkalis are bases, but not all bases are alkalis.
• Salts: Ionic compounds formed by the reaction between an acid and a base (neutralization reaction). They consist of a cation (from the base) and an anion (from the acid).
  • Examples: Sodium chloride (NaCl), Potassium nitrate (KNO₃), Calcium sulphate (CaSO₄), Sodium carbonate (Na₂CO₃).

2. Indicators

Indicators are substances that show a distinct change in colour (or odour) when brought in contact with an acid or a base.

• Natural Indicators:
  • Litmus: Extracted from lichens. Acid: Red; Base: Blue; Neutral: Purple.
  • Turmeric: Acid: Yellow (no change); Base: Reddish-brown.
  • Red Cabbage Extract: Acid: Red/Pink; Base: Green/Yellow.
• Synthetic Indicators:
  • Phenolphthalein: Acid: Colourless; Base: Pink.
  • Methyl Orange: Acid: Red; Base: Yellow.
• Olfactory Indicators: Substances whose odour changes in acidic or basic media. Useful for visually impaired students.
  • Onion Extract: Acid: Retains smell; Base: Loses smell.
  • Vanilla Extract: Acid: Retains smell; Base: Loses smell.
  • Clove Oil: Acid: Retains smell; Base: Loses smell.

3. Chemical Properties of Acids

• Reaction with Metals: Acids react with active metals (above hydrogen in reactivity series) to produce hydrogen gas (H₂) and the corresponding metal salt.
  • Acid + Metal → Salt + Hydrogen Gas
  • Example: 2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)
  • Test for H₂ gas: Burns with a pop sound when a burning candle is brought near it.
  • Note: Acids like HNO₃ are strong oxidizing agents and usually oxidize the H₂ produced to H₂O, except with Mg and Mn (very dilute HNO₃). Curd and sour substances should not be kept in metal containers (like copper or brass) as they react to form toxic compounds.
• Reaction with Metal Carbonates and Metal Hydrogencarbonates (Bicarbonates): Acids react to produce the corresponding salt, carbon dioxide gas (CO₂), and water.
  • Acid + Metal Carbonate → Salt + Carbon Dioxide + Water
  • Example: 2HCl(aq) + Na₂CO₃(s) → 2NaCl(aq) + CO₂(g) + H₂O(l)
  • Acid + Metal Hydrogencarbonate → Salt + Carbon Dioxide + Water
  • Example: HCl(aq) + NaHCO₃(s) → NaCl(aq) + CO₂(g) + H₂O(l)
  • Test for CO₂ gas: Turns lime water (Ca(OH)₂) milky due to the formation of insoluble CaCO₃.
    • Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) [White ppt] + H₂O(l)
  • On passing excess CO₂: The milkiness disappears due to the formation of soluble calcium hydrogencarbonate.
    • CaCO₃(s) + H₂O(l) + CO₂(g) [Excess] → Ca(HCO₃)₂(aq) [Soluble]
• Reaction with Metal Oxides (Basic Oxides): Acids react with metal oxides (which are generally basic in nature) to form salt and water. This is similar to a neutralization reaction.
  • Acid + Metal Oxide → Salt + Water
  • Example: 2HCl(aq) + CuO(s) [Black] → CuCl₂(aq) [Blue-green] + H₂O(l)
• Conductivity: Aqueous solutions of acids conduct electricity due to the presence of mobile ions (H⁺ or H₃O⁺ and the corresponding anion).
  • HCl(aq) → H⁺(aq) + Cl⁻(aq)
  • H⁺ + H₂O → H₃O⁺ (Hydronium ion)
  • Note: Dry HCl gas does not show acidic properties (doesn't turn dry blue litmus red) because H⁺ ions are produced only in the presence of water.
• Strength of Acids:
  • Strong Acids: Ionize completely in water to produce a high concentration of H⁺ ions (e.g., HCl, H₂SO₄, HNO₃).
  • Weak Acids: Ionize partially in water to produce a low concentration of H⁺ ions (e.g., CH₃COOH, H₂CO₃, Citric Acid).
• Concentration vs. Dilution:
  • Concentrated Acid: Contains a large amount of acid dissolved in a small amount of water.
  • Dilute Acid: Contains a small amount of acid dissolved in a large amount of water.
  • Dilution Process: Always add acid slowly to water with constant stirring. Never add water to acid. This process is highly exothermic.

4. Chemical Properties of Bases

• Reaction with Metals: Strong bases react with certain active metals (like Zinc, Aluminium) to produce hydrogen gas and a salt.
  • Base + Metal → Salt + Hydrogen Gas
  • Example: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) [Sodium Zincate] + H₂(g)
• Reaction with Non-metal Oxides (Acidic Oxides): Bases react with non-metal oxides (which are generally acidic in nature) to form salt and water. This is also a type of neutralization reaction.
  • Base + Non-metal Oxide → Salt + Water
  • Example: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (Lime water test)
• Conductivity: Aqueous solutions of bases conduct electricity due to the presence of mobile ions (OH⁻ and the corresponding cation).
  • NaOH(aq) → Na⁺(aq) + OH⁻(aq)
• Strength of Bases:
  • Strong Bases: Ionize completely in water to produce a high concentration of OH⁻ ions (e.g., NaOH, KOH).
  • Weak Bases: Ionize partially in water to produce a low concentration of OH⁻ ions (e.g., Mg(OH)₂, NH₄OH).

5. Neutralization Reaction

• The reaction between an acid and a base to form salt and water is called a neutralization reaction.
  • Acid + Base → Salt + Water
  • Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
  • Ionic form: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)
  • Net ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)
  • Heat is usually evolved (exothermic reaction).

6. pH Scale

• A scale for measuring the hydrogen ion (H⁺) concentration in a solution. 'p' in pH stands for 'potenz' (German word for power).
• Range: 0 to 14.
  • pH = 7: Neutral solution (e.g., pure water)
  • pH < 7: Acidic solution (Lower the pH, stronger the acid)
  • pH > 7: Basic (Alkaline) solution (Higher the pH, stronger the base)
• Universal Indicator: A mixture of several indicators that gives different colours at different pH values, allowing for estimation of the pH. (Often impregnated on paper strips - pH paper).

7. Importance of pH in Everyday Life

• Digestive System: Our stomach produces HCl (pH 1.5-3.5) for digestion. Acidity (excess acid) causes pain and irritation. Antacids (mild bases like Mg(OH)₂ - Milk of Magnesia, or NaHCO₃) neutralize excess acid.
• Tooth Decay: Starts when the pH of the mouth falls below 5.5. Bacteria present in the mouth produce acids by degradation of sugar and food particles. Using toothpaste (generally basic) neutralizes the acid and prevents decay. Tooth enamel (Calcium hydroxyapatite - Ca₅(PO₄)₃OH) is the hardest substance in the body but corrodes below pH 5.5.
• Soil pH and Plant Growth: Plants require a specific pH range (often close to neutral) for healthy growth. Soil pH can be adjusted using substances like quick lime (CaO), slaked lime (Ca(OH)₂), or chalk (CaCO₃) if too acidic, or by adding organic matter if too alkaline.
• Aquatic Life: Survival of aquatic animals becomes difficult when the pH of river/lake water decreases significantly (e.g., due to acid rain).
• Self-defense by Animals and Plants:
  • Bee sting injects methanoic acid (formic acid), causing pain. Applying a mild base like baking soda provides relief.
  • Wasp sting injects an alkaline substance. Applying a mild acid like vinegar provides relief.
  • Nettle leaves have stinging hairs that inject methanoic acid. Relief can be obtained by rubbing the leaves of the dock plant (often grows nearby), which are basic.

8. Salts

• Formation: Formed from neutralization reactions.
• Family of Salts: Salts having the same positive ion (cation) or the same negative ion (anion) belong to the same family (e.g., NaCl, KCl - chloride family; Na₂SO₄, K₂SO₄ - sulphate family; NaCl, Na₂SO₄ - sodium salt family).
• pH of Salt Solutions:
  • Neutral Salts (pH ≈ 7): Formed from strong acid + strong base (e.g., NaCl, KNO₃, Na₂SO₄).
  • Acidic Salts (pH < 7): Formed from strong acid + weak base (e.g., NH₄Cl, CuSO₄). The cation from the weak base hydrolyzes water to produce H⁺ ions.
  • Basic Salts (pH > 7): Formed from weak acid + strong base (e.g., CH₃COONa, Na₂CO₃). The anion from the weak acid hydrolyzes water to produce OH⁻ ions.

9. Important Chemical Compounds (Preparation, Properties, Uses)

• Sodium Hydroxide (NaOH) - Caustic Soda:
  • Preparation (Chlor-alkali process): Electrolysis of an aqueous solution of sodium chloride (brine).
    • 2NaCl(aq) + 2H₂O(l) --(Electrolysis)--> 2NaOH(aq) + Cl₂(g) + H₂(g)
    • Products: NaOH (near cathode), Cl₂ gas (at anode), H₂ gas (at cathode).
  • Uses: Manufacturing soaps and detergents, paper making, artificial fibres, degreasing metals. (Cl₂ used for water treatment, PVC, pesticides; H₂ used as fuel, margarine, ammonia for fertilizers).
• Bleaching Powder (CaOCl₂) - Calcium Oxychloride:
  • Preparation: Action of chlorine gas (from chlor-alkali process) on dry slaked lime (Ca(OH)₂).
    • Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)
  • Properties: Yellowish-white powder, gives smell of chlorine. Actual composition is complex.
  • Uses: Bleaching cotton and linen in textile industry, bleaching wood pulp in paper industry, bleaching washed clothes in laundry, oxidizing agent in chemical industries, disinfecting drinking water.
• Baking Soda (NaHCO₃) - Sodium Hydrogencarbonate:
  • Preparation: Using NaCl, H₂O, CO₂, and NH₃ (Solvay process intermediate).
    • NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃ (Sodium hydrogencarbonate precipitates out as it is sparingly soluble).
  • Properties: Mild non-corrosive base. Action of heat:
    • 2NaHCO₃(s) --(Heat)--> Na₂CO₃(s) + H₂O(l) + CO₂(g)
  • Uses:
    • Ingredient in antacids (being alkaline).
    • In soda-acid fire extinguishers (reacts with acid to produce CO₂).
    • For making baking powder (mixture of baking soda NaHCO₃ and a mild edible acid like tartaric acid). When heated or mixed with water, CO₂ is produced, causing bread/cake to rise and become soft/spongy.
      • NaHCO₃ + H⁺ (from acid) → Na⁺ (salt of acid) + H₂O + CO₂
    • Tartaric acid neutralizes Na₂CO₃ formed upon heating, preventing a bitter taste.
• Washing Soda (Na₂CO₃·10H₂O) - Sodium Carbonate Decahydrate:
  • Preparation: Recrystallization of sodium carbonate (obtained by heating baking soda).
    • Na₂CO₃(s) + 10H₂O(l) → Na₂CO₃·10H₂O(s)
  • Properties: Basic salt, transparent crystalline solid. Efflorescent (loses water of crystallisation on exposure to air).
  • Uses: In glass, soap, and paper industries; manufacturing sodium compounds like Borax; cleaning agent for domestic purposes; removing permanent hardness of water.
• Plaster of Paris (POP) (CaSO₄·½H₂O) - Calcium Sulphate Hemihydrate:
  • Preparation: Heating Gypsum (CaSO₄·2H₂O - Calcium Sulphate Dihydrate) carefully at 373 K (100 °C).
    • CaSO₄·2H₂O(s) --(Heat at 373 K)--> CaSO₄·½H₂O(s) + 1½H₂O(g)
    • Caution: Heating above 373 K leads to formation of anhydrous CaSO₄ (dead-burnt plaster), which does not set like POP.
  • Properties: White powder. On mixing with water, it sets into a hard solid mass (Gypsum) within half an hour.
    • CaSO₄·½H₂O(s) + 1½H₂O(l) → CaSO₄·2H₂O(s) [Hard solid - Gypsum]
  • Uses: Setting fractured bones in the right position, making toys, materials for decoration, making surfaces smooth, fire-proofing materials. Should be stored in moisture-proof containers.

10. Water of Crystallisation

• The fixed number of water molecules present in one formula unit of a salt in its crystalline form.
• Examples:
  • Copper Sulphate: CuSO₄·5H₂O (Blue crystals). Anhydrous CuSO₄ is white. On heating, blue crystals lose water and turn white. Adding water restores the blue colour.
  • Washing Soda: Na₂CO₃·10H₂O
  • Gypsum: CaSO₄·2H₂O
  • Plaster of Paris: CaSO₄·½H₂O
• Salts containing water of crystallisation are called hydrated salts.

Multiple Choice Questions (MCQs)

1. Which of the following substances will turn red litmus blue?
   (a) Vinegar solution
   (b) Baking soda solution
   (c) Lemon juice
   (d) Dry HCl gas

2. A solution reacts with crushed egg-shells (mainly Calcium Carbonate) to give a gas that turns lime-water milky. The solution contains:
   (a) NaCl
   (b) HCl
   (c) LiCl
   (d) KCl

3. Which gas is usually liberated when an acid reacts with a metal?
   (a) Carbon dioxide
   (b) Oxygen
   (c) Hydrogen
   (d) Nitrogen dioxide

4. What happens when a solution of an acid is mixed with a solution of a base in a test tube?
   (i) The temperature of the solution increases
   (ii) The temperature of the solution decreases
   (iii) The temperature of the solution remains the same
   (iv) Salt formation takes place
   (a) (i) only
   (b) (i) and (iii)
   (c) (ii) and (iv)
   (d) (i) and (iv)

5. During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through a guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to:
   (a) Absorb the evolved gas
   (b) Moisten the gas
   (c) Absorb moisture from the gas
   (d) Absorb Cl⁻ ions from the evolved gas

6. Which of the following salts does NOT contain water of crystallisation?
   (a) Blue vitriol (CuSO₄·5H₂O)
   (b) Baking soda (NaHCO₃)
   (c) Washing soda (Na₂CO₃·10H₂O)
   (d) Gypsum (CaSO₄·2H₂O)

7. Sodium carbonate is a basic salt because it is a salt of a:
   (a) Strong acid and strong base
   (b) Weak acid and weak base
   (c) Strong acid and weak base
   (d) Weak acid and strong base

8. Calcium phosphate is present in tooth enamel. Its nature is:
   (a) Basic
   (b) Acidic
   (c) Neutral
   (d) Amphoteric

9. The chemical formula for Plaster of Paris is:
   (a) CaSO₄·2H₂O
   (b) CaSO₄·½H₂O
   (c) CaSO₄
   (d) Ca(OH)₂

10. Which of the following is used for the dissolution of gold?
    (a) Hydrochloric acid
    (b) Sulphuric acid
    (c) Nitric acid
    (d) Aqua regia

Answer Key for MCQs:

1. (b) Baking soda solution (NaHCO₃) is basic.
2. (b) HCl reacts with CaCO₃ to produce CO₂.
3. (c) Hydrogen gas is liberated.
4. (d) Neutralization is exothermic (temperature increases) and forms salt.
5. (c) Calcium chloride is a drying agent (absorbs moisture).
6. (b) Baking soda (Sodium hydrogencarbonate) is anhydrous.
7. (d) Na₂CO₃ is formed from NaOH (Strong Base) and H₂CO₃ (Weak Acid).
8. (a) Calcium phosphate (Calcium hydroxyapatite) is basic in nature.
9. (b) CaSO₄·½H₂O is Plaster of Paris (Calcium Sulphate Hemihydrate).
10. (d) Aqua regia (a mixture of conc. HCl and conc. HNO₃ in 3:1 ratio) can dissolve noble metals like gold.

Make sure you understand the concepts behind each point and reaction. Practice writing the chemical equations and learn the common names, chemical names, and formulas of the important compounds. Good luck with your preparation!