Acids, Bases and Salts (CBSE Class 10 Science)

📘 Chapter 2: Acids, Bases and Salts (CBSE Class 10 Science – Full Notes)

🔷 I. Introduction

Substances are categorized as Acids, Bases, or Salts based on their chemical properties, especially their interaction with water and indicators.

🔷 II. Acids

Definition: Substances that release H⁺ ions in aqueous solution.

Types:

  • Strong Acids: Completely ionize (e.g., HCl, H₂SO₄, HNO₃)
  • Weak Acids: Partially ionize (e.g., CH₃COOH, H₂CO₃)

Properties:

  • Sour taste
  • Turn blue litmus red
  • Conduct electricity
  • React with metals to release H₂ gas
    (e.g., Zn + H₂SO₄ → ZnSO₄ + H₂↑)
  • pH less than 7

Examples:

  • Mineral Acids: HCl, H₂SO₄
  • Organic Acids: Citric, Acetic, Oxalic

🔷 III. Bases

Definition: Substances that release OH⁻ ions in aqueous solution.

Types:

  • Strong Bases: NaOH, KOH
  • Weak Bases: NH₄OH, Ca(OH)₂

Properties:

  • Bitter taste
  • Soapy to touch
  • Turn red litmus blue
  • React with acids to form salt and water
    (e.g., NaOH + HCl → NaCl + H₂O)
  • pH greater than 7

🔷 IV. Indicators

Natural Indicators:

  • Litmus: Red in acid, Blue in base
  • Turmeric: Yellow in acid, Reddish-brown in base

Synthetic Indicators:

  • Phenolphthalein: Colorless in acid, Pink in base
  • Methyl orange: Red in acid, Yellow in base

Olfactory Indicators:

  • Onion, vanilla: Lose smell in base, retain in acid

🔷 V. Reaction of Acids and Bases

  1. Reaction with Metals:
    • Acid + Metal → Salt + H₂
    • Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑
  2. Reaction with Metal Carbonates and Bicarbonates:
    • Acid + Metal carbonate → Salt + CO₂ + H₂O
    • Example: Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O
  3. Neutralization Reaction:
    • Acid + Base → Salt + Water
    • HCl + NaOH → NaCl + H₂O
  4. Reaction with Metal Oxides:
    • Acid + Metal oxide → Salt + Water
    • Example: 2HCl + CuO → CuCl₂ + H₂O
  5. Reaction with Non-metal Oxides:
    • Non-metal oxide + Base → Salt + Water
    • Example: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O

🔷 VI. pH Scale

  • Measures the concentration of H⁺ ions
  • Ranges from 0 to 14
    • Acidic: pH < 7
    • Neutral: pH = 7
    • Basic: pH > 7

Important Uses:

  • Soil pH
  • Tooth decay (below pH 5.5)
  • Antacid usage (pH neutralization)

🔷 VII. Salts

Definition: A compound formed by the neutralization of an acid with a base.

Types of Salts:

SaltFormulaParent AcidParent Base
Sodium chlorideNaClHClNaOH
Sodium carbonateNa₂CO₃H₂CO₃NaOH
Sodium bicarbonateNaHCO₃H₂CO₃NaOH
Bleaching powderCaOCl₂—Ca(OH)₂
Plaster of ParisCaSO₄·½H₂O——

🔷 VIII. Important Compounds

  1. Bleaching Powder (CaOCl₂):
    • Prepared by: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
    • Used for: disinfecting, bleaching, water purification
  2. Baking Soda (NaHCO₃):
    • Used in baking and as an antacid
    • Produces CO₂ when heated:
      2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O
  3. Washing Soda (Na₂CO₃·10H₂O):
    • Made by heating baking soda
      Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
  4. Plaster of Paris (CaSO₄·½H₂O):
    • Formed by heating gypsum (CaSO₄·2H₂O)
      CaSO₄·2H₂O → CaSO₄·½H₂O + 1.5H₂O
    • Reacts with water to form gypsum again

🔷 IX. MCQs (with Answers)

🔹 Basic Concept MCQs

  1. Acids produce ______ ions in aqueous solution.
    a) OH⁻
    b) H⁺
    c) Na⁺
    d) Cl⁻
    ✅ Answer: b
  2. Phenolphthalein in basic solution turns:
    a) Red
    b) Yellow
    c) Pink
    d) Colorless
    ✅ Answer: c
  3. Which of the following is not an indicator?
    a) Turmeric
    b) Vinegar
    c) Methyl orange
    d) Litmus
    ✅ Answer: b

🔹 Application-Based MCQs

  1. What is formed when sodium carbonate reacts with hydrochloric acid?
    a) NaOH + H₂O
    b) NaCl + CO₂ + H₂O
    c) NaHCO₃ + H₂O
    d) Na + HCl
    ✅ Answer: b
  2. Which gas is released when acids react with metal carbonates?
    a) O₂
    b) H₂
    c) CO₂
    d) SO₂
    ✅ Answer: c
  3. What is the pH of pure water?
    a) 0
    b) 7
    c) 10
    d) 14
    ✅ Answer: b

🔹 Assertion-Reason Type MCQs

  1. Assertion: Tooth enamel gets damaged when the mouth’s pH falls below 5.5.
     Reason: Acidic food increases acidity in the mouth.
    a) Both A and R are true; R explains A
    b) Both A and R are true; R doesn’t explain A
    c) A is true, R is false
    d) A is false, R is true
    ✅ Answer: a
  2. Assertion: Baking soda is used in baking.
     Reason: It releases CO₂ on heating making food fluffy.
    ✅ Answer: a

🔹 HOTS-Based MCQs

  1. Which compound is used to manufacture POP?
    a) CaCl₂
    b) CaSO₄·2H₂O
    c) CaOCl₂
    d) Na₂SO₄
    ✅ Answer: b
  2. Which of these will not change blue litmus to red?
    a) Lemon juice
    b) Vinegar
    c) Common salt solution
    d) Orange juice
    ✅ Answer: c

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