Class 10 Science Notes Chapter 2 (Acids; bases and salts) – Science

Okay, let's prepare detailed notes for Chapter 2: Acids, Bases, and Salts from the NCERT Class 10 Science textbook, focusing on aspects relevant for government exam preparation.

Chapter 2: Acids, Bases, and Salts (NCERT Class 10 Science)

Key Concepts for Government Exams:

• Definitions and Properties of Acids and Bases
• Indicators (Types, Colour Changes)
• Chemical Reactions of Acids and Bases (with metals, carbonates, oxides, neutralization)
• Concept of H⁺ and OH⁻ ions in solution
• pH Scale (Definition, Range, Significance, Everyday Applications)
• Salts (Definition, Families, pH of Salts)
• Important Sodium Compounds (NaOH, Bleaching Powder, Baking Soda, Washing Soda) - Preparation and Uses
• Plaster of Paris - Preparation and Uses
• Water of Crystallisation

1. Introduction to Acids and Bases

• Acids:
  • Substances that are sour in taste.
  • Turn blue litmus red.
  • Give H⁺ ions (or H₃O⁺ ions - hydronium ions) in aqueous solution.
  • Examples:
    • Strong Acids: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃). (Dissociate completely in water)
    • Weak Acids: Acetic acid (CH₃COOH), Carbonic acid (H₂CO₃), Oxalic acid, Lactic acid. (Dissociate partially in water)
    • Organic Acids (Naturally Occurring):
      • Vinegar: Acetic acid
      • Orange/Lemon: Citric acid
      • Tamarind: Tartaric acid
      • Tomato: Oxalic acid
      • Sour milk/Curd: Lactic acid
      • Ant sting/Nettle sting: Methanoic acid (Formic acid)
• Bases:
  • Substances that are bitter in taste and soapy to touch.
  • Turn red litmus blue.
  • Give OH⁻ ions (hydroxide ions) in aqueous solution.
  • Examples:
    • Strong Bases: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium hydroxide (Ca(OH)₂). (Dissociate completely in water)
    • Weak Bases: Ammonium hydroxide (NH₄OH), Magnesium hydroxide (Mg(OH)₂ - Milk of Magnesia). (Dissociate partially in water)
  • Alkalis: Bases that are soluble in water (e.g., NaOH, KOH). All alkalis are bases, but not all bases are alkalis.

2. Indicators

• Substances that change their colour or odour when added to an acidic or basic solution.

• Types:

  • Natural Indicators: Litmus (from lichens), Turmeric, Red Cabbage Leaves, Petals of Hydrangea, Petunia, Geranium.
  • Synthetic Indicators: Methyl Orange, Phenolphthalein.
  • Olfactory Indicators: Substances whose odour changes in acidic or basic media (e.g., Onion, Vanilla essence, Clove oil). Useful for visually impaired students.

• Common Indicator Colour Changes (Important for Exams):

  Indicator	Original Colour	Colour in Acid	Colour in Base
  Litmus (Solution/Paper)	Purple/Blue/Red	Red	Blue
  Turmeric	Yellow	Yellow	Reddish-brown
  Phenolphthalein	Colourless	Colourless	Pink
  Methyl Orange	Orange	Red	Yellow
  Red Cabbage Juice	Purple	Reddish	Greenish-Yellow
  Onion (Olfactory)	Characteristic Smell	Retains Smell	Smell Vanishes
  Vanilla (Olfactory)	Characteristic Smell	Retains Smell	Smell Vanishes

3. Chemical Properties of Acids

• (i) Reaction with Metals:
  • Acid + Active Metal → Salt + Hydrogen Gas
  • Example: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
  • Example: 2Na(s) + 2HCl(aq) → 2NaCl(aq) + H₂(g)
  • Test for Hydrogen Gas (H₂): Burns with a pop sound when a burning candle/splinter is brought near it.
  • Note: Acids like HNO₃ are strong oxidizing agents and usually oxidize H₂ produced to H₂O, except with Mg and Mn (very dilute HNO₃).
• (ii) Reaction with Metal Carbonates and Metal Hydrogen Carbonates:
  • Acid + Metal Carbonate → Salt + Water + Carbon Dioxide
  • Example: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
  • Acid + Metal Hydrogen Carbonate → Salt + Water + Carbon Dioxide
  • Example: NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
  • Test for Carbon Dioxide Gas (CO₂): Turns lime water (Ca(OH)₂) milky.
    • Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) [White Precipitate] + H₂O(l)
  • If excess CO₂ is passed, milkiness disappears due to the formation of soluble calcium hydrogen carbonate.
    • CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq) [Soluble]
• (iii) Reaction with Bases (Neutralization):
  • Acid + Base → Salt + Water + Heat
  • This is called a Neutralization Reaction. It is generally exothermic.
  • Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
  • Example: H₂SO₄(aq) + Ca(OH)₂(aq) → CaSO₄(s) + 2H₂O(l)
• (iv) Reaction with Metallic Oxides:
  • Acid + Metallic Oxide → Salt + Water
  • Example: CuO(s) + 2HCl(aq) → CuCl₂(aq) [Blue-green] + H₂O(l)
  • Conclusion: Metallic oxides are generally basic in nature.

4. Chemical Properties of Bases

• (i) Reaction with Metals:
  • Strong Base + Certain Metals (like Zn, Al) → Salt + Hydrogen Gas
  • Example: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) [Sodium Zincate] + H₂(g)
  • Note: Not all metals react with bases.
• (ii) Reaction with Acids (Neutralization): (Covered above)
  • Base + Acid → Salt + Water + Heat
• (iii) Reaction with Non-metallic Oxides:
  • Base + Non-metallic Oxide → Salt + Water
  • Example: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (Lime water test)
  • Example: 2NaOH(aq) + SO₂(g) → Na₂SO₃(aq) + H₂O(l)
  • Conclusion: Non-metallic oxides are generally acidic in nature.

5. What do all Acids and Bases have in Common?

• Acids produce Hydrogen ions (H⁺) when dissolved in water. H⁺ ions cannot exist alone; they combine with water molecules to form Hydronium ions (H₃O⁺).
  • HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq) or simply HCl(aq) → H⁺(aq) + Cl⁻(aq)
• Bases produce Hydroxide ions (OH⁻) when dissolved in water.
  • NaOH(s) + H₂O(l) → Na⁺(aq) + OH⁻(aq)
• The presence of these ions (H⁺ or OH⁻) is responsible for the acidic or basic properties, respectively, and allows their aqueous solutions to conduct electricity.
• Dilution: Mixing an acid or base with water decreases the concentration of ions (H₃O⁺ or OH⁻) per unit volume. This process is highly exothermic, especially for strong acids like H₂SO₄ and strong bases like NaOH.
  • Safety Precaution: Always add acid/base slowly to water with constant stirring, NEVER water to acid/base, to dissipate the heat generated gradually.

6. Strength of Acids and Bases - The pH Scale

• Strength depends on the concentration of H⁺(aq) or OH⁻(aq) ions produced in solution.
  • Strong Acids/Bases: Dissociate completely in water, producing a high concentration of H⁺/OH⁻ ions.
  • Weak Acids/Bases: Dissociate partially in water, producing a low concentration of H⁺/OH⁻ ions.
• pH Scale: A scale for measuring hydrogen ion concentration in a solution. 'p' stands for 'potenz' (German for power). Developed by S.P.L. Sorensen.
  • Range: 0 to 14.
  • pH = 7: Neutral solution (e.g., Pure Water). [H⁺] = [OH⁻]
  • pH < 7: Acidic solution. Lower the pH, stronger the acid. [H⁺] > [OH⁻]
  • pH > 7: Basic (Alkaline) solution. Higher the pH, stronger the base. [H⁺] < [OH⁻]
• Universal Indicator: A mixture of several indicators that gives different colours at different pH values, providing an estimate of the pH.

7. Importance of pH in Everyday Life

• Living Organisms: Our body works within a narrow pH range of 7.0 to 7.8. Aquatic life survives in water bodies within a specific pH range. Acid rain (pH < 5.6) lowers the pH of rivers/lakes, harming aquatic life.
• Soil pH: Plants require a specific pH range for healthy growth. Farmers add substances like quicklime (CaO), slaked lime (Ca(OH)₂), or chalk (CaCO₃) to neutralize acidic soil, or add organic matter to neutralize alkaline soil.
• Digestive System: Our stomach produces HCl (pH approx. 1.5-3.5) for digestion. Excess acid causes indigestion/acidity. Antacids (mild bases like Milk of Magnesia [Mg(OH)₂] or Baking Soda [NaHCO₃]) are used to neutralize excess acid.
• Tooth Decay: Starts when the pH of the mouth falls below 5.5. Bacteria present in the mouth produce acids by degrading sugar and food particles. Using basic toothpaste helps neutralize the acid and prevent decay. Tooth enamel is made of Calcium Hydroxyapatite (crystalline form of calcium phosphate), the hardest substance in the body, but it corrodes below pH 5.5.
• Self-defense by Animals and Plants:
  • Bee sting: Releases an acid (Methanoic acid), causing pain. Relief by applying a mild base like baking soda.
  • Wasp sting: Releases a base, causing pain. Relief by applying a mild acid like vinegar.
  • Nettle leaves: Have stinging hairs that inject Methanoic acid. The traditional remedy is rubbing the area with the leaf of the Dock plant, which often grows nearby and is basic in nature.

8. Salts

• Definition: An ionic compound formed from the reaction between an acid and a base (neutralization). Composed of a cation (from the base) and an anion (from the acid).
• Family of Salts: Salts having the same positive ion (cation) or the same negative ion (anion) belong to a family.
  • Example: NaCl, Na₂SO₄ (Sodium salts)
  • Example: NaCl, KCl (Chloride salts)
• pH of Salt Solutions:
  • Salts of Strong Acid + Strong Base: Neutral (pH = 7). Example: NaCl, KNO₃.
  • Salts of Strong Acid + Weak Base: Acidic (pH < 7). Example: NH₄Cl, CuSO₄. (Due to hydrolysis of the cation)
  • Salts of Weak Acid + Strong Base: Basic (pH > 7). Example: CH₃COONa, Na₂CO₃. (Due to hydrolysis of the anion)
  • Salts of Weak Acid + Weak Base: Can be acidic, basic, or neutral depending on the relative strengths of the acid and base. (Generally considered neutral at Class 10 level, e.g., CH₃COONH₄).

9. Important Chemicals from Common Salt (Sodium Chloride, NaCl)

• Common salt is a raw material for various chemicals.
• (i) Sodium Hydroxide (NaOH) - Caustic Soda:
  • Process: Chlor-alkali process (Electrolysis of aqueous solution of NaCl - Brine).
  • Reaction: 2NaCl(aq) + 2H₂O(l) --(Electrolysis)--> 2NaOH(aq) + Cl₂(g) + H₂(g)
  • Products & Uses:
    • NaOH (at Cathode): Soaps & detergents, paper making, artificial fibres, degreasing metals.
    • Cl₂ (at Anode): Water treatment, PVC, CFCs, pesticides, Bleaching powder.
    • H₂ (at Cathode): Fuels, margarine production, ammonia for fertilizers.
• (ii) Bleaching Powder (CaOCl₂) - Calcium Oxychloride:
  • Production: Action of chlorine gas (from Chlor-alkali process) on dry slaked lime [Ca(OH)₂].
  • Reaction: Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)
  • Uses: Bleaching cotton/linen in textile industry, bleaching wood pulp in paper industry, disinfecting drinking water, oxidizing agent in chemical industries.
• (iii) Baking Soda (NaHCO₃) - Sodium Hydrogen Carbonate:
  • Production: Using NaCl, H₂O, CO₂, and NH₃ (Part of Solvay Process).
    • NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
  • Properties: Mild non-corrosive base. Action of heat:
    • 2NaHCO₃(s) --(Heat)--> Na₂CO₃(s) + H₂O(l) + CO₂(g) (CO₂ causes cakes/bread to rise)
  • Uses:
    • Ingredient in baking powder (Baking soda + mild edible acid like tartaric acid. The acid reacts with NaHCO₃ to produce CO₂ faster).
    • Antacid (being alkaline).
    • In soda-acid fire extinguishers (reacts with acid to produce CO₂).
• (iv) Washing Soda (Na₂CO₃·10H₂O) - Sodium Carbonate Decahydrate:
  • Production: Recrystallization of Sodium Carbonate (Na₂CO₃) obtained by heating baking soda.
    • Na₂CO₃(s) + 10H₂O(l) → Na₂CO₃·10H₂O(s)
  • Properties: Basic salt.
  • Uses: Glass, soap, and paper industries; manufacture of Borax; cleaning agent for domestic purposes; removing permanent hardness of water.

10. Water of Crystallisation

• The fixed number of water molecules present in one formula unit of a salt in its crystalline form.
• These water molecules give the crystal its shape and sometimes colour.
• Examples:
  • Copper Sulphate: CuSO₄·5H₂O (Blue crystals). Becomes white anhydrous CuSO₄ on heating.
  • Washing Soda: Na₂CO₃·10H₂O (White crystals).
  • Gypsum: CaSO₄·2H₂O (Calcium Sulphate Dihydrate).
  • Ferrous Sulphate: FeSO₄·7H₂O (Green crystals).
• Action of Heat: Hydrated salts lose their water of crystallisation upon heating and often change colour and state (become amorphous powder).
  • CuSO₄·5H₂O (Blue) --(Heat)--> CuSO₄ (White) + 5H₂O

11. Plaster of Paris (POP) (CaSO₄·½H₂O) - Calcium Sulphate Hemihydrate

• Production: By heating Gypsum (CaSO₄·2H₂O) at 373 K (100 °C) carefully.
  • Reaction: CaSO₄·2H₂O(s) --(Heat at 373 K)--> CaSO₄·½H₂O(s) + 1½H₂O(g)
  • Note: Heating above 373 K leads to the 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) again.
  • Reaction (Setting): CaSO₄·½H₂O(s) + 1½H₂O(l) → CaSO₄·2H₂O(s) [Hard solid Gypsum]
• Uses:
  • Supporting fractured bones in the right position.
  • Making toys, decorative materials, statues.
  • Making surfaces smooth before painting.
  • Used in laboratories for sealing air gaps.

Exam Preparation Tips:

• Memorize the chemical names and formulas of all important compounds mentioned (especially NaOH, CaOCl₂, NaHCO₃, Na₂CO₃·10H₂O, CaSO₄·½H₂O, CaSO₄·2H₂O).
• Understand and practice writing balanced chemical equations for all reactions discussed.
• Know the tests for H₂ and CO₂ gases.
• Remember the colour changes of common indicators.
• Understand the pH scale and its applications.
• Focus on the preparation and uses of the five key salts derived from common salt and gypsum.
• Relate the concepts to everyday life examples provided in the textbook.
• Solve NCERT exercises and exemplar problems.

This comprehensive overview covers the essential points of Chapter 2 for government exam preparation based on the NCERT Class 10 curriculum. Good luck!