Chapter 2: Acids, Bases and Salts
Introduction to Acids and Bases
Acids and bases are fundamental to chemistry, influencing taste, reactions, and daily applications. Acids taste sour (e.g., lemon juice), while bases taste bitter (e.g., baking soda). This chapter explores their properties, reactions, and applications.
Indicators: Substances that change color to identify acids or bases.
- Natural Indicators: Litmus (purple dye from lichen, purple in neutral, red in acid, blue in base), turmeric (yellow in acid, reddish-brown in base), red cabbage, flower petals (Hydrangea, Petunia, Geranium).
- Synthetic Indicators: Phenolphthalein, methyl orange.
- Olfactory Indicators: Substances like onion, vanilla, clove change odor in acidic/basic media.
Application: For acidity (excess stomach acid), use a base like baking soda (antacid) to neutralize, not acidic lemon juice or vinegar.
“Acids and bases shape our world, from digestion to industrial processes.”
Chemical Properties of Acids and Bases
2.1.1 Acids and Bases in the Laboratory
Activity 2.1: Test solutions (HCl, H₂SO₄, HNO₃, CH₃COOH, NaOH, Ca(OH)₂, KOH, Mg(OH)₂, NH₄OH) with indicators.
| Indicator | Acid | Base |
|---|
| Red Litmus | No change | Turns blue |
| Blue Litmus | Turns red | No change |
| Phenolphthalein | Colorless | Pink |
| Methyl Orange | Red | Yellow |
Activity 2.2: Olfactory indicators (onion, vanilla, clove).
- Onion cloth: Loses odor in HCl, retains in NaOH.
- Vanilla: Odor diminishes in NaOH, persists in HCl.
- Clove: Odor changes in NaOH, persists in HCl.
- Conclusion: Onion and vanilla are effective olfactory indicators.
2.1.2 Reaction with Metals
Activity 2.3: Zinc + dilute H₂SO₄ produces hydrogen gas.
- Observation: Bubbles on zinc, soap solution bubbles, burning candle pops (H₂).
- Reaction: Acid + Metal → Salt + H₂(g)
- Examples:
- Zn + H₂SO₄ → ZnSO₄ + H₂
- Zn + 2HCl → ZnCl₂ + H₂
Activity 2.4: Zinc + NaOH (heated) produces hydrogen.
- Reaction: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g) (Sodium zincate).
- Note: Only some metals (e.g., Zn, Al) react with bases.
2.1.3 Reaction with Metal Carbonates/Hydrogencarbonates
Activity 2.5: Na₂CO₃/NaHCO₃ + HCl produces CO₂.
- Reactions:
- Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
- NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
- Test: CO₂ turns lime water milky: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
- Excess CO₂: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq).
- General: Metal carbonate/hydrogencarbonate + Acid → Salt + CO₂ + H₂O.
2.1.4 Neutralization Reaction
Activity 2.6: NaOH + phenolphthalein (pink) + HCl → colorless, NaOH restores pink.
- Reaction: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).
- General: Base + Acid → Salt + H₂O.
- Neutralization: Acid and base cancel each other’s effects.
2.1.5 Reaction of Metallic Oxides with Acids
Activity 2.7: CuO + HCl → blue-green solution (CuCl₂).
- Reaction: CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l).
- General: Metal oxide + Acid → Salt + H₂O.
- Note: Metallic oxides are basic.
2.1.6 Reaction of Non-metallic Oxides with Bases
Example: CO₂ + Ca(OH)₂.
- Reaction: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
- Note: Non-metallic oxides (e.g., CO₂) are acidic, reacting like acids with bases.
Common Properties of Acids and Bases
2.2.1 Ions in Solution
Activity 2.8: Test conductivity of HCl, H₂SO₄, glucose, alcohol.
- Observation: Acids (HCl, H₂SO₄) make bulb glow (conduct electricity); glucose, alcohol do not.
- Reason: Acids produce H⁺(aq) ions, which conduct electricity. Glucose/alcohol lack ions.
- Acids: Contain H⁺ (e.g., HCl → H⁺ + Cl⁻, H₂SO₄ → 2H⁺ + SO₄²⁻).
- Bases: Produce OH⁻ (e.g., NaOH → Na⁺ + OH⁻).
Activity 2.9: Dry HCl gas vs. HCl solution.
- Observation: Dry HCl gas doesn’t change dry litmus; HCl solution turns blue litmus red.
- Conclusion: HCl produces H⁺ only in water: HCl + H₂O → H₃O⁺ + Cl⁻.
- Hydronium Ion: H⁺ combines with H₂O to form H₃O⁺.
Neutralization (Ionic): H⁺(aq) + OH⁻(aq) → H₂O(l).
Activity 2.10: Diluting H₂SO₄/NaOH in water.
- Observation: Beaker warms (exothermic).
- Dilution: Reduces H₃O⁺/OH⁻ concentration per unit volume.
- Caution: Add acid to water slowly to prevent splashing/breaking due to heat.
Acids show acidic behavior only in water due to ion formation.
Strength of Acids and Bases
2.3.1 pH Scale
Measures H⁺ concentration (0 to 14).
- pH = 7: Neutral (e.g., distilled water).
- pH < 7: Acidic (lower pH = more H⁺).
- pH > 7: Basic (higher pH = more OH⁻).
- Universal Indicator: Shows color changes for pH (e.g., red for acidic, blue for basic).
Activity 2.11: Test pH of saliva, lemon juice, NaOH, HCl, etc.
- Examples: Lemon juice (pH ~2, acidic), NaOH (pH ~14, basic), tap water (pH ~7, neutral).
Strong vs. Weak:
- Strong Acids: Fully ionize, more H⁺ (e.g., HCl).
- Weak Acids: Partially ionize, less H⁺ (e.g., CH₃COOH).
- Strong Bases: Fully dissociate, more OH⁻ (e.g., NaOH).
- Weak Bases: Partially dissociate, less OH⁻ (e.g., NH₄OH).
2.3.2 Importance of pH
- Human Body: pH 7.0–7.8; stomach acid (HCl) aids digestion, excess causes indigestion (neutralized by antacids like Mg(OH)₂).
- Tooth Decay: pH < 5.5 corrodes enamel; basic toothpaste neutralizes acid.
- Soil pH: Plants need specific pH (Activity 2.12: Test soil pH with universal indicator).
- Acid Rain: pH < 5.6 harms aquatic life.
- Venus: Sulphuric acid clouds (low pH) make life unlikely.
Self-Defense: Bee/ant stings (methanoic acid, pH < 7) relieved by baking soda; nettle stings (methanoic acid) also acidic.
Salts
2.4.1 Family of Salts
Activity 2.13: Write formulae and identify acids/bases.
| Salt | Formula | Acid | Base |
|---|
| Potassium sulphate | K₂SO₄ | H₂SO₄ | KOH |
| Sodium sulphate | Na₂SO₄ | H₂SO₄ | NaOH |
| Calcium sulphate | CaSO₄ | H₂SO₄ | Ca(OH)₂ |
| Magnesium sulphate | MgSO₄ | H₂SO₄ | Mg(OH)₂ |
| Copper sulphate | CuSO₄ | H₂SO₄ | Cu(OH)₂ |
| Sodium chloride | NaCl | HCl | NaOH |
| Sodium nitrate | NaNO₃ | HNO₃ | NaOH |
| Sodium carbonate | Na₂CO₃ | H₂CO₃ | NaOH |
| Ammonium chloride | NH₄Cl | HCl | NH₄OH |
Families: Sodium salts (NaCl, Na₂SO₄, NaNO₃, Na₂CO₃), sulphate salts (K₂SO₄, Na₂SO₄, CaSO₄, MgSO₄, CuSO₄), chloride salts (NaCl, NH₄Cl).
2.4.2 pH of Salts
Activity 2.14: Test pH of salts.
| Salt | pH | Nature | Acid | Base |
|---|
| NaCl | 7 | Neutral | HCl (strong) | NaOH (strong) |
| KNO₃ | 7 | Neutral | HNO₃ (strong) | KOH (strong) |
| AlCl₃ | <7 | Acidic | HCl (strong) | Al(OH)₃ (weak) |
| ZnSO₄ | <7 | Acidic | H₂SO₄ (strong) | Zn(OH)₂ (weak) |
| CuSO₄ | <7 | Acidic | H₂SO₄ (strong) | Cu(OH)₂ (weak) |
| NaCH₃COO | >7 | Basic | CH₃COOH (weak) | NaOH (strong) |
| Na₂CO₃ | >7 | Basic | H₂CO₃ (weak) | NaOH (strong) |
| NaHCO₃ | >7 | Basic | H₂CO₃ (weak) | NaOH (strong) |
Rule: Strong acid + strong base → neutral (pH 7); strong acid + weak base → acidic (pH < 7); weak acid + strong base → basic (pH > 7).
2.4.3 Chemicals from Common Salt
Sodium Chloride (NaCl): Obtained from seawater or rock salt, used in food and industry.
- Chlor-Alkali Process: Electrolysis of brine: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g).
- Products: NaOH (caustic soda), Cl₂, H₂.
- Uses: NaOH in soaps, Cl₂ in disinfectants, H₂ as fuel.
- Bleaching Powder (Ca(ClO)₂): 2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O.
- Uses: Bleaching textiles, paper pulp, laundry; oxidizing agent; water purification.
- Baking Soda (NaHCO₃): NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃.
- Uses: Baking powder (with tartaric acid, releases CO₂), antacid, fire extinguishers.
- Heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
- Washing Soda (Na₂CO₃·10H₂O): Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O.
- Uses: Glass, soap, paper industries; borax production; cleaning; softening hard water.
2.4.4 Water of Crystallization
Fixed water molecules in salt crystals.
- CuSO₄·5H₂O: Blue, turns white on heating, blue again with water.
- Gypsum (CaSO₄·2H₂O): Heated at 373 K → Plaster of Paris (CaSO₄·½H₂O).
- Plaster of Paris: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O.
- Uses: Bone support, toys, decoration, smooth surfaces.
Key Terms
- Acid: Produces H⁺(aq), sour, turns blue litmus red.
- Base: Produces OH⁻(aq), bitter, turns red litmus blue.
- Alkali: Water-soluble base.
- Neutralization: Acid + base → salt + water.
- pH: Measures H⁺ concentration (0–14).
- Water of Crystallization: Water molecules in salt crystals.
- Antacid: Base to neutralize stomach acid.