Acids, Bases and Salts — Class 10 Physical Science
"From the hydrochloric acid in your stomach to the baking soda in your kitchen, acids and bases are EVERYWHERE."
1. Arrhenius Theory
ACID: Substance that produces H⁺ (hydrogen) ions in WATER. HCl → H⁺ + Cl⁻. H₂SO₄ → 2H⁺ + SO₄²⁻. BASE: Substance that produces OH⁻ (hydroxide) ions in WATER. NaOH → Na⁺ + OH⁻.
Properties of Acids
SOUR taste. Turn BLUE litmus RED. React with METALS → salt + H₂↑. React with CARBONATES/BICARBONATES → salt + CO₂↑ + H₂O. React with BASES → salt + water (NEUTRALISATION).
Properties of Bases
BITTER taste. SOAPY to touch. Turn RED litmus BLUE. React with acids → neutralisation.
2. Indicators
| Indicator | In Acid | In Neutral | In Base |
|---|---|---|---|
| Litmus | RED | PURPLE | BLUE |
| Phenolphthalein | COLOURLESS | Colourless | PINK |
| Methyl Orange | RED | Orange | YELLOW |
| Red Cabbage | RED | Purple | GREEN/YELLOW |
| Turmeric | YELLOW | Yellow | REDDISH BROWN |
Olfactory Indicators
Substances whose SMELL changes in acid/base. Onion: loses smell in base (NaOH). Vanilla: loses smell in base. Clove oil: loses smell in base.
3. pH Scale — 0 to 14
pH = −log[H⁺]. pH < 7: ACIDIC. pH = 7: NEUTRAL. pH > 7: BASIC. 'pH measures the CONCENTRATION of H⁺ ions. LOWER pH = HIGHER H⁺ concentration = STRONGER acid.' pH in daily life: Stomach acid (1-2). Lemon juice (2-3). Vinegar (3). Pure water (7). Blood (7.35-7.45). Baking soda (9). Household ammonia (11).
pH of Soil and Plants
Most plants grow best at pH 6.5-7.5. Acidic soil → add LIME (CaO). Basic soil → add ORGANIC MATTER.
pH and Tooth Decay
Tooth enamel (calcium phosphate) starts dissolving at pH < 5.5. Bacteria in mouth produce acid from sugar → pH drops → cavities. 'Brush your teeth. Reduce sugary foods. This is chemistry — not just dental advice.'
4. Neutralisation: Acid + Base → Salt + Water + Heat
EXOTHERMIC reaction. HCl + NaOH → NaCl + H₂O. H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.
5. Important Salts
| Salt | Formula | Preparation | Key Uses |
|---|---|---|---|
| Baking Soda | NaHCO₃ | NaCl + H₂O + CO₂ + NH₃ → NaHCO₃ + NH₄Cl (Solvay process) | Antacid (neutralises excess stomach acid). Baking powder (NaHCO₃ + mild acid — releases CO₂ on heating → cakes RISE). Fire extinguisher. |
| Washing Soda | Na₂CO₃·10H₂O | Heating baking soda → Na₂CO₃. Recrystallised with water. | Glass manufacture. Soap and detergent. Water softening. |
| Bleaching Powder | CaOCl₂ | Cl₂ + Ca(OH)₂ (dry slaked lime) → CaOCl₂ + H₂O | Disinfectant (kills germs). Textile and paper bleaching. |
| Plaster of Paris (POP) | CaSO₄·½H₂O | Heating GYPSUM (CaSO₄·2H₂O) at 373 K (100°C) | Bone fracture casts. Moulds for statues and toys. Decorative ceiling work. |
| Gypsum | CaSO₄·2H₂O | Naturally occurring mineral | Cement manufacture. Added to cement to slow down setting. |
6. Water of Crystallisation
Water molecules CHEMICALLY BONDED within a salt crystal. CuSO₄·5H₂O (BLUE — hydrated copper sulphate). Heat → CuSO₄ (WHITE — anhydrous) + 5H₂O. Add water → BLUE again. 'The colour change is REVERSIBLE — a test for water.'
7. Common Mistakes
- 'All acids are dangerous' — Citric acid (lemons), acetic acid (vinegar), and ascorbic acid (Vitamin C) are CONSUMED safely. Some acids are weak and essential for life.
- 'pH increases with acidity' — pH DECREASES with acidity. pH=1 is MORE acidic than pH=6.
- 'Water is never acidic' — PURE water is neutral (pH 7). But rainwater is slightly acidic (pH ~5.6 — CO₂ dissolves to form carbonic acid). Acid rain (pH < 5.6) from SO₂, NO₂ pollution.
8. AP SSC Exam Focus
| Topic | Marks |
|---|---|
| pH scale and indicators | 3-4 |
| Neutralisation | 2-3 |
| Important salts (table) | 4-5 |
| Water of crystallisation | 2-3 |
9. Worked Examples — pH and Dilution
Example 1: A solution has [H⁺] = 1 × 10⁻³ M. Find its pH. Solution: pH = −log[H⁺] = −log(10⁻³) = 3. The solution is ACIDIC.
Example 2: The pH of a solution is 5. What is the [H⁺] concentration? Solution: [H⁺] = 10⁻ᵖᴴ = 10⁻⁵ M.
Example 3: A solution with pH 2 is diluted 100 times. What is the new pH? Solution: Dilution reduces [H⁺] by factor of 100. Original [H⁺] = 10⁻² M. New [H⁺] = 10⁻⁴ M. New pH = 4. 'A tenfold dilution changes pH by 1 unit.'
10. More Chemical Equations
Reaction of Acids with Metals
Zn + H₂SO₄ → ZnSO₄ + H₂↑. Mg + 2HCl → MgCl₂ + H₂↑. 2Al + 6HCl → 2AlCl₃ + 3H₂↑. 'The pop test (burning splinter near the mouth of test tube → POP sound) confirms HYDROGEN gas.'
Reaction of Acids with Carbonates and Bicarbonates
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑. NaHCO₃ + HCl → NaCl + H₂O + CO₂↑. 'CO₂ turns lime water MILKY: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O.'
Reaction of Metal Oxides with Acids
Metal oxides are BASIC in nature. CuO + 2HCl → CuCl₂ + H₂O (black CuO dissolves → blue-green CuCl₂ solution).
Reaction of Non-metal Oxides with Bases
Non-metal oxides are ACIDIC in nature. CO₂ + 2NaOH → Na₂CO₃ + H₂O. SO₂ + 2NaOH → Na₂SO₃ + H₂O.
11. Natural Indicators — How to Make Them
Red Cabbage Indicator: Boil chopped red cabbage in water. The purple extract: RED in acid, GREEN in base. Turmeric: Yellow turmeric turns REDDISH BROWN in soap (basic). Flower Indicators: Hydrangea flowers are BLUE in acidic soil, PINK in basic soil. 'Nature is a chemistry lab. Many coloured flowers contain pigments that act as pH indicators.'
12. Concentration of Acids and Bases
Strong Acid: COMPLETELY ionises in water (HCl → H⁺ + Cl⁻). Example: HCl, H₂SO₄, HNO₃. Weak Acid: PARTIALLY ionises (CH₃COOH ⇌ CH₃COO⁻ + H⁺). Example: Acetic acid, citric acid. Strong Base: Completely ionises (NaOH → Na⁺ + OH⁻). Example: NaOH, KOH. Weak Base: Partially ionises (NH₄OH ⇌ NH₄⁺ + OH⁻). Example: NH₄OH (ammonium hydroxide).
'Strength depends on IONISATION, not concentration. A DILUTE strong acid is still strong (completely ionised). A CONCENTRATED weak acid is still weak (partially ionised).'
13. Detailed Preparation of Important Salts
Baking Soda — Solvay Process
NaCl + H₂O + NH₃ + CO₂ → NaHCO₃↓ + NH₄Cl. 'The NaHCO₃ is sparingly soluble — it precipitates and is filtered.' On heating: 2NaHCO₃ →(heat)→ Na₂CO₃ + H₂O + CO₂↑. This CO₂ makes cakes and bread RISE.
Washing Soda
Na₂CO₃·10H₂O is the DECAHYDRATE (10 water molecules). Efflorescent — loses water of crystallisation on exposure to air. Uses: Removing permanent hardness of water. Cleaning agent. Manufacture of glass, soap, paper.
Bleaching Powder
The 'available chlorine' content determines quality. On exposure to air: CaOCl₂ + CO₂ → CaCO₃ + Cl₂↑. 'The chlorine gas is what bleaches — it OXIDISES coloured substances to colourless ones.'
Plaster of Paris
CaSO₄·½H₂O — '½' means one water molecule for every TWO CaSO₄ units. Mixing with water: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (GYPSUM) — SETS as hard solid. 'The setting is a REHYDRATION reaction. The volume EXPANDS SLIGHTLY — that's why POP makes perfect casts for moulds.'
14. Self-Test
Q1: A solution turns blue litmus RED. Is it acidic or basic? What is its approximate pH range? A1: ACIDIC. pH < 7 (approximately 0-6).
Q2: Why does baking soda (NaHCO₃) relieve acidity in the stomach? A2: Baking soda is a BASE. It NEUTRALISES excess HCl in the stomach: NaHCO₃ + HCl → NaCl + H₂O + CO₂. The CO₂ may cause burping — but the acid is neutralised.
Q3: Calculate the pH of 0.0001 M HCl solution. A3: HCl → H⁺ + Cl⁻ completely. [H⁺] = 10⁻⁴ M. pH = −log(10⁻⁴) = 4.
Q4: Why does blue copper sulphate turn white on heating? A4: Hydrated CuSO₄·5H₂O has water of crystallisation (blue). Heating removes this water → anhydrous CuSO₄ (WHITE). Adding water reverses the change.
Q5: Differentiate between baking soda and washing soda. A5: Baking soda = NaHCO₃ (sodium hydrogencarbonate). Washing soda = Na₂CO₃·10H₂O (sodium carbonate decahydrate). Baking soda is used in baking and as antacid. Washing soda is used in glass manufacture and water softening.
Q6: Why do detergent micelles not form scum in hard water? A6: Detergents contain SULPHONATE groups (R−SO₃⁻Na⁺) instead of carboxylate groups. Calcium and magnesium salts of sulphonates are SOLUBLE — so no scum.
Q7: A student adds universal indicator to three solutions: A (pH 2), B (pH 7), C (pH 12). What colours will she see? A7: A → RED (strongly acidic). B → GREEN (neutral). C → VIOLET (strongly basic).
