📑 Topics on this page (Jump to Topic):
📌 1. Physical & Chemical Changes
| Physical change | Chemical change | |
|---|---|---|
| New substance | Not formed | Formed |
| Reversible | Usually yes | Usually no |
| Examples | Ice melting, water boiling, salt dissolving, glass breaking, magnetisation | Rusting of iron, curdling of milk, digestion, burning paper/candle wick, ripening of fruit |
- Burning candle shows both: melting wax is physical, combustion is chemical.
- Law of conservation of mass — Lavoisier; balanced equations rest on it. Lavoisier = father of modern chemistry.
📌 2. Types of Reactions
| Type | Form | Example |
|---|---|---|
| Combination | A + B → AB | CaO + H₂O → Ca(OH)₂ (slaked lime, heat released) |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ (thermal); electrolysis of water; photolysis of AgBr (photography) |
| Displacement | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu (blue turns green) |
| Double displacement | AB + CD → AD + CB | Na₂SO₄ + BaCl₂ → BaSO₄↓ (white precipitate) + 2NaCl |
| Neutralisation | Acid + base → salt + water | HCl + NaOH → NaCl + H₂O |
- Exothermic (heat released): combustion, respiration, slaking of lime, dissolving acid in water. Endothermic (heat absorbed): photosynthesis, decomposition of CaCO₃, dissolving ammonium chloride in water (cooling).
- Respiration is exothermic — glucose + O₂ → CO₂ + H₂O + energy.
📌 3. Oxidation, Reduction, Corrosion & Catalysts
- Oxidation = gain of oxygen / loss of hydrogen / loss of electrons. Reduction = the reverse. Both together = redox (CuO + H₂ → Cu + H₂O).
- Rust = hydrated iron oxide (Fe₂O₃·xH₂O); it needs both moisture and oxygen. Prevention: paint, oil, grease, galvanisation (zinc coating), chromium plating, alloying (stainless steel).
- Green coating on copper (basic copper carbonate), black coating on silver (silver sulphide) — corrosion.
- Rancidity: oxidation of oils/fats — prevented by filling chip packets with nitrogen, antioxidants, airtight containers, refrigeration.
- A catalyst changes the reaction rate and remains unchanged. Haber process (NH₃) — iron; hydrogenation of oils (vanaspati) — nickel; contact process (H₂SO₄) — V₂O₅; enzymes are biological catalysts.
📌 4. Chemical Bonding
| Bond | How it forms | Properties / examples |
|---|---|---|
| Ionic (electrovalent) | Complete transfer of electrons (metal → non-metal) | High m.p., water-soluble, conduct when molten/dissolved — NaCl, MgO |
| Covalent | Sharing of electrons | Low m.p., usually non-conductors — H₂, O₂ (double), N₂ (triple), CH₄, H₂O |
| Hydrogen bond | Weak attraction between H and F, O or N | High b.p. of water, low density of ice, DNA double helix |
| Metallic | "Sea" of electrons | Conductivity, malleability |
⚠️ Exam Traps:
- Rusting needs both water and oxygen.
- Curdling of milk is a chemical change and is not reversible.
- A catalyst does not shift equilibrium; it only changes the rate.
📌 5. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. Which of the following is a chemical change? Practice
Correct answer: (D)
Rust is a new substance (iron oxide).
Rust is a new substance (iron oxide).
Q2. Rusting of iron requires: Practice
Correct answer: (B)
No rust forms in dry air or oxygen-free water.
No rust forms in dry air or oxygen-free water.
Q3. Coating iron with zinc is called: Practice
Correct answer: (C)
Zinc corrodes first and protects iron.
Zinc corrodes first and protects iron.
Q4. Which gas is filled in chips packets? Practice
Correct answer: (A)
Nitrogen prevents oxidation (rancidity).
Nitrogen prevents oxidation (rancidity).
Q5. Photosynthesis is which type of reaction? Practice
Correct answer: (B)
Plants absorb energy from sunlight.
Plants absorb energy from sunlight.
Q6. Adding water to quicklime (CaO) is a: Practice
Correct answer: (B)
CaO + H₂O → Ca(OH)₂ + heat.
CaO + H₂O → Ca(OH)₂ + heat.
Q7. When an iron nail is put in copper sulphate solution, the colour changes from: Practice
Correct answer: (D)
Fe displaces Cu forming FeSO₄ (green).
Fe displaces Cu forming FeSO₄ (green).
Q8. Which catalyst is used in making vanaspati ghee? Practice
Correct answer: (C)
Hydrogenation of oil in the presence of Ni.
Hydrogenation of oil in the presence of Ni.
Q9. The Haber process manufactures: Practice
Correct answer: (C)
N₂ + 3H₂ → 2NH₃ (catalyst: iron).
N₂ + 3H₂ → 2NH₃ (catalyst: iron).
Q10. Oxidation involves: Practice
Correct answer: (A)
OIL RIG: Oxidation Is Loss, Reduction Is Gain.
OIL RIG: Oxidation Is Loss, Reduction Is Gain.
Q11. What type of bond is present in NaCl? Practice
Correct answer: (B)
Na gives an electron, Cl takes it.
Na gives an electron, Cl takes it.
Q12. In a nitrogen molecule (N₂) the atoms are joined by a: Practice
Correct answer: (C)
N≡N — that is why N₂ is very stable.
N≡N — that is why N₂ is very stable.
Q13. Ice floats on water because of: Practice
Correct answer: (B)
Hydrogen bonds form an open lattice; density is lower.
Hydrogen bonds form an open lattice; density is lower.
Q14. Curdling of milk is a: Practice
Correct answer: (D)
Lactobacillus turns lactose into lactic acid.
Lactobacillus turns lactose into lactic acid.
Q15. The law of conservation of mass was given by: Practice
Correct answer: (C)
Total mass stays constant in a chemical reaction.
Total mass stays constant in a chemical reaction.
Q16. In the body, enzymes act as: Practice
Correct answer: (A)
They speed up reactions.
They speed up reactions.
Q17. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl is a: Practice
Correct answer: (D)
Ions are exchanged; BaSO₄ is a white precipitate.
Ions are exchanged; BaSO₄ is a white precipitate.
Q18. In CuO + H₂ → Cu + H₂O, the substance reduced is: Practice
Correct answer: (A)
CuO loses oxygen (reduction); H₂ is oxidised.
CuO loses oxygen (reduction); H₂ is oxidised.
Q19. Why do silver articles turn black? Practice
Correct answer: (A)
Reaction with H₂S in air.
Reaction with H₂S in air.
Q20. Dissolving ammonium chloride in water makes the beaker cold. This is: Practice
Correct answer: (D)
Heat is absorbed from the surroundings while dissolving.
Heat is absorbed from the surroundings while dissolving.