What is a Chemical Reaction?
A chemical reaction occurs when one or more substances (reactants) are converted into one or more different substances (products).
Signs That a Chemical Reaction Has Occurred
- Change in colour (iron rusting turns orange-brown)
- Evolution of a gas (CO₂ bubbles when vinegar meets baking soda)
- Change in temperature
- Formation of a precipitate
- Change in smell
Writing a Chemical Equation
Word equation: Magnesium + Oxygen → Magnesium oxide
Chemical equation: 2Mg + O₂ → 2MgO
Rules for Balancing Equations
1. Count atoms of each element on both sides 2. Add coefficients to balance — never change subscripts 3. Start with the most complex molecule
Types of Chemical Reactions
1. Combination Reaction
Two or more substances combine to form a single product.
A + B → AB
Example: 2Mg + O₂ → 2MgO
2. Decomposition Reaction
A compound breaks down into simpler substances.
Example: 2H₂O₂ → 2H₂O + O₂
3. Displacement Reaction
A more reactive element displaces a less reactive element.
Example: Fe + CuSO₄ → FeSO₄ + Cu
4. Double Displacement Reaction
Two compounds exchange ions, usually forming a precipitate.
Example: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
5. Oxidation–Reduction (Redox) Reactions
- Oxidation = loss of electrons / gain of oxygen
- Reduction = gain of electrons / loss of oxygen
Key Formulas to Remember
| Reaction | Equation |
|---|---|
| Magnesium in air | 2Mg + O₂ → 2MgO |
| Water electrolysis | 2H₂O → 2H₂ + O₂ |
| Iron + copper sulphate | Fe + CuSO₄ → FeSO₄ + Cu |
Board Exam Tips
- 5–6 marks typically come from this chapter
- Always state the type of reaction when asked
- Write balanced equations — unbalanced equations lose marks
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Frequently Asked Questions
A chemical reaction is a process in which substances called reactants are transformed into different substances called products through breaking and forming of chemical bonds.