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Class 10 Chemical Reactions and Equations: cheat sheet
CBSE Science. How to write and balance chemical equations, and the main types of reaction: combination, decomposition, displacement, double displacement and redox.
Key ideas
- Signs of a chemical reaction: a change of state or colour, a gas given off, a change in temperature, or a precipitate.
- An equation is balanced when each element has the same number of atoms on both sides (mass is conserved).
- Combination: two or more substances form one product. Decomposition: one substance breaks into two or more.
- Decomposition needs energy: heat (thermal), electricity (electrolysis) or light.
- Displacement: a more reactive element pushes a less reactive one out of its compound.
- Double displacement: two compounds swap ions, often making a precipitate.
- Oxidation is gain of oxygen or loss of hydrogen; reduction is the opposite. When both happen together it is a redox reaction.
- Corrosion (rusting of iron) and rancidity (oils and fats going stale) are everyday oxidation.
Remember
| Combination | 2Mg + O₂ → 2MgO |
| Thermal decomposition | CaCO₃ → CaO + CO₂ (on heating) |
| Displacement | Fe + CuSO₄ → FeSO₄ + Cu |
| Double displacement | Na₂SO₄ + BaCl₂ → BaSO₄ (white precipitate) + 2NaCl |
| Redox | CuO + H₂ → Cu + H₂O (CuO is reduced, H₂ is oxidised) |
Watch out
- Changing the small numbers in a formula to balance an equation: only change the numbers in front.
- Mixing up displacement (an element and a compound) with double displacement (two compounds).
- Saying the substance that gains oxygen is reduced (it is oxidised).
Worked example
For the reaction CaO + H₂O → Ca(OH)₂, find the masses of the other substances when 252 g of CaO reacts completely. (Atomic masses: H = 1 u, O = 16 u, Ca = 40 u)
Working: molar masses CaO = 56, H₂O = 18, Ca(OH)₂ = 74; CaO: 252 g is 4.5 mol
Answer: 81 g of H₂O; 333 g of Ca(OH)₂
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