CBSE Class 10 Chemical Reactions — All Five Types with Examples
"Chemical Reactions and Equations" is the first chapter of the CBSE Class 10 science book, and it is the chapter the rest of chemistry stands on. If you can look at an equation and name its type, and if you can balance it without guessing, you have already earned most of the marks this chapter offers. This guide covers all five reaction types with balanced equations you can trust, a step-by-step balancing method, and the errors that examiners see most often.
First, what a balanced equation means
A chemical equation is balanced when the number of atoms of every element is the same on both sides. This is not a rule invented for exams — it is the law of conservation of mass. Atoms are rearranged in a reaction; they are never created or destroyed.
You may change only the coefficients (the big numbers written in front of a formula). You may never change a subscript, because changing H₂O to H₂O₂ changes the substance itself.
1. Combination reaction
Two or more substances combine to form a single product. General form: A + B → AB.
2Mg + O₂ → 2MgO
2H₂ + O₂ → 2H₂O
C + O₂ → CO₂
The quicklime reaction, CaO + H₂O → Ca(OH)₂, is the one CBSE asks about most. It is also strongly exothermic — the beaker becomes hot — and the slaked lime produced, Ca(OH)₂, is used to whitewash walls.
2. Decomposition reaction
A single compound breaks into two or more simpler substances: AB → A + B. Because bonds must be broken, decomposition needs energy, and the source of that energy gives the reaction its name.
| Type | Energy source | Balanced equation |
|---|---|---|
| Thermal decomposition | Heat | CaCO₃ → CaO + CO₂ |
| Thermal decomposition | Heat | 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ |
| Thermal decomposition | Heat | 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ |
| Electrolytic decomposition | Electricity | 2H₂O → 2H₂ + O₂ |
| Photolytic decomposition | Light | 2AgCl → 2Ag + Cl₂ |
| Photolytic decomposition | Light | 2AgBr → 2Ag + Br₂ |
Check the ferrous sulphate equation yourself: on the left there are 2 Fe, 2 S and 8 O; on the right 2 Fe, 2 S and 3 + 2 + 3 = 8 O. It balances. The colour change from green crystals to a reddish-brown solid is the observation examiners want.
3. Displacement reaction
A more reactive element pushes a less reactive element out of its compound: A + BC → AC + B. The reactivity series decides who displaces whom.
Zn + 2HCl → ZnCl₂ + H₂
2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
In the first reaction the blue colour of copper sulphate fades to pale green, and a reddish-brown copper deposit appears on the iron nail. Iron is above copper in the reactivity series, so it can displace copper — but copper cannot displace iron, and writing that reverse reaction is an automatic loss of marks.
4. Double displacement reaction
Two compounds exchange their ions: AB + CD → AD + CB. Nothing is oxidised or reduced; the ions simply swap partners. When one of the products is insoluble it separates out as a solid, and the reaction is also called a precipitation reaction.
Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃ (yellow precipitate)
NaOH + HCl → NaCl + H₂O (neutralisation)
Neutralisation is a double displacement reaction too — the H⁺ of the acid pairs with the OH⁻ of the base to give water, and the leftover ions form the salt.
5. Oxidation and reduction (redox)
In Class 10 the working definition is simple: oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen. Both always happen together, so the reaction is called a redox reaction.
ZnO + C → Zn + CO
MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O
Reading CuO + H₂ → Cu + H₂O:
CuO loses oxygen → CuO is reduced; H₂ is the reducing agent.
H₂ gains oxygen → H₂ is oxidised; CuO is the oxidising agent.
Observation: black copper oxide turns reddish-brown.
Two everyday consequences of redox appear in the same chapter: corrosion (iron slowly forming reddish-brown rust in moist air) and rancidity (fats and oils oxidising, giving food a bad smell and taste — which is why chips packets are flushed with nitrogen).
Balancing, step by step
Take the classic exam equation: Fe + H₂O → Fe₃O₄ + H₂.
Step 1 — count atoms as written. Left: Fe 1, H 2, O 1. Right: Fe 3, O 4, H 2.
Step 2 — balance the element with the largest count first. Oxygen: put 4 in front of H₂O → Fe + 4H₂O → Fe₃O₄ + H₂. Now O is 4 = 4.
Step 3 — balance the metal. Put 3 in front of Fe → 3Fe + 4H₂O → Fe₃O₄ + H₂.
Step 4 — balance hydrogen last. Left has 4 × 2 = 8 H, so put 4 in front of H₂ → 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Step 5 — final check. Fe 3 = 3, H 8 = 8, O 4 = 4. Balanced.
Leave hydrogen and oxygen for the end whenever they appear in more than one compound — it saves a lot of rubbing out.
Common mistakes that cost marks
- Changing a subscript to balance. Turning H₂O into H₂O₂ balances the oxygen but changes water into hydrogen peroxide. Only coefficients may change.
- Forgetting state symbols. Many questions specifically ask for (s), (l), (g) and (aq). Write them when asked.
- Confusing displacement with double displacement. Displacement always has a free element on the left (Fe, Zn, Al); double displacement has two compounds on both sides.
- Calling every heating reaction "decomposition". 2Mg + O₂ → 2MgO is heated too, but two reactants become one product, so it is combination.
- Missing the observation. Colour change, gas evolved, precipitate formed or temperature change is usually a separate mark — write it.
Quick revision table
| Type | General form | One example to remember | Give-away clue |
|---|---|---|---|
| Combination | A + B → AB | CaO + H₂O → Ca(OH)₂ | Only one product |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ | Only one reactant; needs heat, light or electricity |
| Displacement | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu | A free element on each side |
| Double displacement | AB + CD → AD + CB | Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃ | Ions swap; often a precipitate |
| Redox | oxidation + reduction together | CuO + H₂ → Cu + H₂O | Oxygen or hydrogen is gained and lost |
Check every equation you balance. The free Chemical Equation Balancer takes an unbalanced equation, balances it, and shows the atom count on both sides — so you can see exactly where your own attempt went wrong instead of only being told it is wrong.
Open the Chemical Equation Balancer →Moving into Class 11 chemistry next year? ABC Chemistry runs Class 11–12 chemistry coaching at the Gurugram centre and online classes across India, with home tuition also available in the Delhi–Gurugram region — details at abcchemistry.in.