⚗️ Chemical Reactions & Equations – Full Breakdown

AMU Class 11 Science Chemistry June 2026

        Chemical reactions are the heart of chemistry. AMU asks about types of reactions (combination, decomposition, displacement, double displacement, redox), balancing equations, and oxidation-reduction. The most common AMU questions are "identify the type of reaction" and "which is oxidised/reduced." I've organised this so you can instantly recognise any reaction type.
    

1) Writing Chemical Equations

Reactants → Products

Must be balanced — same number of each atom on both sides (law of conservation of mass).

Steps to Balance:
1. Write skeleton equation (correct formulas)
2. Count atoms on each side
3. Add coefficients (never change subscripts!)
4. Check and repeat

Example — Balancing:
Unbalanced: Fe + H₂O → Fe₃O₄ + H₂
Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂

2) Types of Chemical Reactions — The BIG 5 for AMU

1. Combination Reaction

Two or more reactants → ONE product.

CaO + H₂O → Ca(OH)₂ + heat (slaked lime)
2Mg + O₂ → 2MgO (magnesium burns in air)
2H₂ + O₂ → 2H₂O

2. Decomposition Reaction

ONE reactant → two or more products.

2H₂O → 2H₂ + O₂ (electrolysis of water)
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (heating lead nitrate — brown fumes)
2KClO₃ → 2KCl + 3O₂ (with MnO₂ as catalyst)
CaCO₃ → CaO + CO₂ (calcination)

Types of Decomposition:

Thermal: Heat breaks down (CaCO₃ → CaO + CO₂)
Electrolytic: Electricity breaks down (2H₂O → 2H₂ + O₂)
Photolytic: Light breaks down (2AgCl → 2Ag + Cl₂ — used in photography)

⚠️ AMU Direct: "When lead nitrate is heated, what colour fumes are produced?" Answer: Brown fumes of NO₂. Also: "What type of reaction is photography based on?" → Photolytic decomposition of AgCl/AgBr.

3. Displacement Reaction

A more reactive element displaces a less reactive element from its compound.

Fe + CuSO₄ → FeSO₄ + Cu (iron displaces copper — blue to green)
Zn + 2HCl → ZnCl₂ + H₂
2Na + 2H₂O → 2NaOH + H₂

⚠️ AMU Classic: "Zn + CuSO₄ → ZnSO₄ + Cu. Identify the type of reaction." Answer: Displacement reaction. Also: "The colour of CuSO₄ changes from blue to colourless. Why?" → Cu²⁺ (blue) is displaced by Zn and forms Zn²⁺ (colourless).

4. Double Displacement Reaction

Two compounds exchange their ions (precipitation or neutralisation).

AgNO₃ + NaCl → AgCl↓ + NaNO₃ (white precipitate of AgCl)
BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl (white precipitate of BaSO₄)
NaOH + HCl → NaCl + H₂O (neutralisation — no visible change but forms water)

🧠 Trick — Single vs Double Displacement:
Single: Element + Compound → Different element + Different compound (one swap)
Double: Compound + Compound → Different compound + Different compound (two swaps)
Look for the precipitate (↓) — if one product is insoluble, it's usually double displacement.

5. Oxidation & Reduction (Redox)

Oxidation: Gain of oxygen OR loss of hydrogen.
Reduction: Loss of oxygen OR gain of hydrogen.

ZnO + C → Zn + CO
ZnO → Zn: Oxygen removed → Reduction (of ZnO)
C → CO: Oxygen gained → Oxidation (of C)

Carbon is the reducing agent (it reduces ZnO and gets oxidised itself)
ZnO is the oxidising agent (it oxidises C and gets reduced itself)

⚠️ AMU Gold: "In CuO + H₂ → Cu + H₂O, which is oxidised and which is reduced?" Answer: H₂ is oxidised (gains O → H₂O), CuO is reduced (loses O → Cu). H₂ is reducing agent, CuO is oxidising agent.

Redox Mnemonic

OIL RIG: Oxidation Is Loss (of electrons/hydrogen), Reduction Is Gain (of electrons/hydrogen).

3) Corrosion & Rancidity

Corrosion

Gradual destruction of metals due to reaction with air, moisture, or chemicals.

Rusting of Iron: Fe + O₂ + H₂O → Fe₂O₃·xH₂O (rust)

Conditions: Oxygen + Water (both needed)
Prevention: Painting, oiling, galvanisation, alloying

⚠️ AMU Question: "What is necessary for rusting of iron?" Answer: Oxygen and Water (both). "Which metal is used to galvanise iron?" → Zinc.

Rancidity

Oxidation of fats/oils in food — causes bad smell and taste.

Prevention: Adding antioxidants (BHA/BHT), keeping in airtight containers, refrigeration, flushing with N₂ gas.

4) Common AMU-Style MCQs

Q1:

2Mg + O₂ → 2MgO is which type of reaction?

(a) Decomposition (b) Combination (c) Displacement (d) Redox

Answer: (b) Combination (also redox, but combination is the primary classification)

Q2:

What is the colour of FeSO₄ solution after Fe displaces Cu from CuSO₄?

(a) Blue (b) Green (c) Colourless (d) Brown

Answer: (b) Green (Fe²⁺ gives green colour)

Q3:

In the reaction 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂, the gas evolved is:

(a) Oxygen (b) Nitrogen (c) Brown fumes of NO₂ (d) Hydrogen

Answer: (c) Brown fumes of NO₂

Q4:

Which of the following is a double displacement reaction?

(a) 2H₂ + O₂ → 2H₂O (b) AgNO₃ + NaCl → AgCl + NaNO₃ (c) Zn + H₂SO₄ → ZnSO₄ + H₂ (d) CaCO₃ → CaO + CO₂

Answer: (b)

Q5:

In CuO + H₂ → Cu + H₂O, the reducing agent is:

(a) CuO (b) H₂ (c) Cu (d) H₂O

Answer: (b) H₂

5) Quick Revision

Combination: A + B → AB. Decomposition: AB → A + B.
Displacement: A + BC → AC + B. Double displacement: AB + CD → AD + CB.
Oxidation: gain O / lose H. Reduction: lose O / gain H.
OIL RIG: Oxidation Is Loss, Reduction Is Gain (of e⁻/H).
Rusting: Fe + O₂ + H₂O → rust. Needs both O₂ and H₂O.
Rancidity: oxidation of fats → antioxidants + air-tight storage prevent it.

🧠 Final tip: For reaction type questions, look at the pattern quickly: 1 product = combination, 1 reactant = decomposition, element + compound = displacement, compound + compound = double displacement.