📘 CHEMICAL REACTION & EQUATION

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Physical Change

A physical change is a change in which no new substance is formed, and the change is usually reversible. It only affects the physical properties like shape, size, state, or appearance.

1. Melting of ice
2. Boiling of water
3. Breaking of glass
4. Dissolving sugar in water
5. Cutting of paper

Chemical Change

A chemical change is a change in which a new substance is formed with new chemical properties. It is usually irreversible and involves a chemical reaction.

1. Burning of paper
2. Rusting of iron
3. Digestion of food
4. Souring of milk
5. Cooking of food

Chemical Reaction

A chemical reaction is a process in which one or more substances (called reactants) are converted into new substances (called products) with different properties.

General Form: Reactants → Products

Characteristics of a Chemical Reaction

1. Change in State
2H₂(g)+O₂(g)→2H₂O

2. Change in Colour
Fe(s)+CuSO₄(aq)→FeSO₄(aq)+Cu(s)

3. Evolution of Gas
Zn(s)+2HCl(aq)→ZnCl₂(aq)+H₂(g)

4. Change in Temperature
2Mg(s)+O₂(g)→2MgO(s)+Heat & Light

5. Formation of Precipitate
BaCl₂(aq)+Na₂SO₄(aq)→BaSO₄(s)↓+2NaCl(aq)

Chemical Equation

A chemical equation is a symbolic representation of a chemical reaction in the form of symbols and formulae of the reactants and products.

H₂+O₂→H₂O

Types of Chemical Equations

Unbalanced: H₂+O₂→H₂O

Balanced: 2H₂+O₂→2H₂O

Why balance? To follow law of conservation of mass.

Balancing Chemical Equations

• 2Mg + O₂ → 2MgO
• Zn + 2HCl → ZnCl₂ + H₂
• 3Fe + 4H₂O → Fe₃O₄ + 4H₂
• 4Al + 3O₂ → 2Al₂O₃
• 2Na + 2H₂O → 2NaOH + H₂
• 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
• Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
• C + O₂ → CO₂
• Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O
• CH₄ + 2O₂ → CO₂ + 2H₂O

Types of Chemical Reactions

CaO + H₂O → Ca(OH)₂

CaCO₃ → CaO + CO₂

Zn + CuSO₄ → ZnSO₄ + Cu

Na₂SO₄ + BaCl₂ → BaSO₄ ↓ + 2NaCl

Redox Reaction

• Oxidation: Loss of electrons / gain of oxygen
• Reduction: Gain of electrons / loss of oxygen

Zn + CuSO₄ → ZnSO₄ + Cu

Corrosion

Fe + O₂ + H₂O → Fe₂O₃·xH₂O

Rancidity

Rancidity is spoilage of oils and fats due to oxidation.

Activities

2Mg + O₂ → 2MgO

Pb(NO₃)₂ + 2KI → PbI₂ ↓ + 2KNO₃

Fe + CuSO₄ → FeSO₄ + Cu

2FeSO₄ → Fe₂O₃ + SO₂ + SO₃

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

2AgCl → 2Ag + Cl₂

Zn + 2HCl → ZnCl₂ + H₂

Ca(OH)₂ + CO₂ → CaCO₃ ↓ + H₂O

NCERT Questions

• Magnesium cleaned to remove oxide layer
• X = CaO
• Gas double = Hydrogen
• Fe + CuSO₄ → FeSO₄ + Cu
• H₂ + Cl₂ → 2HCl

Quick Revision

• Chemical reaction = new substance
• Balanced equation important
• Oxidation = oxygen gain
• Reduction = oxygen loss