chem

Definition: A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products).

Indicators of a Chemical Reaction:

  • Change in state

  • Change in colour

  • Evolution of gas

  • Change in temperature (heat released or absorbed)

  • Formation of precipitate

Example:

  • Magnesium + Oxygen → Magnesium oxide
    2Mg+O2→2MgO2Mg+O2​→2MgO

2. Chemical Equations

Word Equation: Shows reactants and products in words.
Example:
Hydrogen + Oxygen → Water

Balanced Chemical Equation:
Follows the Law of Conservation of Mass:
Number of atoms of each element is the same on both sides.

Balancing Example:
Fe+H2O→Fe3O4+H2Fe+H2​O→Fe3​O4​+H2​
Balanced:
3Fe+4H2O→Fe3O4+4H23Fe+4H2​O→Fe3​O4​+4H2​

Symbols Used:

  • (s): solid

  • (l): liquid

  • (g): gas

  • (aq): aqueous

  • ↑ gas evolved

  • ↓ precipitate

3. Types of Chemical Reactions

a. Combination Reaction

Two or more substances combine to form one product.
CaO+H2O→Ca(OH)2+HeatCaO+H2​O→Ca(OH)2​+Heat

b. Decomposition Reaction

One substance breaks into two or more substances.

i. Thermal Decomposition:
CaCO3→heatCaO+CO2CaCO3​heat​CaO+CO2​

ii. Photodecomposition:
2AgCl→sunlight2Ag+Cl22AgClsunlight​2Ag+Cl2​

iii. Electrolytic Decomposition:
2H2O→electricity2H2+O22H2​Oelectricity​2H2​+O2​

c. Displacement Reaction

A more reactive element displaces a less reactive one.
Zn+CuSO4→ZnSO4+CuZn+CuSO4​→ZnSO4​+Cu

d. Double Displacement Reaction

Two compounds exchange ions.
Na2SO4+BaCl2→BaSO4↓+2NaClNa2​SO4​+BaCl2​→BaSO4​↓+2NaCl

e. Redox Reaction

Oxidation and reduction occur together.
Zn+CuSO4→ZnSO4+CuZn+CuSO4​→ZnSO4​+Cu
Zn: Oxidized, Cu²⁺: Reduced

4. Oxidation and Reduction

  • Oxidation: Gain of oxygen or loss of hydrogen

  • Reduction: Loss of oxygen or gain of hydrogen

Example:
CuO+H2→Cu+H2OCuO+H2​→Cu+H2​O
CuO is reduced; H₂ is oxidized.

5. Corrosion

Metal reacts with air, moisture forming oxide layer (e.g. rusting of iron)
4Fe+3O2+6H2O→4Fe(OH)34Fe+3O2​+6H2​O→4Fe(OH)3​

6. Rancidity

Spoiling of food due to oxidation of fats/oils. Prevented by:

  • Storing in airtight containers

  • Refrigeration

  • Using antioxidants

🔬 Activities

  1. Mg Ribbon in Flame → Bright white light (Magnesium oxide)

  2. Ferrous sulphate heating → Green to brown, SO₂ & SO₃ gas

  3. Lead nitrate heating → Yellow ppt, brown NO₂ gas

  4. AgCl in sunlight → Turns grey (silver)