What is thermal decomposition?
A single compound breaks down into two or more simpler substances when it is heated. Bonds must be broken, so energy is taken in. Thermal decomposition is endothermic and stops when the heating stops.
Experiment 1: ferrous sulphate crystals
Ferrous sulphate (iron(II) sulfate) forms green crystals because it holds water of crystallisation: FeSO4·7H2O. NCERT activity 1.5.
FeSO4·7H2O(s) → FeSO4(s) + 7H2O(g)
Stage 1: the crystals lose water and turn from green to white. The water condenses as droplets on the cooler glass near the mouth of the tube.
2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g)
Stage 2: the white solid turns reddish-brown. Sulphur dioxide has a sharp smell like burning sulphur.
Experiment 2: lead nitrate
2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)
NCERT activity 1.6. White crystals give a yellow solid, brown fumes and oxygen.
- The crystals crackle first. Tiny pockets of water trapped inside expand and shatter them. This is decrepitation.
- Lead(II) oxide is orange-brown while hot and yellow when cool.
- Nitrogen dioxide is a dense, brown, toxic gas. Oxygen is colourless.
Gases and their tests
| Gas | Appearance and test |
|---|
| H2O vapour | Condenses to droplets. Turns white anhydrous copper(II) sulfate blue. |
| SO2 sulphur dioxide | Colourless, sharp smell. Turns damp blue litmus red. Turns acidified potassium dichromate(VI) from orange to green. |
| SO3 sulphur trioxide | Makes white acidic fumes in moist air. Turns damp blue litmus red. |
| NO2 nitrogen dioxide | Brown gas. Turns damp blue litmus red. |
| O2 oxygen | Colourless. Relights a glowing splint. |
Why the temperatures differ
Different compounds break down at different temperatures. Lead nitrate decomposes more easily than ferrous sulphate, so ferrous sulphate needs the High flame. The temperatures in this simulation are estimates for teaching.
Safety
- Use a fume cupboard or a very well ventilated room. The gases are harmful.
- Use about 2 g of solid, hold the boiling tube at an angle and point its mouth away from everyone.
- Lead compounds are poisonous. Wear goggles and gloves and wash your hands after.
- Never sniff the gas directly. Waft it towards you.
Write it like the mark scheme
- Give the colour changes: green to white to reddish-brown, and white to yellow.
- Name each gas and give its colour or test.
- Write state symbols and balance the equations.
- Say “thermal decomposition” and “endothermic”.
Syllabus links
CBSE class 10 science, chapter 1, activities 1.5 and 1.6, decomposition reactions. IGCSE 0620: types of reaction, energy changes, thermal decomposition and tests for gases.