📖 Theory & Concepts▼

Thermal Decomposition

Many compounds decompose on heating to form simpler substances. The color change of the residue and the nature of the gas evolved help identify the original compound.

CompoundOriginal ColorResidue ColorGas
CuCO₃Light GreenBlack (CuO)CO₂
ZnCO₃WhiteYellow (hot) → White (cold)CO₂
Pb(NO₃)₂WhiteBrownish-Yellow (PbO)NO₂ + O₂

Gas Evolved Tests

  • CO₂: Colorless, odorless. Turns lime water milky.
  • NO₂: Reddish-brown fumes with a pungent suffocating smell.
  • O₂: Colorless, odorless. Relights a glowing splint.

Select Compound

Predict before you heat

Use the theory table to reason out what heating should produce.

⚠️ Safety

  • Lead nitrate gives off toxic brown NO₂ on heating — do this in a fume cupboard and never inhale the fumes.
  • Strong heating: test tube, residue and tongs stay hot for a while — hold with a holder and let them cool before handling.
  • Point the mouth of the heated tube away from yourself and others; solids can spit (decrepitation).
  • Lead compounds are toxic — wash hands after use and dispose of residues in the labelled waste.
Ready to heat
Lime Water Copper Carbonate (CuCO₃)
Lab Journal
Waiting for experiment...
Original Color---
Residue Color---
Gas Confirmed---