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Chemistry Decomposition


A decomposition reaction is the breakdown of a compound into two or more components.

  • Usually achieved by strong heating of compound.
  • To constant weight.
  • Gravimetric analysis (change in mass to find composition)


2NaHCO3(s) → CO2(g) + H2O(l)  + 2NaCO3(s)

(Baking Soda)

CuSO4.5H2O(s) → CuSO4(s) + 5H2O(l)


  • Compound
  • Boiling tube
  • Electronic scales
  • Clamp stand
  • Bunsen burner
  • Tongs
  • Safety glasses


  1. Weigh a clean, dry boiling tube, using a balance (accurate to 2 d.p.).
  2. Record this mass (and all following masses).
  3. Add about 2 g of compound and reweigh accurately the boiling tube and contents. Record the total mass.
  4. Clamp boiling tube horizontally then heat boiling tube, gently at first (for a few minutes) and then strongly for 10 minutes turning boiling tube in clamp periodically.
  5. Allow to cool, reweigh and record the mass.
  6. Heat strongly for another 5 minutes, cool, reweigh and record the mass again.
  7. If the mass in step 5 is within 0.05 g of the mass in step 4, use this value for the calculations below. Otherwise, repeat step 5, until two masses within 0.05 g are achieved (constant weight).

Safety Precautions

Safety glasses should be worn while conducting this experiment. Also try to point boiling tube in direction away from others when heating.