Thermal Decomposition of Ferrous Sulfate: Chemical Analysis

Thermal Decomposition of Ferrous Sulfate

Observations

• Crystals: FeSO₄ crystals are light green.
• Initial Heating: Steamy fumes condense on the cooler parts of the boiling tube.
• Liquid Analysis:
  • Blue litmus paper: No change.
  • Red litmus paper: No change.
  • Cobalt chloride paper: Turns pink (indicates water).
• Strong Heating: FeSO₄ crystals crumble into a white powder, which eventually turns reddish-brown. A colorless gas is released that fumes strongly in air.
• Gas Analysis:
  • Acidified potassium dichromate paper: Turns from orange to green.
  • Moist blue litmus paper: Turns red.
  • Moist red litmus paper: No change.

Conclusions

1. FeSO₄ crystals are light green.
2. Steamy fumes represent the water of crystallization, which evaporates and condenses as water droplets.
3. The colorless liquid is neutral water, confirmed by the cobalt chloride test.
4. Strong heating causes decomposition into ferric oxide, sulfur dioxide, and sulfur trioxide.

Chemical Reaction

2FeSO₄·7H₂O _(s) → Fe₂O_{3 (s)} + SO_{2 (g)} + SO_{3 (g)} + 14H₂O _(g)

Gas Properties

SO₂ has a pungent, choking smell similar to burning sulfur. SO₃ gas dissolves in water vapor to produce sulfuric acid mist, causing the strong fuming effect.

Analysis Summary

• Potassium Dichromate: Turning green confirms the presence of SO₂.
• Litmus Test: Moist blue litmus turning red confirms the acidic nature of the vapors.
• Residue: The reddish-brown residue is identified as ferric oxide (Fe₂O₃).