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Experiment: Water Treatment – Coagulation


14 June 2007 by Quest 1. Links in with Water Treatment.


To accelerate the settling of dirt particles from muddy water.


Half fill three test tubes with muddy water. Ensure their levels are the same.

In two other test tubes make up solutions of 1 measure of FeSO4 with one pipette of water, and 3 measures of FeSO4 with one pipette of water.

Add one of these solutions to each of two of the muddy water test tubes.

Top the third test tube up to the same level with pure water – it is our control test tube.

Stir to mix then leave to settle.

Shake to see if the FeSO4 has had any effect.


  • Iron(II) sulphate (FeSO4)
  • Water.
  • Mud.
  • Test tubes, water dropper (known as a pipette).
  • Metal spatula.
  • Cloth for mopping up spills.


The ferrous sulphate crystals when dissolved in water gave the water a pale yellow colour.

Nothing seemed to happen for a long time of settling, and each of the three test tubes seemed to be settling equally quickly. However, a small difference was visible with the 3 measure tube.

When the test tubes were stoppered and shaken it was obvious that the test tube with 3 measures of FeSO4 had particles that settled out very quickly, as opposed to the other test tubes which stayed muddy after the shaking. The 3 measure test tube still had cloudy water though.


Iron(II) sulphate makes a big difference in making dirt particles settle out of water, but only if enough is used.

Very muddy water may need a lot of FeSO4 to work.


17 September 2008: After leaving the test tubes for ages (one and a quarter years), one was stained a yellow colour from excess ferrous sulphate. Where the settled mud at the the bottom of the test tube had collected was not stained. This shows it's not just porcelain that's affected by this sort of staining.