Experiment 30: Testing impurities in milk


  • To check the milk for impurities


Materials required:

  • 20mL milk
  • 1% iodine solution
  • test tube
  • soybean/ arhar powder
  • red litmus paper
  • dil. sulphuric acid

Procedure - Test for Starch

  • Add a few drops of tincture of Iodine or Iodine solution.
  • Formation of blue colour indicates the presence of starch.

Procedure - Test for Urea

  • Take a teaspoon of milk in a test tube.
  • Add ½ teaspoon of soybean or arhar powder.
  • Mix up the contents thoroughly by shaking the test tube.
  • After 5 minutes, dip a red litmus paper in it.
  • Remove the paper after ½ a minute.
  • A change in colour from red to blue indicates the presence of urea in the milk.

Procedure - Test for Formalin

  • Take 10 ml of milk in a tests tube and add 5 ml of conc. sulphuric acid from the sides of the wall without shaking.
  • If a violet or blue ring appears at the intersection of two layers then it shows presence of formalin.

Observations and Findings:

  • After the solvent has risen about 15 cm you will notice two different spots of blue and red colors on the filter paper. The pattern is called chromatograph.


  • Over time, we see a pattern composed of various colors / components of the ink


  • Household milk generally contains starch an impurity. In some cases, it might have urea or formalin (added to increase shelf life of milk).