This test uses two chemicals, sodium hydroxide (colourless) and copper sulphate (light blue).
Proteins are polypeptides which are chains of amino acids held together by peptide bonds. The sodium hydroxide hydrolyses the peptides bonds cutting the protein up into short peptide chains. As these bonds are broken, some react with the copper (II) ions in the copper sulphate forming lilac/purple coloured compounds.
Note: Sometimes the sodium hydroxide and copper sulphate are premixed and called Biuret solution.
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