Answer: The blue colour of a biuret solution changes to violet.
Explanation: Biuret solution is a reagent that is used to test for the presence of protein in a substance.
A Biuret solution is composed of sodium hydroxide and copper II sulphate. Copper ions give the reagent its characteristic blue colour. Copper ions react with peptide bonds in the protein in an alkaline solution to form a Biuret compound with a violet colour. The copper ions which have two positive charges are reduced to a positive charge, thereby causing a colour change.