A student heats a test tube containing a large amount of protien and biuret reagent. She notices a color change from violet to blue. Explain why.

Biuret reagent is normally blue but it changes colour to violet in the presence of protein because Cu²⁺ of the reagent binds to the peptide bonds of protein. So, a mixture of protein and Biuret reagent is violet. However, when proteins are heated, they denature, which means their peptide bonds are broken down. Also, the complex between Biuret reagent and now destroyed peptide bonds does not exist any more, the reagent is free again and therefore, blue.