When copper sulphate (CuSO4) and sodium hydroxide (NaOH) are used for protein test, they react to form a compound called copper hydroxide (Cu(OH)2). This compound has a characteristic deep blue colour, and its formation indicates the presence of protein in the sample being tested.

The reaction between CuSO4 and NaOH can be represented as follows:

CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4

In the presence of protein, the copper hydroxide formed reacts with the protein to form a complex compound called the biuret complex. This complex also has a deep blue colour, and its formation is the basis of the biuret test for protein. The biuret test is a widely used qualitative test for the presence of protein in a sample.