This is because it contains copper(II) ions, which are blue in color. Fehling's solution is actually a mixture of two separate solutions:
* Fehling's A: Aqueous solution of copper(II) sulfate (CuSO4)
* Fehling's B: Aqueous solution of sodium potassium tartrate (also known as Rochelle salt) and a strong alkali like sodium hydroxide (NaOH)
When these two solutions are mixed, they form the deep blue solution that is used in the Fehling's test.
