The biuret test for proteins gives
- A. a violet colour with copper two ions in alkaline solution
- B. a brick red precipitate
- C. a silver mirror
- D. a blue black colour
Explanation
Copper two ions form a violet complex with the peptide linkages, so the test detects the presence of peptide bonds rather than any particular amino acid and requires at least two of them. A blue black colour with iodine is the test for starch and a brick red precipitate with Fehling's solution indicates a reducing sugar. The ninhydrin test is the alternative for free amino acids.