A purple colour was obtained when sodium hydroxide solution and a drop of copper sulphate solution were added…

Step 1: Analyze the question The question describes a food test. NaOH solution plus a drop of copper sulphate solution gives a purple colour. You need to identify which food class this test detects.

Step 2: Explain the concept This is the Biuret test, used to detect proteins.

• In an alkaline medium (NaOH), copper(II) ions (Cu²⁺) form a complex with the peptide bonds (-CO-NH-) in proteins.
• This complex gives a purple/violet colour.
• If there is no protein, the solution stays blue.

Step 3: Compare with other food tests

| Food class | Test | Positive result | |---|---|---| | Starch (carbohydrate) | Iodine solution | Blue-black | | Reducing sugar | Benedict's/Fehling's, heat | Brick-red precipitate | | Fat/oil | Ethanol emulsion test, or brown paper | Milky emulsion, or translucent spot | | Protein | Biuret (NaOH + CuSO₄) | Purple/violet |

Step 4: Identify the answer

• A. Carbohydrate: starch gives blue-black with iodine, and sugars give a red precipitate on heating with Benedict's. Neither gives purple here.
• B. Fat: it needs the ethanol emulsion test.
• C. Protein: correct, because the Biuret test gives purple.
• D. Sugar: it needs Benedict's or Fehling's solution and heat.

Step 5: Exam tip Remember "Biuret = Blue to Purple = Protein". Both start with P: Purple and Protein. Note that the copper sulphate is added only a drop at a time, since too much will mask the purple with its own blue colour.