Give a test that can be used to differentiate chemically between butter and cooking oil.

Answer: Add a few drops of bromine water (or alkaline potassium permanganate) to each: cooking oil decolourises it, butter does not.

Cooking (vegetable) oils contain mostly unsaturated fats, which have C=C double bonds. Butter contains mostly saturated fats.

• Take a little cooking oil and a little melted butter in two test tubes.
• Add a few drops of bromine water (orange-red) to each and shake.
• In the oil, the orange colour disappears, because bromine adds across the double bonds (an addition reaction).
• In butter, the colour of bromine water remains (practically no change), because saturated fats do not undergo addition.

Alkaline \(\mathrm{KMnO_4}\) (pink) can be used in the same way: oil decolourises it, butter does not.