Butter is an animal fat which contains saturated fatty acids. On the other hand, cooking oil is a vegetable oil which contains unsaturated fatty acids. Now, unsaturated compounds decolourise bromine water whereas saturated compound do not. So we can distinguish between butter and cooking oil by the bromine water test. We take a little of butter and cooking oil in two separate test tuber and add some red brown coloured bromine water to them. The test tube in which the bromine water gets decolourised contains cooking oil. The test tube in which the bromine water does not get decolourised contains butter.
