The substance commonly used to test for an unsaturated hydrocarbon is bromine water. Bromine water is a reddish-brown liquid that reacts specifically with unsaturated hydrocarbons that contain double or triple carbon-carbon bonds. When bromine water is added to an unsaturated hydrocarbon, the orange-brown color of the bromine water disappears as the bromine adds across the double or triple bonds in the hydrocarbon. This decolorization indicates the presence of unsaturation. In contrast, saturated hydrocarbons (with only single bonds) do not react with bromine water, and the color remains unchanged. Additionally, potassium permanganate (KMnO4) in alkaline solution is also used as a test reagent for unsaturation, known as Baeyer’s test, where the purple color of KMnO4 disappears and a brown precipitate forms if unsaturation is present. But the primary and most widely used test is the bromine water test due to its simplicity and clear visual indication of unsaturation in hydrocarbons.