What is the best method to distinguish unsaturated compounds from saturated compounds?

The best method to distinguish unsaturated compounds from saturated compounds is using bromine solution. This technique takes advantage of the reactivity of unsaturated compounds, which contain double or triple bonds, allowing them to undergo addition reactions with bromine.

When bromine solution, which is typically a reddish-brown liquid, is added to an unsaturated compound, the color of the bromine will fade or disappear as it reacts with the double or triple bonds in the compound. This reaction indicates the presence of unsaturation. In contrast, saturated compounds, which do not have these multiple bonds, will not react with bromine in the same way; thus, the bromine's color will remain unchanged.

Using litmus paper or a pH indicator primarily detects acidity or basicity and does not specifically identify the saturation level of a compound. Heat is a general technique but does not provide a direct or clear distinction between saturated and unsaturated compounds. Therefore, bromine solution is by far the most effective method for this purpose.