Which test is commonly used for unsaturation in organic compounds?

The bromine test is widely used to identify unsaturated compounds, such as alkenes and alkynes, due to the presence of double or triple carbon-carbon bonds. When bromine water (which is a solution of bromine in water) is added to an organic compound that is unsaturated, the reddish-brown color of the bromine will disappear. This occurs because the bromine reacts with the double or triple bonds through an addition reaction, forming a colorless dibromo compound. The decolorization of bromine indicates the presence of unsaturation in the molecule.

In contrast, the other tests mentioned do not specifically indicate unsaturation. The alcohol test is typically used to identify alcohol functional groups, the silver nitrate test is useful for identifying alkyl halides, and the litmus test is primarily employed to test for acidity or basicity in solutions rather than to determine the presence of unsaturation in organic compounds.