Which chemical test is used to detect the presence of a C=C double bond in alkenes?

• A. Bromine water
• B. Benedict's solution
• C. Litmus paper
• D. Iodine number

Answer: (A)

Bromine reacts with the carbon–carbon double bond by adding across it, turning the orange-brown bromine water colorless. This happens because the pi electrons in the C=C bond attack Br2, forming a vicinal dibromide and breaking the Br–Br bond, so the solution decolorizes. If there is no C=C double bond, the bromine water doesn’t react under these conditions, and the color remains orange-brown. The other options don’t test for unsaturation in the same quick, visual way: Benedict’s solution detects reducing sugars, litmus paper checks acidity/basicity, and iodine number is a more indirect measurement of unsaturation in substances like fats rather than a simple qualitative test for alkenes. So, bromine water decolorization is the test used to detect the presence of a C=C double bond in alkenes.