Here's why:
* Bromine (Br2) in water is a reddish-brown solution.
* When a hydrocarbon is added, the bromine reacts with it in a halogenation reaction. This reaction typically involves the addition of bromine across a double or triple bond in the hydrocarbon.
* The product of this reaction is a brominated hydrocarbon, which is usually colorless.
* As the bromine reacts with the hydrocarbon, the orange-brown color of the bromine solution fades and eventually disappears, leaving a colorless solution.
Important Note: This color change is a common indication of the presence of an unsaturated hydrocarbon (one with double or triple bonds). However, it's not a definitive test for unsaturation, as some other reactions can also consume bromine.
