Unsaturated fats change bromine water from orange to colourless. How?

Unsaturated fats usually have a C=C double bond, e.g. vegetable oil. So, it's 'unstable' because of the high concentration of negative charge (electrons). Bromine (B2) is not polarised. However, as it approaches the double bond, it polarises. We can this a permanent dipole to induced dipole reaction. Bromine than attaches to the hydrocarbon creating halogenated hydrocarbons (Bromine is a halogen). Since bromine is added on, we call this an addition reaction. This helps separate alkanes from alkenes. Diagram will help explain.