Lycopene is the molecule responsible for much of the red colour of tomatoes.
When bromine water is added to tomato juice, various addition reactions occur, with bromine molecules reacting with some of the double bonds in the lycopene molecules.
The wavelengths of light absorbed by the brominated products probably depends on the number of conjugated double bonds remaining, i.e. not reacted with bromine. In any case, many different colours are seen ranging from yellow to green and blue.
Here’s an animation of one such experiment. First bromine water is added to tomato juice in a measuring cylinder and then the mixture is stirred with a glass rod.
Almost a rainbow!
We also wanted tp see what happened when chlorine water and tincture of iodine were added to tomato juice.
Unsurprisingly, the chlorine water bleached the tomato juice white, whilst the tincture of iodine produced some interesting, if quite dark, green and blue colours after some time.
In summary, some interesting colours were seen in the tomato juice, although these demonstration experiments must be carried out in a fume cupboard. They can lead onto a discussion of addition reactions in alkenes and a test for alkenes, such as ethene, by decolourising bromine water.