|MadSci Network: Chemistry|
Most substances dissolved in water will tend to reduce the rate of evaporation and increase the boiling point. This effect will be greater the larger the number of molecules dissolved in water, so for something like food colouring where a very small number of molecules is involved it is unlikely you would see any effect.
The rate of evaporation is reduced because the main driving force behind evaporation is that the vapour form of a substance will be much more disordered than the liquid form. Adding something to a pure liquid makes it more disordered, so there will be less of a difference between the liquid and the vapour and the rate of evaporation will be slower.
You might want to try making a fairly concentrated solution (for example, of salt or sugar) and see if you can observe any differences in evaporation rates between them and distilled water. You will need to make sure that you use identical vessels and conditions for the pure water and the solutions; you can never be too careful when collecting data.
Try the links in the MadSci Library for more information on Chemistry.