Molecules moving in warm and cold water

Add food dye to warm or cold water, to observe the rate of mixing at different temperatures.
Science content
Chemistry: States of Matter, Properties of Materials (K-7)
Chemistry: Atoms, Molecules (3-7)
Chemistry: Physical Changes, Solutions, Mixtures and Separating (2, 4, 5, 6)
  • tap water, hot and cold. just boiled water is most effective for the hot.
  • clear-sided containers, heat proof to stand boiled water
  • drops of blue and yellow food dye, or just use one colour

Add hot water to one container, and cold water to another. Allow to sit for 30 seconds so that the water is not sloshing in the container.
Drip one drop of yellow food dye at one side of each container, and one drop of blue food colouring to the other side of each. (Alternatively, just a drop of blue in each.)
Watch how the food colouring mixes through each.
The blue and yellow will mix much faster to make a green colour in the glass of hot water. (With blue alone it will mix into the warmer water faster.) This is because the hot water has currents in it (as water evaporates from the surface). The food dye molecules get carried along by these currents.
The colder water has slower currents, so the food dye does not mix into the water as quickly.

Note that this is not diffusion, but showing the water currents in hot and cold water.


Video explaining why this activity is not diffusion:

Grades taught
Gr 4
Gr 5