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 at all.
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 molecules are moving around faster and bump into the food dye molecules more frequently, thus moving them through the water more quickly. The term for molecules moving and mixing together in a liquid or gas is "diffusion".

With older students, try acting out the molecules in cold and warm water:
Give students each a coloured card, so there are equal numbers of two different card colours.
Ask students to move around slowly, as if they are in a cold liquid. Stop them after a minute, and see that the colours are not mixed much.
Then repeat with a warmer temperature and the students moving around faster for the same amount of time (one minute). This time, the colours will be much more mixed up.


Show molecular motion in solid water too - freeze a drop of food dye in ice and see that it does not move at all. (?)

Grades taught
Gr 4
Gr 5