Swirling colours – an experiment that demonstrates Brownian motion and the movement of water molecules.
This experiment is a triple whammy! It not only looks great, but it’s also a simple example of Brownian motion and demonstrates how temperature affects the movement of water molecules.
You could ask students to use four cocktail sticks (two for each water temperature) to ensure that their experiment is a fair test and takes in to account control variables.
Each student will need:
- Food colouring (two different colours)
- Two cocktail sticks or wooden skewers
- A glass containing cold water
- A glass containing hot (not boiling) water
Student instructions
Try this colourful experiment in your kitchen. Safety first: do not use boiling water, hot water from the tap is fine and is less likely to burn you. Even so, be careful as the glass will get hot.
- Dip the end of one cocktail stick into one bottle of food colouring and one into the other. The cocktail sticks will soak up a tiny bit of the food colouring,
- Hold both sticks, one at each side of the glass, and dip them into the cold water.
- Now, quickly repeat steps 1-2 but do this in the hot water.
Now try to answer these questions:
What differences did you see between the hot and cold water?
Students will see that the food colouring in the hot water mixes much more quickly than the food colouring in the cold water. The water molecules are randomly moving and bumping into the food colouring molecules causing the food colouring to diffuse.
Explain why this happened. Remember what happens to the movement of water molecules when they are heated.
This is an example of Brownian motion. In the hot water the water molecules are moving faster, bumping into the food colouring molecules more often and with more force so diffusion happens much more quickly compared to the cold water.
Further interest:
Why not show your students the free video resource: Random Force and Brownian Motion?