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Activity

Molecules moving in warm and cold water

Summary: 
Add food dye to warm or cold water, to observe the rate of mixing at different temperatures.
Science content (2016 curriculum): 
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)
Lessons activity is in: 
Properties of materials: density, stickiness, elasticity, foaming
Materials: 
  • 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
Procedure: 

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 is "diffusion".

Notes: 

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 5
Teaching site: 
ingridscience afterschool