Coloured Ice and Density Experiment
Density can be a difficult idea to wrap your mind around whether you are a child or an adult. It isn’t possible to see density, just by looking at a particular liquid and so we need to find other ways to help kids to understand that different liquids have different numbers of molecules to each other.
One way to see density is to look at 2 liquids with different densities and the way that they interact. There are lots of different ways to do this, and I have several on this blog. I read about this particular experiment on ABC Science and thought I would share it. I have simplified the explanation here so that it is a little easier to explain than the original post! Best of all, this experiment is super safe so it can be done with children of all ages.
- 2 identical glasses
- Cooking salt (use cooking rather than table salt as table salt has a caking agent that will make the water cloudy)
- Food colour
The night before you want to do this experiment:
- Put ½ a teaspoon of food colour into a cup of water and stir.
- Pour the water into an ice-cube tray and put it into the freezer.
- Boil some water in the kettle.
- Pour the boiled water into a glass and fill it about 2/3 full.
- While the water is still hot, add salt one tablespoon at a time and stir until it is fully dissolved. Keep adding the salt until there is a bit of residue at the bottom of the glass that won’t dissolve.
- Fill another glass to the same level of water and place both glasses in the fridge overnight.
Conducting the experiment
- Place the glasses of water side by side. It doesn’t matter whether you know which one is which.
- Take 2 of the coloured ice-cubes and place one in each glass.
The water in the glass without salt will take on the colour of the ice-cube as it melts. The water in the ice-cube is the same density as the plain water and the two mix evenly as the ice-cube melts.
The glass with the salt will look quite different. As the ice-cube melts some of the colour will fall and tint the salt water but, the plain water in the ice-cube is less dense than the salt water and it will therefore collect at the top of the glass.
Cold water is more dense than warm water and by chilling the glasses of water, you increase their density, this increases the effect of the plain water collecting at the top of the glass of salt water.
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