Part of the show The Science of Nanotechnology
What you Need
A wooden spoon
Water ~ 250ml
Some Cornflour (Cornstarch) ~ a few teaspoons
What to do
Put the water in the Saucepan
Add a couple of teaspoons of cornflour to the water
Heat the water until it is simmering
THIS SHOULD BE DONE WITH AN ADULT
Keep stirring until something happens
Add some cornflour to the hot water, how does this behave?
What may happen
When the mixture gets hot and you stir it, it will get thicker and thicker
If you add the cornflour to the hot water it will turn into a lump covered in a jelly like substance.
We also heated some cornflour particles under a microscope with a hot air blower, which allows you to see what is going on.
Here is the same process as a video.
Why does it happen?
Cornflour is made up of tiny starch particles (< 0.01mm across). These are made up of long starch molecules all wrapped up tight, a bit like a ball of wool. In cold water these are stable.
If you heat them up in water the starch molecules start to dissolve, they unwrap and swell up hugely.
If you then stir the result the starch molecules from the different particles tangle with each other producing a great big tangled network. This flows slowly because the molecules take a while to untangle, so thickening the liquid. This is how you thicken soups and gravy.
If you add a lump of cornflour the particles on the outside swell and turn to a jelly like substance, which is actually quite waterproof, protecting the cornflour in the centre from the water, and creating a dry lump. So if you want to thicken gravy, make sure you mix the cornflour with cold water before you heat it up.