Water Droplets

What you need

  • A hotplate
  • Beakers of water, both clear and coloured (with a few drops of food colouring)
  • Droppers

What you do

  1. Turn the temperature up on the hotplate so that it sits at about 100C.
  2. Drop some water onto the hot plate and watch it boil away.
  3. Turn the hotplate up to a temperature in excess of about 160C.
  4. Drop a few drops of coloured water onto the very hot surface and watch them skitter around.

What's going on?

This is an example of what is called the Leidenfrost effect.

A drop of water on a hot plate at 100C flattens into a splat as the water boils away.

When the hot plate is much hotter the first part of the water droplet begins to evaporate before the droplet starts to deform. This water vapour cushions the rest of the droplet and insulates it from the hotplate. The result is a drop of liquid sitting on the hotplate in much the same way as the drop would sit on a cool table. The layer of insulating vapour makes the droplet float around on the hotplate like a hovercraft.

At much lower temperatures you can see the same effect with liquid nitrogen dropped on a table - the drops move in the same way.

Special Safety advice

Protect eyes and hands from the boiling drops of water and be aware that the hotplate needs to be very hot in this experiment.