Description

Ice cubes float on cooking oil but, on melting, the water that is produced sinks.


Apparatus

One 1dm measuring cylinder.


Chemicals

The quantities given are for one demonstration.

Water.

A few ice cubes. These may be made with a little food colouring (blue is good) for better visibility.

400 cm of cooking oil.

Method

Make the ice cubes with a few drops of food colouring per cube. Ensure that they are completely frozen. Partly frozen cubes may have liquid water trapped inside which will affect their density.

Check that ice cubes do actually float on the brand of cooking oil to be used.

The Demonstration

Place about 400cm of water and 400cm of cooking oil in the measuring cylinder. Allow the two layers to separate fully; the oil will be on the top. Drop an ice cube into the cylinder. It will float (just) on top of the oil. Watch the cube. As it melts, the water that is formed makes a droplet attached to the cube. Eventually this detaches itself from the cube and sinks, joining the water layer below. This illustrates the anomalously greater density of water compared to ice.