This is an interesting demonstration which can be conducted as a practical. It shows the difference between the density of water and the density of frozen water (ice) very graphically.
Some technician preparation is required for this practical. Firstly ice cubes need to be made using a small amount of food colouring to colour the ice. A dark blue will stand out best against the oil. Secondly a large measuring cylinder (1l) needs to be filled with half water and half cooking oil (or slightly less than half of each, leaving a gap at the top)
Equipment required (per set):
The cooking oil will sit on top of the water layer. On the surface of the oil is placed one of the coloured ice cubes. This should float on the oil showing that the oil is more dense than the cube. As the cube melts, water should form a droplet that then breaks through the oil layer and combines with the water layer. This shows that the water is more dense than the oil.
- 1l Measuring clyinder
- Cooking oil
- Coloured ice cubes (one per demonstration)
It works because The density of ice is about 0.92g cm-3 and water is about 1.00g cm-3 at 0°C. The cooking oil has a density somewhere between the two. This allows the ice to float on the oil and the water to sink through the oil layer.
The difference in density is due to the increase in volume as the water freezes.
When water freezes, the volume increases by about 9%. This increase can be shown by freezing 100ml of water in a measuring cylinder. When fully frozen the level should have increased by around 9%.