This practical can be conducted as a demonstration or can easily be set up for a science club type event. Although not necessarily part of the curriculum, some of the principles involved can be related to curriculum chemistry.
Equipment typically required (per set):
- A clear drinking glass
- 1/4 cup vegetable oil
- 1 teaspoon of salt
- Red food colouring (optional)
The glass needs to be filled about 3/4 with water and a few drops of red food colouring are added. Colour is optional but does add to the effect.
The oil needs to be slowly poured over the surface of the water. This will float on the water. Leave to settle until you can see two distinct layers.
Gently sprinkle some of the salt on top of the oil layer. This should make the oil blobs initially fall but then rise in the glass.
The way this works is that the salt is heavier than the oil and so when it sinks, it takes some of the oil with it to the bottom of the glass. Once individual grains of salt dissolve in the water, the drops of oil rise again. To repeat the effect, keep adding small amounts of salt.
To make this into a true experiment, students could try to work out if the effect continues as long as salt is being added. They could try adding sand or sugar instead of the salt and different types (different viscosity) of oil could be used.
A similar practical is "the rise and fall of a peanut" which can be accessed by clicking the link below.