This week we are going to make our very own lava lamps!
You will need…
- A clear plastic bottle – I used an old 2L Cola bottle (washed up of course)
- A bottle of vegetable oil
- Alka Seltzer
- Food colouring (a bright colour is best!)
Fill the bottle a quarter full with water. Top up to the (near) top with the vegetable oil.
The oil and water should separate into two layers, water at the bottom and oil on the top.
Add about 6-8 drops of food colouring once they have separated. The colour will mix with the water at the bottom.
Pop in half an alka seltzer tablet and watch the bubbles form. Add more alka seltzer bit by bit to keep the bubbles rising and falling.
The Science bit
Firstly water and oil will not mix – this is because we say that water is a polar molecule – it’s structure means that is has a positive charge one end and a negative charge the other. Water molecules stick together because the positive end of one water molecule is attracted to the negative end of another.Oil molecule structure is different – it is non polar meaning that its charge is more evenly spread out, so the oil is not attracted to water – in fact we call it hydrophobic (water fearing) so it tries to get as far away from water as possible and will not mix. The reason that oil rests on top of the water rather than underneath is because it has a different density to water.
As the alka seltzer is added (this is made of citric acid and sodium bicarbonate) it reacts with the water and form carbon dioxide gas and Sodium citrate. It is the carbon dioxide bubbles that carry the coloured water to the top.