All you need is:
See through container
Fill your container about 2/3’s full with your oil. Then fill the other 3rd with water. Add some food colouring. When ready break the Alka-Seltzer tablet into a few pieces and add a piece to the container and watch what happens. When things slow back down just add more of the Alka-Seltzer tablet.
Things to talk about during and after this experiment
Oil & water do not mix. The molecules of oil do not like to mix with the water molecules. Water has a higher density than oil. Basically it is heavier which is why it sinks to the bottom of your container.
The food colouring doesn’t mix with the oil just the water.
The Alka-Seltzer when added to water makes bubbles of carbon dioxide. The bubbles attach to some water molecules and float to the surface, when they reach the surface the bubble pops and the coloured water sinks back to the bottom.
N.B Warning Alka-Seltzer is a medicine. This activity must be done with adult supervision. Please undertake this activity at your own risk.