You will need:
- Food colouring (the little squeeze bottles are very handy here)
- A tall glass or vase filled with water
- Vegetable oil
- Somewhere where you run the experiment without worrying about oil spills
Carefully pour some vegetable oil into a tall glass or vase filled with water. Keep pouring the oil until you have at least 2 cm of oil floating above the water.
You should find that the oil floats above the water in the glass.
Oil does not mix with water because oil is hydrophobic. This is because water , H2O, is made of 1 oxygen atom which carries two negative charges and the two hydrogen atoms each carry a positive charge. This causes water molecules to be polar in charge, which means that the charge across each molecule is uneven. Oils on the other hand are non-polar, which means their charges are spread evenly across the oil molecule and as such are repelled by the polar water molecules.
Also, the oil floats above the water because it is less dense than water (ie. less mass per volume). Try this density column science activity to look even further at floating liquids!
Gently squeeze the bottles of food colouring above the oil. Do this with as many colours as possible!
Firstly, the food colour beads drop through the oil as they are denser than the oil.
You should find that the food colour droplets form bead shapes at the boundary between the oil and water layers.
The bead shapes form as the water-based food colouring is repelled by the oil and this repulsion from the oil plus the surface tension in the food colouring around each droplet pulls them into a sphere shape.
Once the food colouring droplets break the surface tension of the water layer, they stream downwards towards the bottom of the glass. This is because food colouring is denser than water.
You will notice that the food colouring spreads out as it drops through the water. This is because the food colouring is water-based and so is able to mix in water. The food colouring will continue to spread out in the glass as the food colour moves from high concentration to low concentration.
The food colour beads will continue to drop out of the oil, producing the classic ‘fireworks in a glass’ image. Keep adding more food colouring and watch the action!
Eventually, the food colouring will spread out throughout the water in the glass, leaving the lighter coloured oil floating above it.
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Variables to test
- Try different oil above the water – do the food colour droplets fall through each oil at the same speed?
- What happens if you add salt or sugar to the water first?
- What happens if you change the temperature of the oil, water or food colouring?
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I love how the fireworks in the glass stood out and l learnt that the charges make a positive charge so the veggie oil and water don’t mix!
I added little bit to much oil and It took longer to go though
Yes! This will happen for sure. Try different types of oils, which types of oils let the food colouring drops fall down faster? Let us know!
It would have to take a long time
Not too long 🙂