Fireworks in a glass : Fizzics Education

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Fireworks in a glass

Fireworks in a glass

Follow FizzicsEd 150 Science Experiments:

You will need:

  • Food colouring (the little squeeze bottles are very handy here)
  • A tall glass or vase filled with water
  • Vegetable oil

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Vegetable oil, 4 food colouring squeeze bottles and a vase filled 2/3 with water
1 Pouring vegetable oil into a water-filled glass vase

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.

2 Vegetable oil floating in 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!

3

Gently squeeze the bottles of food colouring above the oil. Do this with as many colours as possible!

4 Droplets of food colouring suspended between the oil and water layers in a glass.

You should find that the water droplets initially drop through the oil and form beads of food colouring at the boundary between the oil and water layers.

The beads form as the water-based food colouring is repelled by the oil and this repulsion from the water plus the surface tension around each droplet pulls them into a sphere shape. These food colour beads drop through the oil as they are denser than the oil.

5 Yellow and blue food colouring falling through water from a layer of oil. The droplets are spreading through the water.

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.

6 Red, green, yellow and blue food colouring from underneath oil floating above water in a glass vase

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!

7 Dark coloured water below a layer of ligh yellow coloured oil floating above it.

Eventually, the food colouring will spread out throughout the water in the glass, leaving the lighter coloured oil floating above it.

8

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?

More on variables here

Learn more!

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