This lovely, colourful experiment is super easy to set up and carry out but looks amazing! As well as being fun to watch it’s also great for learning about density ( the oil later floats on top of the water ). 

Printable Instructions for a Firework in a glass investigation #Bonfirenight #scienceforkids

Colourful Jar Experiment

You’ll need

A jar or glass

Warm water


Food Colouring


a glass of water and food colouring ready for a colourful water experiment


Fill the glass or jar about 3/4 full of warm water

Carefully pour a small layer of oil onto the surface of the water and leave to settle. Note that oil and water don’t mix!

Use a pipette to carefully drop small amounts of food colouring into the jar. You should see the food colouring drop through the oil into the water leaving colourful trails behind.

Image of droplets of food colouring sitting on top of a layer of water.

Why do oil and water not mix?

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. The structure of an oil molecule structure is different – it is non polar, which means its charge is more evenly spread out . In fact oil is hydrophobic (water fearing) so it tries to get as far away from water as possible.

Why does oil sit on top of water?

The reason that oil rests on top of the water rather than underneath is because oil is less dense than water.

oil, water and food colouring

Why does food colouring not mix with oil?

The food colouring we used was water based and therefore does not mix with the oil, but sinks through the oil into the water below.

Why does the food colouring leave trails?

Since the addition of the colouring makes the food colouring heavier than the water it sinks to the bottom leaving trails (resembling fireworks) as some of the colour diffuses into the water.

Firework in a glass or colourful jar experiments for kids. Watch as the food colouring bursts through the oil layer into the water #scienceforkids #fireworkcraft

More Science Experiments with Water

Rookie Parenting has a brilliant travelling water demonstration.

Try my easy density trick using salt, water and food colouring.

salt water density trick

Discover the best material for waterproofing a dinosaur!

Try one of my many other water science experiments for kids!

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Originally posted at Science Sparks