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Lava fizz

By David Shaw, 12 July 2018

Lava lamps look super cool, but they can be expensive and hard to find. With this activity, you can make your own bubbly lava and learn a bit about oil and water!

You will need

• Water
• Tall glass
• Food colouring
• Spoon
• Vegetable oil
• Effervescent electrolyte tablets, such as Hydralyte tablets

What to do

1. Put a few centimetres of water in the bottom of a glass.
2. Add a few drops of food colouring to the water and stir.
3. Add vegetable oil to the glass until it’s a few centimetres from the top.
4. Wait a minute or two for the water and oil layers to separate.
5. Drop an effervescent tablet into the glass, and watch as the water bubbles and floats!

What’s happening?

Water and oil seem pretty similar at first glance. They’re both liquids, they’re both see-through, and a litre of each weighs about a kilogram. So why don’t they mix?

Chemically, oil and water are very different. Water molecules are a type of polar molecule. Polar molecules have a positively charged end and a negatively charged end. Oil molecules do not have charged ends – which makes oil a non-polar liquid.

Polar molecules are attracted to polar molecules, and non-polar to non-polar, which explains why oil and water don’t really mix well. Instead they separate out into layers. Water is a bit denser than vegetable oil, so the water settles to the bottom, and the oil floats on top.

Effervescent tablets react differently to oil and water. The fizzing of these tablets is typically caused by citric acid and sodium bicarbonate. These two substances are stable when they are solids, and they don’t do much in oil either.

But charged water molecules will rip citric acid and sodium bicarbonate apart. Floating freely, the acid reacts with the bicarbonate to produce liquid water and carbon dioxide gas.

Carbon dioxide gas bubbles are very light, but they are surrounded by a layer of water. As they float up, they drag that water with them. Eventually, the bubble makes it to the surface and pops. The water is no longer buoyed by a gas bubble, so it sinks back down to the bottom of the glass creating a lava lamp effect.

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