Activity: Make a lava lamp

What you need:

• A clear container or bottle
• Clear vegetable oil/baby oil
• Food colouring
• Water
• Fizzy tablet

What to do:

1. Fill your bottle a little more than halfway with baby oil and food colouring.
2. Add a splash of water to the bottle. Make sure you leave a small empty space at the top of the bottle. What do you notice? What happens to the water? Does the water float above or sink below the oil?
   
3. Now break off a little bit from your fizzy tablet and drop it in the bottle. Does the tablet float or sink? What happens next?
4. To restart the reaction, add another bit of fizzy tablet. Wait a while for the oil to settle if it gets too frothy.

How did your lava lamp work?

When you add water to the bottle, it sinks beneath the oil. This is because water is more dense than oil. Density is how heavy or light something is for its size. The oil floats because it is lighter (less dense) than the same amount of water.

Food colouring has the same density as water, so it sinks through the oil and mixes with the water, making it colourful.

The fizzy tablet is denser than both the oil and water. It sinks to the bottom and dissolves in the water, making a gas. The gas bubbles are less dense than water and oil. This makes them float to the top, carrying some coloured water with them. When the blobs of water reach the top, the gas escapes into the air and the dense water sinks again.

What more can you do?

• Store your lava lamp bottle by putting on its cap. You can bring it back to life anytime with another bit of fizzy tablet.
• Create a really cool ‘lava lamp’ effect by shining a bright light under the bottle! Groovy!
• Try adding salt instead of a fizzy tablet – what happens?
• Try making a bigger lava lamp using a bigger bottle for an even better effect!
• Have you ever blown bubbles under the water? What happens to the bubbles?