Homemade Lava Lamp Experiment: Why Do the Blobs Rise and Fall?
By Ahmed, founder
A homemade lava lamp is one of the most-searched kids’ science projects, and the reason is simple: it looks wonderful. But there’s a real science question hiding in the blobs: why do they go up, and why do they come back down?
What you need
- A tall clear glass or jar, about 300 mL (10 oz). Clear plastic is safest.
- Vegetable or canola oil
- Water
- Liquid food colouring
- Fizzy tablets, such as store-brand antacid or fizzy vitamin C (a grown-up handles and breaks them)
The method
- Fill the glass about three quarters full with oil.
- Pour in water until it’s nearly full. Watch what happens: the water sinks and sits under the oil.
- Add about 8 drops of food colouring. The drops drift down through the oil and then burst into the water.
- Break a fizzy tablet into a few pieces and drop one in.
- Watch the blobs. They rise, pop at the top and sink again.
When the show slows down, add another piece of tablet. Keep a tray under the glass and don’t drink it or seal it with a lid, since gas is being made.
Why the oil and water don’t mix
Oil floats on water because oil is less dense. Density is how much stuff is packed into a space. A cup of water has more mass than a cup of oil, so the water sinks below it. Also, water molecules prefer to stay with other water molecules, so the two don’t blend.
The food colouring is water-based, which is why it falls through the oil, then stays in the water layer.
Why the blobs go up and down
When the tablet hits the water, it fizzes. It is making bubbles of carbon dioxide gas. The gas bubbles stick to blobs of coloured water and carry them upward, because the bubbles are very light. At the surface, the bubbles pop and release the gas into the air. Now the blob is heavy again, and it sinks.
That cycle repeats until the tablet runs out. It’s a lava lamp without the lamp, and without a hot bulb.
A fair test: does warm water change the show?
Make two identical lamps. Use cold tap water in one and warm tap water in the other (a grown-up runs the tap, and the water should be comfortably warm to touch, never hot). Drop in the same size piece of tablet at the same time.
Which one fizzes faster? Warmer water usually makes the reaction a little quicker, but the difference can be modest, so watch for several seconds and count the bubbles or time the first blob reaching the top. Run it twice to see whether the difference holds. That’s a real fair test, with a clear variable and a measurement.
This is the experiment in our lesson Why do the blobs in a lava lamp go up and down?, which includes a version for younger kids and one for older kids.
Questions kids ask
Why doesn’t the oil turn colours? The colouring dissolves in water, not oil.
Is it a chemical reaction? The fizzing is. The tablet reacts with water to make carbon dioxide. The layers, on the other hand, are just physical: density and separating liquids.
Can I use salt instead of a tablet? Yes. A teaspoon of salt sprinkled on the oil sinks, carrying oil droplets down with it, and then the oil rises back. It’s slower and calmer, but it works.
Safety notes
Keep tablets away from small children. Don’t taste anything. A grown-up should handle the tablets, wipe up spills (oil is slippery) and dispose of oil in the bin rather than the sink.
More ideas
For more experiments like this one, look at Kitchen Chemistry for Families, or try the oobleck experiment. You can sign up for three free lessons or see everything on the parents page.