What it shows
Oil floats on water because it is less dense, and the two do not mix. A piece of fizzing antacid tablet (ENO or similar) dropped in makes carbon dioxide bubbles that lift blobs of coloured water up through the oil and let them fall again, like a lava lamp.
The science
Density is how much mass is packed into a space. Oil is less dense than water, so it floats on top, and because oil and water molecules do not attract each other they stay in separate layers. The tablet contains citric acid and sodium bicarbonate that react only when they reach the water, releasing carbon dioxide. A bubble attaches to a drop of coloured water, making the pair lighter than oil, so it rises; at the surface the gas escapes and the drop sinks back down.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A clear plastic bottle
- Cooking oil to fill three quarters of the bottle
- Water to fill the rest
- A few drops of food colour
- An effervescent antacid tablet or a spoon of ENO fruit salt, broken into pieces
- A torch to shine through, optional
Safety
For this project: Do the activity on a tray, and do not drink from the bottle afterwards. Keep the tablet pieces away from small children who might swallow them.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Pour the water into the bottle first, about a quarter of the way up.
Add the food colour and swirl so the water is strongly coloured.
Slowly pour the oil down the inside of the bottle until it is nearly full, and wait for the layers to settle.
Notice that the colour stays in the water layer and does not spread into the oil.
Drop in a small piece of tablet and watch the bubbles form at the bottom where it touches the water.
Shine the torch through the bottle from below for the full lava-lamp effect.
Add another piece when the fizzing stops; the bottle can be reused many times by adding fresh tablet.
Cap the bottle loosely, never tightly, while the tablet is fizzing so the gas can escape.
What you should see
Coloured blobs rise through the oil, hang at the surface, then sink in a steady cycle for a minute or two per tablet piece, with the two layers staying unmixed.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does oil float on water?
Oil is less dense than water.
2What makes the blobs rise?
Carbon dioxide bubbles stick to the coloured water drops and lift them.
3Why does the colour not spread into the oil?
Food colour dissolves in water, not in oil, and the two liquids do not mix.
4Why must the cap stay loose?
The gas needs to escape; a sealed bottle would build up pressure.
Take it further
Add a spoon of salt instead of the tablet and explain why the blobs rise and fall differently.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Easy Science Projects for Class 1 to 5
Simple, safe projects young children can make with a little help
Science projects
Safe Chemistry Projects for School
Experiments and models with kitchen materials and proper safety