What it shows
A soap bubble is a thin film of water held between two layers of soap. Adding glycerine or sugar slows the evaporation of the water, so the bubbles live longer. You time bubble lifetimes for different recipes.
The science
A bubble pops when its wall thins and tears. Water in the film evaporates and the film drains downwards under gravity. Glycerine and sugar hold onto water and make the liquid thicker, so the film thins more slowly and survives longer. Soap lowers surface tension and stops the film from tearing as quickly.
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.
- Dishwashing liquid
- Water
- Glycerine from a chemist, or sugar
- Straw or a wire loop
- Four cups
- A stopwatch
- A tray or plate for catching bubbles
Safety
For this project: Do not drink the bubble mixture and keep it away from the eyes. Wipe up spills because soapy floors are slippery.
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
7 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.
Make four mixtures of 200 ml water with two spoons of dish soap, each in a separate cup.
Add nothing to cup A, one spoon of glycerine to cup B, two spoons to cup C and two spoons of sugar to cup D.
Stir gently so that no foam forms and rest for ten minutes.
Blow a bubble on a wet plate with a straw and start the stopwatch.
Record how many seconds it lasts until it pops.
Repeat ten times for each recipe and take the average.
Draw a bar chart of the average lifetime for each recipe.
What you should see
Plain soap bubbles last a few seconds, while those with glycerine or sugar last longer. Two spoons of glycerine should give the best average lifetime.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is a bubble made of?
A thin film of soapy water around air.
2Why does a bubble pop?
The film thins as water evaporates and drains away.
3Why does glycerine help?
It holds water and slows evaporation.
4Why average ten bubbles?
To reduce chance errors.
Take it further
Test bubbles in humid monsoon air and in dry winter air and explain the difference.
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
Science Projects for Class 6 to 8
Working models matched to middle-school science topics
Science projects
Safe Chemistry Projects for School
Experiments and models with kitchen materials and proper safety