You will need
- Newspaper or wipe down tablecoth
- Ketchup sachets
- Clear plastic water bottles with caps
Before you begin
- You may want to run this experiment as one of a few bases.
- Choose ketchup sachets where the ketchup is densely packed in, or ones made from both plastic and foil.
- Cover a table with newspaper or a wipe down tablecloth. You could also run the experiment outside.
Make ketchup bottles
- Split into small groups.
- The person leading the experiment should give everyone a clear plastic bottle. Everyone should fill it to the top with water.
- Everyone should fold a ketchup sachet in half lengthways and slide it into the bottle. If it sinks straight away, the air bubble inside is too small – take it out and try another sachet.
- Everyone should screw the lid onto the bottle.
- Everyone should gently squeeze the bottle and watch as the ketchup sachet begins to sink to the bottom. They should release their grip and watch it rise to the top again.
- Everyone should keep going until they’ve seen the sachet rise and sink. People could add in some drama by pretending to be a magician and commanding the sachet to rise and sink. Can anyone make their sachet float in the middle of the bottle?
The science behind the magic
- Everyone should gather together. Everyone should think about how the ketchup bottles worked, and people should share their ideas.
- The person leading the experiment should pass a ketchup sachet around and everyone should squeeze it gently. They should be able to move the ketchup around a little bit because there’s a pocket of air inside the sachet.
- The person leading the experiment should explain that when someone squeezes the bottle, the air in the ketchup sachet is also squeezed. It gets packed in to a smaller space, so it becomes denser and sinks.
- Everyone should think about other items that float in water. They may think of things like bath toys, sticks, ice, or wax. Everyone should try to explain why they float. The person leading the experiment should explain that items that are less dense than water float. Items full of air are usually less dense than water.
This activity gave everyone a chance to problem solve when they worked out the science behind the magic of the experiment. Did anyone have an unruly sachet that didn’t float or sink like the others? What did they do? How did people control how quickly the sachets rose and sank—how did their actions change the results? What else could people use instead of ketchup sachets to create a similar experiment?
This activity also involved using science skills to do an experiment and watch what happened. Some people may have found it easier to hear or read all the instructions before they started, while others may have preferred to figure it out as they went along. People could talk about what they preferred. What did everyone think would happen when they first put their sachet in the bottle? Was it the same as what happened?
- Rubbish and recycling
All items should be clean and suitable for this activity.
- Water games and activities
Be careful when doing activities with, in, or near water. Check surfaces and reduce the risk of slipping where possible. Make sure you have appropriate supervision for this activity.