This experiment takes a few minutes to set up, but once done you can do the experiment over and over. Kids will enjoyed taking turns making the scuba diver move up and down in the bottle.
- Empty Plastic Two Liter Bottle
- Drinking Straw
- Small Paper Clip
- Play-dough or Reusable Adhesive Putty
- Thick Foil (I used a Foil Pan)
Step 1 – Cut out your diver using the same shape and size indicated above. I lightly sketched my diver on my foil before I cut him out. Note: That is a small paper clip. The diver should be approximated an inch and a half tall.
Helpful Tip: Download the Template for the Bottle Diver Here
Step 2 – Cut the straw and position it on the paper-clip as shown above. Each end should be secured by the ends of the paper clip.
Step 3 – Slowly slide the straw onto the diver as shown above. The diver should look like he’s wearing a scuba tank. Then place a small piece of play-dough or putty on the diver’s feet.
Step 4 – Fill a glass with water and put the diver in. This is to test to make sure it floats. It should float as shown above. If it doesn’t float, your straw may have a hole in it. Try again with a new straw.
Step 5 – Fill the two-liter bottle with water. Make sure to fill it to the top, otherwise, the experiment won’t work. Carefully place the diver into the bottle and screw on the lid.
Step 6 – What do you think will happen when you squeeze the bottle? What about when you stop squeezing the bottle. Write down your hypothesis (prediction) and try it out and observe what happens.
Do you know why then you squeeze the bottle the diver sinks to the bottom and when you stop squeezing the diver floats back to the top.
Find out the answer in the how does this experiment work section below.
This experiment is also referred to as the Cartesian Diver Experiment and it is a simple and fun way to teach kids about density and how it impacts whether an object will sink or float.
Density is a measure of the mass per unit volume of a substance. Water has a density of 1 g/mL (g/cm3). Objects will float in water if their density is less than 1 g/mL. Objects will sink in water if their density is greater than 1 g/mL.
When you first put the diver into the bottle, the combined density of the diver, straw, paperclip and playdough is slightly less than the density of water, so it floats. A small bubble of air gets trapped in the straw when you put the diver in the bottle. When you squeeze the bottle, you increase the pressure of the water in the bottle, so water is forced up into the straw compressing the air bubble in the straw. As the air bubble gets smaller, the density of the diver increases and the diver begins to sink. When you release the bottle, the pressure lessens and the water moves back out of the straw. The air bubble in the straw returns to its original size causing the diver to become less dense and float back to the top of the bottle.
Other Ideas to Try
Try this experiment in different ways – try it with cold water in the bottle and then try it with warm water in the bottle. Does temperature affect the density of the diver?
I hope you enjoyed the experiment. Here are some printable instructions: