Oceans - Life and Livelihoods - STEM Challenge
Sink or Swim - Water Density
Experiment
This experiment is a simple and fun way to learn 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.
Less Dense
More Dense
Water has a density of 1 g/mL (or 1 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.
Gases have a low density. In a gas, the distance between molecules is very large. In contrast, the molecules in liquids are very close together. This means that molecules of gases are more easily compressed than molecules of liquids.
Let’s find out how changing the density of an object can change its buoyancy by making a cartesian diver! This experiment is named after René Descartes (1596-1650), a French scientist and mathematician who used the diver to demonstrate gas laws and buoyancy.
The Experiment
Materials:
Empty Plastic Two Litre Bottle
Drinking Straw (try and find a reclaimed one)
Small Paper Clip
Play-dough or Reusable Adhesive Putty
Thick Foil (ex: Foil Pan)
Scissors
Water
Step 1 – Take the foil and cut out your diver using the template. You can lightly sketch your diver on your foil before you cut him/her out. Note: The diver should be approximately an inch and a half tall.
Step 2 – Cut the straw and position it on the paper-clip as shown in the photo. Each end should be secured by the ends of the paper clip.
Step 3 – Slowly slide the straw onto the diver as shown in the photo. The diver should look like he’s wearing a scuba tank!
Step 4 – Then place a small piece of play-dough or putty on the diver’s feet. This will be his counter weight.
Step 5 – Fill a glass with water and drop the diver in. This will test to make sure it floats. It should float as shown in the photo. If it doesn’t float, your straw may have a hole in it, or you may need to adjust the amount of playdough on the feet.
Step 5 – Fill a glass with water and drop the diver in. This will test to make sure it floats. It should float as shown in the photo. If it doesn’t float, your straw may have a hole in it, or you may need to adjust the amount of playdough on the feet.
Step 6 – Fill the two-liter bottle with water. Make sure to fill it to the top! Carefully place the diver into the bottle (feet first) and screw on the lid.
Step 7 – What do you think will happen when you squeeze the bottle? What about when you stop squeezing the bottle? Write down your hypothesis, then try it out and observe what happens.
Conclusion
Why does the diver sink to the bottom when you squeeze the bottle?
And why does the diver float back to the top when you stop squeezing?
Challenges that you may encounter
If the scuba diver is too heavy and sinks right away, try with less play dough. ( he is more dense than the water).
If the play dough won't stick to the diver - try wetting play dough a bit first, or try bluetack.
Be sure to insert your diver feet first into the bottle of water.
Explanation
When you first put the diver into the bottle, the combined density of the diver, straw, paperclip and play dough is slightly less than the density of water, so it floats. A small bubble of air is 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. Water is forced up into the straw compressing the air bubble in the straw. As the air bubble gets smaller, its density increases, and the density of the diver does too. The diver begins to sink because it’s density is higher than the water’s density.
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 and density. That causes the diver to become less dense and float back to the top of the bottle.
Most fish keep themselves from either sinking or floating to the surface by using muscles to squeeze or relax a small air bladder or sac (with a small air bubble inside). These fish have this sac inside their bodies. By squeezing the sac smaller, the fish will sink. By relaxing their muscles, the sac increases in size, displaces more water, and a fish will begin to rise to the surface.
Humans use this same principle to control the buoyancy of a submarine. By pumping water in and out of tanks stored in the submarine, a submarine can be made to rise and sink.
