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One of the interesting facts of water relates to its density. You should remember that water becomes less dense as it freezes. This is the reason that ice cubes will float in a glass of water. Remember that things which are denser than water will sink. Things less dense than water will float.

Your challenge in this activity will be to create a "diver" which will float, but can be made to sink as well. Remember that floating occurs because your "diver" is less dense than water. When you compress the 2 liter bottle you force water into the dropper and increase the mass of your diver. This also compresses the air at the top of the dropper. The density increased and your floating "diver" sinks.

You will create a contained environment where the volume of a given container of air can be reduced by exerting pressure in it. You will demonstrate that you can compress air, making it more dense so that your container can sink in water. Drag your mouse over the link to see a video demonstration. Remember to close the window to return to this page.



  • Copper wire (thin for glass medicine dropper and thick for plastic medicine dropper)
  • Wire cutters
  • Medicine dropper (glass or plastic)
  • Large, clear plastic bottle with an air tight lid (such as an empty 2-liter soda bottle)
  • Drinking glass
  • Water
  • Paper for observations
Safety concerns: icon Be sure to keep all glassware, and sharp object safety rules. Remember not to put a stopper into any flask unless given direct instruction by your teacher.


  1. Wrap several turns of thin wire around the glass medicine dropper or cut a length of thick wire and stick it into the plastic dropper's tube.
  2. Place the dropper in the glass of water. It should barely float, with only the top showing.
  3. If the dropper floats too high, add more wire. If it sinks, remove some wire.
  4. Fill the bottle with water to the top.
  5. Put the dropper into the bottle (water will spill out).
  6. Screw the cap on tightly. No air or water should spill out when it is squeezed.
  7. Squeeze the sides of the bottle. Take note of what happens. If the dropper does not move, take it out and add more wire. Record what you observe the dropper doing when the sides of the bottle are squeezed.
  8. Release the sides of the bottle. What happens to the dropper when the sides of the bottle are released?

Analysis and Conclusions

  1. When you squeeze the bottle some of the water is pushed up into the dropper. Why?
  2. When you squeeze the bottle, the volume of the dropper stays the same.  What happens to its mass? (Think about the fact that some water is pushed up into the dropper.)
  3. Why does the dropper sink when you squeeze the sides of the bottle?
  4. Why does the dropper rise when you release the sides of the bottle?
  5. How is the density of an object related to how well it floats in water?


Now that you have experimented with density, it is time to check and see how well you understand density! Read each question carefully and then drag your mouse over the answer you have chosen. You will discover how well you know density as you check your answers this way.

The Density Quiz

1.  Would you float on water if your mass was 10 g and your volume was 100 mL?

2.  What would happen to the density of an object if you decreased the mass?

3.  What would happen to the density of an object if you decreased the volume?

4.  Which would float on water?

5.  How could you make a piece of steel float?

Review science safety rules here.

Get the plug-ins: Get Adobe Acrobat Reader , and Get Quicktime Player. (The Quicktime plug-in is needed to play sounds and movies correctly.)

Want to share photos of you or your friends doing this activity? Send it in an e-mail with the following information:

  1. The title of the activity
  2. The URL (Internet address)
  3. Your name.

Remember that no pictures can be used that show student faces or student names on it. 

Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE 7th grade science core.

Updated October 24, 2008 by: Glen Westbroek

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