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 more dense than water will sink. Things that are 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 the air in your "diver" is less dense than water. When you compress air (put the same amount of air in a smaller area) you can increase the density of the air. Air compression will cause your floating "diver" to sink.

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.

Materials

• 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 a air tight lid (such as an empty 2-liter soda bottle)
• Drinking glass
• Water

Procedure:

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 than remove some wire.
4. Fill the bottle with water all the way up 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 the bottle 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.
8. Release the sides of the bottle. Take note of what happens.

Observations:

1. Record what you observe the dropper doing when the sides of the bottle are squeezed.
2. 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?

Safety concerns: Be sure to follow all glassware, and sharp object safety rules that are specified by your teacher in all general laboratory experiences.

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?