Density

Wildlife Conservation Center Aquarium 8^th Grade:

Topic: Density

Grade level: 8th
Objective:
Students will determine that gases can be compressed under pressure and how pressure affects the human body and mechanical devises.
Background information
�� Imagine entering a fresh water pond of lake. Take a deep breath and you're likely to float. Exhale, and you'll probably find yourself sinking. These "ups and downs" depend upon the amount of air in your lungs. As the volume of gas increases, you become more buoyant. As the gas volume drops, you lose buoyancy and begin to sink.
�� An unusual behavior of ice can be explained by its density. An ice cube, a solid, floats on water, a liquid. Ice floats because it is less dense than water. As water freezes, the water molecules spread apart. They are less densely packed than near-freezing water. As a result, the same amount of matter takes up more space, or is less dense. Thus, ice floats on water.
Materials needed:
�� Clear plastic bottle with tight fitting lid (1-liter soda bottle)
�� Two inch section of a straw
�� Paper clip
�� Clay
�� Water
Procedure:
1. Fill the soda bottle with water.
2. Fold the straw in half and close with the paper clip. Place the paper clip on the inside walls of the straw.
3. Pace a small amount of clay on the end of the paper clip.
4. Lower the straw unit into the container with clay facing down. Keep the straw unit upright so that air is trapped in the straw.
5. Fill the bottle completely to the top and screw on the lid tightly.
Question:
Ask the students to figure out a way to sink the straw unit without moving (putting it upside down, etc) the soda bottle before giving out the questions.

1. Squeeze the center of the plastic soda bottle. What happens to the straw unit?
2. Release your pressure. What happens now?
3. Why, without squeezing, does the straw unit float?
Conclusion:
What is the relationship between pressure and volume?
Many sauces are denser than water, but it is the air bubble at the top of the sauce that determines whether the packet will sink or swim. Squeezing the bottle causes the bubble to shrink. This smaller bubble is less buoyant and
the packet sinks.