Weighing air
You can make a scale with which to weigh air! A lamp with an ordinary
light bulb will make the warm air you need.
Materials
lamp
paper bags (2, same size)
sticks (2, long)
string
tape
Balance one stick between two chairs or two small tables. Make sure the
stick is steady.
Cut two pieces of string. Tape one end of each piece of string to the
bottom of a paper bag. Then tie one bag to each end of the second stick.
Open the bags all the way.
Balance the second stick across the first stick. Make sure that the
stick is straight and that the bags hang down evenly.
Turn on the lamp. Stand it under one of the paper bags, so that the heat
from the light bulb warms the air in the bag. What happens as the air
gets warm?
When the air inside one bag warms up, the two bags are no longer
balanced. The bag above the lamp moves up, and the other bag moves down.
The heated air in the bag above the lamp expands as it warms up—the
molecules push farther and farther away from each other. Some of the air
is pushed out of the bag.
The air left in the warm-air bag has many fewer molecules than the air
in the cool-air bag. It weighs much less than the cool air. So the bag
of warm, light air rises, and the bag of heavy, cool air sinks.
