Moving molecules
Here is a way to show that hot and cold molecules move at different
speeds.
A hot-and-cold contest
Materials
bowls (2, large)
food coloring
medicine dropper
pan (bigger than bowl)
water
Fill one bowl with the coldest water you can get from the faucet. Then
fill the other bowl with the hottest water you can get. Place both bowls
on a table.
Wait until the water stops moving. Then use the medicine dropper to
put three drops of food coloring in the center of each bowl. (Don’t let
the dropper touch the water.) Do this quickly, so that the drops go into
each bowl at almost the same time. How fast does the food coloring
spread in each bowl?
The food coloring spreads much faster in the hot water than in the
cold water. The fast-moving molecules in the hot water spread the
molecules of food coloring rapidly.
Warming up
Fill one bowl with very cold water. Place it in the pan. When the water
stops moving, put three drops of food coloring in the center of the
bowl.
Watch the food coloring spread for fifteen seconds. (You can count
slowly to fifteen.) Then pour very hot water into the pan in which the
bowl is sitting. What happens?
The food coloring spreads slowly until the hot water is poured into the
pan. Then it spreads faster! The hot water outside the bowl heats the
cold water in the bowl—it makes the molecules speed up.
