WHAT YOU NEED:
2 tin cans (remove the labels and lids)
Salt
Ice
Water
WHAT YOU DO:
Fill each can about half way with ice. Crushed ice works best because you have less empty space in the can, but cubes can work too.
In the can where frost will form, add about 4 tbsp salt and mix well. It doesn't have to be exact; if you're estimating, it's better to add too much than not enough.
In the can where dew will form, pour in enough cool water to cover the ice
That's it! This doesn't happen right away, so this can be a great time to teach your lesson while you wait for the reaction to play out. Or you can set up a time lapse (like mine below) to capture what happens.
Time lapse of an experiment to create frost and dew
WHAT IS HAPPENING:
Salt helps melt ice. Melting requires heat, so it pulls heat from the atmosphere around it which makes the atmosphere cooler around it. Cooling the atmosphere causes the little particles of water vapor in the air to condense onto the surface of the can. The mixture in the can is colder than freezing, so the water that collects on the outside of the can is frozen.
In the other can, the water and ice combination is around 32º and the temperature outside the can is warmer than that. Liquid collects on this can because the reaction we talked about on the other can isn't happening.
DISCUSSION IDEAS:
When do you notice frost more often? In the summer, spring, winter, or fall?
When do you notice dew more often?
