Make Dew and Frost
2 tin cans without a lid (condensed soup cans work well, but peel the labels off)
rock salt or table salt
In one tin can put a mixture of crushed ice about half full and about 4 tablespoons of salt.
Mix it well for about 30 seconds and then let sit.
In the other can put only crushed ice and cool tap water. Fill the can about half way full of ice and then put just enough tap water in the can to cover the ice.
Note the frost forming on the outside of the can with the ice and salt mixture. Compare this with the liquid moisture on the outside of the can which contains ice only.
Why does this happen? The salt wants to absorb water to make a salt solution. To do that, the salt has to melt the ice into water. The heat required to melt the ice comes from the ice itself. The strange effect is caused by the chemical reaction between the salt and the ice. Strange as it seems, melting the ice actually makes the mixture cooler. The salt water mixture inside the can gets below freezing, so the moisture from the air that collects on the outside of the can will freeze. This is why frost forms!
On the other can, dew forms because the mixture of the melting ice and water is just at freezing and the temperature outside the can is warmer causing the dew to form.