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Salem Clock Shop - 1085 Broadway Street NE, Salem, OR 97301 - (503) 581-3803 Fax: (503) 581-3331 |
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You can learn how dew and frost form by experimenting with a few simple items. |
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Making Dew & Frost |
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Fill one tin can half full with crushed ice. Add 4 tablespoons of salt and mix well for about 25 seconds.
Fill the second tin can half full with crushed ice and add enough cold tap water to just cover the ice. Place the thermometer in the can.
Let both cans sit for 2 or 3 minutes. Then look at the outside of both cans; the can with ice and water should have formed a coating of dew, and the can with ice and salt should have formed a layer of frost.
Read the thermometer that was in the can of ice and water. Then, place it in the other can, and after a minute or so, read the new temperature. It should be quite a bit lower. Why?
Explanation: The salt absorbs water creating a salt solution. The process requires heat to melt the ice into water. Surprisingly, that heat comes from the ice itself! This effect is caused by the chemical reaction between the salt and the ice. Therefore, melting the ice actually makes the mixture cooler. The salt water brine inside the can cools below freezing and moisture from the air that collects on the outside of the can will also freeze. This is why frost forms!
In the case of the second can, dew formed because the mixture of the melting ice and water was just at or just above freezing, and the warmer temperature outside the can caused water from the atmosphere (dew) to accumulate in liquid form. |
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Bill of Materials
· 2 tin cans · Thermometer · Rock salt or table salt · Crushed ice
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