by Melissa Jaramillo and Julie Snyder
Take a can or orange juice from the freezer and set it on the counter for a few minutes. That cold, sugary-looking coating on the outside of the can is frost. Scrape some into your hand. Did it melt into water? Where did the frost on the outside of the can come from? Did it appear out of thin air?
Actually, it did -- air has water vapor (water in the form of a gas) in it. You can not see water vapor because it is invisible. Warm air can hold more water vapor than cool air so when air is cooled below its dew point some of the water vapor condenses into liquid water droplets.
If the temperature falls below the freezing point of water (32°F, 0° C), the droplets freeze and ice crystals "grow" on them. Sometimes, the water vapor changes directly into ice crystals. If you live in an area with cold winters, you have probably seen frost on the ground. Frost forms best on clear, windless nights. As air near the earth cools below its dew point, frost begins to form on grass, leaves, and other cold surfaces.
• Crushed ice
• Two empty cans
• Large spoon
1. Label one can as "ice and salt." Add 2 cups crushed ice and 1/2 cup salt. Stir well.
2. Label the second can as "only ice." Put 2 cups crushed ice in it.
3. In a few minutes, touch the outsides of both cans.
4. Wait 30 minutes. Observe both cans again.
What did you learn?
Q: Did adding salt to the ice change its temperature?
A: Salt water has a lower melting point (or freezing point) than plain water, so the can with the salted ice will be coldest.
Q: Why did the dew on one can turn to frost and not the dew on the other can?
A: The dew on the salted ice can turned into frost because the melting point of water/salt is much less than the freezing point of water. It was as if that can had been placed in a freezer. The plain ice can was at the melting point of water, so the dew did not freeze.
Julie Snyder is a mom of six, interested in kids, pregnancy, birth, people and lives in the outlying Seattle area. Melissa Jaramillo is mom to many. She's passionate about building, encouraging, and strengthening families on this adventure known as parenthood!
Copyright © Melissa Jaramillo and Julie Snyder. Permission to republish granted to Pregnancy.org, LLC.