How Does Frost Form?
One of the hallmarks of autumn, besides leaves falling and children returning to school, is frost. Frost is the fine layer of water crystals that forms from water vapor in the air. In August, you might have felt the wet dew covering the grass in the morning. Dew forms when water vapor in the air condenses onto cool surfaces. Condensation is the name for the process of a gas changing to a liquid. If it gets cold enough at night, the water vapor goes directly from a gas to solid. This process is called deposition. The reverse process, sublimation, is when a solid changes directly to a gas.
How does frost form? Jack Frost isn’t the only one who can make frost—you can too!
Problem: How Does Frost Form?
- 8 oz. clear plastic glass or a Pyrex-type glass measuring cup
- 1 cup of ice
- Two tablespoons water
- Paper towel
- 3 ½ tablespoons rock salt
- Thermometer (optional)
- Fan (optional)
- Take the 8 oz. plastic glass or Pyrex measuring cup, and fill it with ice. Why do you need to use a plastic or heat resistant container?
- Add two tablespoons of water, and stir. If you have a thermometer, record the temperature.
- Wipe the outside of the glass with the paper towel, to remove any condensation. What would this liquid be called if it was condensing outside?
- Add 3 ½ tablespoons of rock salt. Stir for thirty seconds.
- For the next five minutes, try scratching against the glass every thirty seconds.
- If you have a thermometer, record the temperature.
- Record what you see and feel.
An icy layer (frost) should form on the side of the cup. It might become thicker with time.
You needed to use plastic or Pyrex cup because the large temperature changes involved in this experiment could cause a regular glass to crack or shatter. When you first added the ice to the cup, water vapor in the air might have condensed on the glass, forming “dew” on the outside of the glass. Note that this water came from the atmosphere, not the water inside the cup. Ice (solid water) normally melts at 32 F. The addition of rock salt lowers the melting point to 30 degrees F or even lower. The liquid inside your cup forming from the ice is so cold that the water vapor in the air does not form a liquid, it forms a solid. Deposition caused gas to go straight to being a solid without being a liquid first. Repeat this experiment; the amount of frost you’ll get is likely to vary depending on the humidity. Humidity is how much moisture is in the air. On days with high humidity you are likely to get a thicker layer of frost.
How does wind affect the formation of frost? If you have a fan, you can find out. Set up the experiment the same way you did the first time, but after you add the salt, set your cup about one yard away from the fan set at low. Compare how long it takes frost to form and how thick it is with no fan. Repeat the experiment again, this time with the fan on high.
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