Frosty: How Does Frost Form?

Problem

How Does Frost Form?

Materials

• 7-ounce (210-ml) plastic glass
• Ice
• Tap water
• Paper towel
• 4 tablespoons (60 ml) rock salt (used to make homemade ice cream)
• Timer

Procedure

1. Fill the glass three-fourths full with ice.
2. Cover the ice with water.
3. Dry the outside of the glass with the paper towel.
4. Sprinkle the salt over the ice.
5. Gently shake the glass back and forth four or five times to mix the ice, water, and salt.
6. Scratch against the outside of the glass with your fingernail every 15 seconds for 2 minutes.

Results

A very thin layer of soft, white ice forms on the outside of the plastic glass, usually during the first 15 to 30 seconds. The frosty layer of ice is thicker after 2 minutes.

Why?

Frost is a light deposit of small, thin crystals of ice that form on cold objects when water vapor changes directly into a solid. Frost occurs when a layer of air comes in contact with a surface having a temperature below freezing (32 degrees Fahrenheit or 0 degrees Celsius). Salt lowers the temperature of the icy water below freezing, which cools the glass to a below-freezing temperature. Air touches the glass and, when this happens, the water vapor in the air changes directly into ice without first changing into water. The change from a gas directly to a solid without forming a liquid is called sublimation. Frost usually forms in nature when the nights are clear, cold, and calm and the air above the surface is relatively moist. Frost, like dew, is not precipitation because it does not fall from the sky, but forms directly on objects. The white color of the frost is due to air trapped inside the ice crystals.

Let's Explore

1. How much does humidity affect the formation of frost? Repeat the experiment on days of varying humidity. The local weather report in the newspaper or on television will give you daily humidity readings.
2. How much does wind speed affect the formation of frost? Repeat the original experiment four times. The first time, place the glass in a box where it will receive no wind. For the remaining trials, set the glass about 1 yard (1 m) from a fan. First perform the experiment with the fan on low speed; then with the fan on medium; then with the fan on high speed.
3. At what temperature does the frost form? Repeat the original experiment placing a thermometer in the glass. Carefully add the ice, water, and salt. Record the temperature at which the frost begins to form.
4. Repeat the original experiment using containers made of different materials such as glass, paper, and metal. Science Fair Hint: The different containers and the results can be used as part of the display.

Show Time!

1. Another way to produce frost is to place an empty drinking glass in a freezer for 15 minutes. Remove the glass and scratch the outside surface with your fingernail for evidence of frost formation. If no frost forms, repeat the experiment on days of varying humidity.
2. Does the temperature difference between air and the object it touches affect the formation of frost? Compare the effects of cold air touching a cold object and warm air touching a cold object by placing a small baby food jar inside a large, wide-mouthed jar. Secure the lid on the large jar and place it in the freezer. After 30 minutes take the jar out. Leave the lid closed, and observe the surface of the two jars for 2 to 3 minutes. You can remove any frost that might form on the outside jar with a wet paper towel in order to observe the surface of the smaller jar inside.
3. Frost is not frozen dew. Prepare dew by filling a plastic glass with ice and covering the ice with water. Allow the glass to stand until large drops of water (dew) form on the outside. This should take about 5 to 10 minutes. Place the glass in a freezer. Remove after one hour and observe the frozen dew. Compare the appearance of frost made in previous experiments to the frozen dew.

Check it Out!

White frost is called hoarfrost. Hoarfrost forms when water vapor touches a very cold surface and freezes instantly, leaving long spiky needles. Hoarfrost usually occurs when the air is humid, the temperature of the air is around the freezing point (32 degrees Fahrenheit or 0 degrees Celsius), and the surface the air touches is significantly below the freezing point. Black frost occurs when the air is relatively dry and the temperature is below the freezing point. Find out more about frost. How is the intensity of frost determined? What is radiation frost? How is fern frost formed?

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