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2.18: Freezing

Difficulty Level: At Grade Created by: CK-12
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The man in this photo is climbing a frozen waterfall. Ice climbing is a dangerous sport that should be attempted only by highly experienced climbers. For one thing, ice is very slippery, which makes it harder than rock to grip. Clinging to the slippery vertical surface takes strength, training, and the right equipment.

From Liquid to Solid

You don’t have to be an ice climber to enjoy ice. Skating and fishing are two other sports that are also done on ice. What is ice? It’s simply water in the solid state. The process in which water or any other liquid changes to a solid is called freezing. Freezing occurs when a liquid cools to a point at which its particles no longer have enough energy to overcome the force of attraction between them. Instead, the particles remain in fixed positions, crowded closely together, as shown in the Figure below.

Freezing Point

The temperature at which a substance freezes is known as its freezing point. Freezing point is a physical property of matter. The freezing point of pure water is 0°C. Below this temperature, water exists as ice. Above this temperature, it exists as liquid water or water vapor. Many other substances have much lower or higher freezing points than water. You can see some examples in the Table below. The freezing point of pure water is included in the table for comparison.

Substance Freezing Point (°C)
Helium -272
Oxygen -222
Nitrogen -210
Pure Water 0
Lead 328
Iron 1535
Carbon 3500

Q: What trend do you see in this table?

A: Substances in the table with freezing points lower than water are gases. Substances in the table with freezing points higher than water are solids.

Q: Sodium is a solid at room temperature. Given this information, what can you infer about its freezing point?

A: You can infer that the freezing point of sodium must be higher than room temperature, which is about 20°C. The freezing point of sodium is actually 98°C.

Summary

  • Freezing is the process in which a liquid changes to a solid. It occurs when a liquid cools to a point at which its particles no longer have enough energy to overcome the force of attraction between them.
  • The freezing point of a substance is the temperature at which it freezes. The freezing point of pure water is 0°C.

Vocabulary

  • freezing: Process in which a liquid changes to a solid.

Practice

Road crews put salt on icy roads, and salt is also used to make ice cream. Do you know why? Read the article at the following URL to find out, and then answer the questions below.

http://science.howstuffworks.com/nature/climate-weather/atmospheric/road-salt.htm

  1. Describe how salt melts road ice.
  2. Make a table, using data in the article, to show how adding salt to water changes its freezing point.
  3. Explain why salt is used to make ice cream.

Review

  1. Define freezing.
  2. What happens to the particles of matter when it changes from a liquid to a solid?
  3. What is the freezing point of a substance? What is the freezing point of water?
  4. Adding antifreeze to water lowers its freezing point. Based on this statement, what can you infer about the freezing point of antifreeze?

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    Vocabulary

    freezing

    Process in which a liquid changes to a solid.

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    Difficulty Level:
    At Grade

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    Grades:
    7 , 8
    Date Created:
    Oct 31, 2012
    Last Modified:
    Sep 13, 2016
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