READ: Changes of State

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.

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.

CK-12 Foundation, Physical Science. http://creativecommons.org/licenses/by-nc-sa/3.0/