Changes of State

A change of state, or phase change, is a transformation of matter from one state to another.

Ice melting in the spring is an example of fusion. When the temperature rises, the ice absorbs heat (energy) and becomes liquid.

Boiling water is an example of rapid vaporization. The water absorbs heat (energy) from the stove and becomes gaseous.

Drying wet clothes on a clothesline is also an example of vaporization. To be more precise, it is evaporation: the water evaporates from the fabric into the air and the clothes become dry. It is a slow process that occurs at a temperature that is lower than water’s boiling point.

Dry ice is carbon dioxide (CO₂) in solid form. Carbon dioxide is solid at temperatures below -78°C and gaseous at temperatures above -78°C. When dry ice is placed in an environment at room temperature, it sublimates. It passes directly from the solid state to the gaseous state, without passing through the liquid state. This is why it is called dry ice.

Water freezing on a lake is an example of solidification. As the temperature drops, the water in the lake loses heat (releases energy) and becomes solid.

The fog that forms on windows is an example of condensation. When water vapour in the air comes into contact with a cold window, it loses heat (releases energy) and becomes liquid.

The frost that forms on the lawn in the morning is a result of deposition. When the water vapour in the air comes into contact with cold grass, it loses heat (releases energy) and becomes solid.

A phase change diagram is a graph showing how the temperature of a substance changes over time. The plateaus correspond to phase changes.

When ice is heated in a closed container until it turns completely to vapour, the graph of water temperature over time is similar to this one.

1. All water is solid below 0°C. It absorbs energy (heat) which increases its temperature.

2. At 0°C, water continues absorbing energy but its temperature does not change. The absorbed energy allows solid water to melt and become liquid. Fusion occurs. During this first plateau, water is present in both solid and liquid states. Therefore, 0°C is the melting point of water. At the end of this plateau, all water becomes liquid. Fusion is complete.

3. Between 0°C and 100°C, all water is liquid. It absorbs energy (heat) which increases its temperature.

4. At 100°C, water continues absorbing energy but its temperature does not change. The absorbed energy allows liquid water to boil and become gaseous. Vaporization occurs. During this second plateau, water is present in both liquid and gaseous states. Therefore, 100°C is the boiling point of water. At the end of this plateau, all water becomes gaseous. Vaporization is complete.

5. All water is gaseous above 100°C. It absorbs energy (heat) which increases its temperature.

Alternatively, when water vapour is cooled in a closed container until it turns completely to ice, the graph of water temperature over time is similar to this one.

1. All water is gaseous above 100°C. It releases energy (heat) which decreases its temperature.

2. At 100°C, water continues releasing energy but its temperature does not change. The released energy allows gaseous water to become liquid. Condensation occurs. During this first plateau, water is present in both gaseous and liquid states. At the end of this plateau, all water becomes liquid. Condensation is complete.

3. Between 100°C and 0°C, all water is liquid. It releases energy (heat) which decreases its temperature.

4. At 0°C, water continues releasing energy but its temperature does not change. The released energy allows liquid water to freeze and become solid. Solidification occurs. During this second plateau, water is present in both liquid and solid states. At the end of this plateau, all water becomes solid. Solidification is complete.

5. All water is solid below 0°C. It releases energy (heat) which decreases its temperature.