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Changes of State

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Both of these photos show the famous Golden Gate Bridge near San Francisco, California. The pictures were taken from about the same point of view on the same day, but they look very different. In the picture on the left, the deck of the bridge is almost completely hidden by a thick layer of fog. In the picture on the right the fog has disappeared, and the deck of the bridge—as well as the water below it—is clearly visible. Fog consists of tiny droplets of liquid water. The fog in the picture is like a cloud at ground level. Where did the fog come from, and where did it go?

What Are Changes of State?

The water droplets of fog form from water vapor in the air. Fog disappears when the water droplets change back to water vapor. These changes are examples of changes of state. A change of state occurs whenever matter changes from one state to another. Common states of matter on Earth are solid, liquid, and gas. Matter may change back and forth between any two of these states.

Changes of state are physical changes in matter. They are reversible changes that do not change matter’s chemical makeup or chemical properties. For example, when fog changes to water vapor, it is still water and can change back to liquid water again.

Processes that Cause Changes of State

Several processes are involved in common changes of state. They include melting, freezing, sublimation, deposition, condensation, and evaporation. The Figure below shows how matter changes in each of these processes.

Q: Which two processes result in matter changing to the solid state?

A: The processes are deposition, in which matter changes from a gas to a solid, and freezing, in which matter changes from a liquid to a solid.

Summary

  • A change of state occurs whenever matter changes from one state to another. Changes of state are physical changes in matter. They are reversible changes that do not change matter’s chemical makeup or chemical properties.
  • Processes involved in changes of state include melting, freezing, sublimation, deposition, condensation, and evaporation.
  • Energy is always involved in changes of state. Particles of matter either absorb or lose energy when matter changes from one state to another.

Practice

Take the changing states quiz at the following URL. Be sure to check your answers.

http://www.bbc.co.uk/apps/ifl/schools/ks2bitesize/science/quizengine?quiz=changingstates&templateStyle=science

Review

  1. Define change of state, and give an example.
  2. Identify processes that change matter to a liquid state.
  3. Why must energy be absorbed to change a liquid to a gas?

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