Heat and Internal Energy

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A series of free Science Lessons for 7th Grade and 8th Grade, KS3 and Checkpoint Science in preparation for GCSE and IGCSE Science.

The following diagram shows the changes in the states of matter: melting, boiling, condensation, freezing, sublimation. Scroll down the page for more examples and solutions.
Changing states of matter

Heat and Internal Energy
What is meant by internal energy?
What happens to internal energy during changes of state?

Internal energy is the total kinetic energy and potential energy of all the particles that make up a system.
When we heat a solid, we increase the internal energy. At some point, the solid will turn into a liquid (melting).
If we continue to heat the liquid then the internal energy will increase. At some point the liquid will turn to a gas (boiling).
If we cool down the gas then we reduce the internal energy. At some point, the gas turns back to liquid (condensation).
If we cool the liquid down further, the internal energy is reduced even more. Eventually, the liquid turns into a solid (freezing).
Sometimes, a solid can turn directly to a gas. It is called sublimation.

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