Change of Phase

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A series of free Online High School Physics Video Lessons.

In this lesson, we will learn

  • evaporation
  • condensation
  • boiling
  • freezing
  • regelation


Evaporation is the vaporization at the surface of liquids. It is related to vapor pressure. When the surface of a liquid gets warm, the additional energy is enough for it to vaporize. Evaporation is seen in the water cycle, as water from rains vaporize from the surfaces of bodies of water.

Understanding the process of evaporation.
How does a liquid become a gas?
We’ll look at how vaporization and evaporation happens, and learn a big mistake that many people make when they deal with the liquid gas phase transition.

When a liquid evaporates or boils to become a gas, the molecules do not come apart. This is true whether you’re talking about evaporation, boiling, or vaporization.


Condensation is the change in phase of a gas upon the surface of a solid due to its loss of energy to that solid. Water droplets often form on the surface of cool substances because the water vapor in the air loses energy when it comes into contact with the cold substance and turns to a liquid.

Understanding the process of condensation.


Boiling is the transition of a substance from the liquid to gaseous state of matter. When something boils its vapor pressure equals the atmospheric pressure. At high altitudes things boil at a much lower temperature because there is less atmospheric pressure acting upon them.

How matter changes phase from liquid to gas.


Freezing is the transition of a substance from the liquid to solid state of matter. Most substances are more dense in their solid form, with the exception of water which expands as it freezes. The opposite of freezing is melting.

How matter changes phase from liquid to solid?


Regelation is the melting and freezing of a substance due to changes in pressure rather than temperature.

Understanding the relationship between pressure and state of matter.

When pressure is applied to ice, its melting point is reduced so it turns to water. When the pressure is removed, however, it turns back into ice. This process is called ‘regelation.’

Freezing by Boiling
Liquids require heat to boil, and if the conditions are right one liquid can be boiled in order to freeze a second. Under a vacuum, the water in an acetone/water mixure can freeze while the acetone boils.

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