# Heat Transfer & Phase Changes

Coming up next: Thermal Expansion & Heat Transfer

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• 0:02 Heat Transfer
• 1:04 Phase Changes
• 3:20 Lesson Summary

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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this lesson, you will be able to explain what heat transfer is and describe the various phase changes that can result from heat transfer in terms of the position of the molecules. You will also be able to give the names of those phase changes. A short quiz will follow.

## Heat Transfer

Heat energy, or thermal energy is the energy of a substance or system in terms of the motion or vibrations of its molecules. The faster the molecules in a substance move, the more heat energy they have. You can transfer heat energy from one object to another, and in fact, a heat transfer will occur naturally whenever a hotter object is in contact with a colder one; heat will always move from the hotter object to the colder one. For example, if you were to put your warm hands against a cold metal pole, the skin on your hands would feel cold. That's because the heat from your hands transfers to the cooler metal. Your hands feel colder because some of your heat has been transferred to the metal.

When you add heat energy to a substance, one of two things can happen: either the temperature of the substance will increase, or the phase (or state) of the substance will change. Temperature is the average kinetic energy of the molecules in a substance. So it's pretty obvious why temperature might increase - heat is the motion of molecules, so if you get the molecules in a substance to move faster, then they'll have a higher temperature. But what about phase change?

## Phase Changes

Put a cup of water in the freezer, and it will change to a solid. Put it out in the sun, and eventually the liquid will 'disappear'. But in fact, it hasn't really gone anywhere - it's just changed into a gas. Solids, liquids and gases are the three main phases (or states) of matter. But what happens to the molecules when phase changes occur?

Whereas temperature is the average kinetic energy of the molecules, the state is related to the spacing (or potential energy) of the molecules. Molecules that are further apart have more potential energy. Solids are substances with tightly-packed molecules in neat rows that can only move by vibrating. Liquids on the other hand are substances with molecules that can slide past one another and aren't as tightly packed as solids. Liquids do not have a consistent shape, and will change shape to fill the container. And gases are substances with molecules that are free to move in every direction. With a gas, not only does it not have a consistent shape, but it doesn't even have a consistent volume - the space it takes up can change.

Every phase change has a name: changing a solid to a liquid is called melting, changing a liquid to a gas is called boiling (or evaporating) , changing a gas to a liquid is called condensing, and changing a liquid to a solid is called freezing.

We can plot a graph to show the energy involved in phase changes: a graph of temperature plotted against heat added. It looks like this. As you can see, there are diagonal areas and flat areas. The diagonal areas are where heat is being added to increase the temperature. The flat areas are the phase changes, where heat is being added, but is going into the potential energy of the molecules, not the kinetic energy - the temperature therefore remains the same during these phase changes. These graphs and the equations that go with them are discussed in more detail in another lesson.

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