Login
Copyright

Heat Transfer & Phase Changes

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: Thermal Expansion & Heat Transfer

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
 Replay
Your next lesson will play in 10 seconds
  • 0:02 Heat Transfer
  • 1:04 Phase Changes
  • 3:20 Lesson Summary
Add to Add to Add to

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Login or Sign up

Timeline
Autoplay
Autoplay
Create an account to start this course today
Try it free for 5 days!
Create An Account

Recommended Lessons and Courses for You

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.

Graph of phase changes

Under certain conditions (the right pressure) it is even possible for a solid to change straight into a gas or the reverse. A solid changing straight into a gas is called sublimation, and a gas changing straight into a solid is called deposition. There are even other phases of matter possible in conditions we don't find on the surface of the Earth, such as plasmas. But all the basic principles behind them are the same.

To unlock this lesson you must be a Study.com Member.
Create your account

Register for a free trial

Are you a student or a teacher?
I am a teacher

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back

Earning College Credit

Did you know… We have over 95 college courses that prepare you to earn credit by exam that is accepted by over 2,000 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it free for 5 days!
Create An Account
Support