What is Melting Point? - Definition, Range & Determination

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: Heptane: Structure, Uses & Formula

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

Take Quiz Watch Next Lesson
Your next lesson will play in 10 seconds
  • 0:00 Phases Changes Vs.…
  • 1:25 Melting Point Definitions
  • 2:31 What Happens During Melting
  • 3:21 Melting Point Ranges
  • 4:02 Big Picture of Melting Point
  • 5:44 Lesson Summary
Save Save Save

Want to watch this again later?

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

Log in or Sign up

Speed Speed Audio mode

Recommended Lessons and Courses for You

Lesson Transcript
Instructor: Nancy Devino
What makes things melt? Why do some substances melt all at once, while others seem to take forever? And how do you measure the exact temperature at which something melts? We'll answer these questions as we explore an important physical property of a compound: its melting point.

Phase Changes vs. Phase Diagrams

Before we get into the details of melting, let's take a closer look at the most common forms, or phases, of matter: solids, liquids, and gases. Scientists have determined the state of matter for lots of different compounds at different pressures and temperatures. A graph of these results of matter states is called a phase diagram and may look something like this:

A phase diagram shows states of matter at different pressures and temperatures.
phase diagram

On a phase diagram, the solid boundary shows the pressure and temperature at which the substance exists in both phases simultaneously. That's because phase changes are equilibrium processes. This can be an equilibrium between solid and liquid, or between liquid and gas.

So what temperature is required to get this compound to melt? It depends! Notice that the upward pointing line in the middle leans a little to the right. This tells us that at a pressure of 260 atmospheres, which is the upper horizontal dotted line, this compound melts at about 200 Kelvin. But when the pressure is dropped, the compound melts at a lower temperature. Can the melting temperature really change? Yes!

Melting Point Definitions

Now that we know something about phase diagrams, it will make more sense why scientists define melting point the way they do. First, in everyday terms, the melting point is the temperature at which a substance melts or freezes. Did you catch that? The melting point is the same as the freezing point! In other words, it goes from a solid to a liquid at the same temperature that it goes from a liquid to a solid.

Let's take a closer look at the everyday part of this definition. In science, everyday working conditions are usually defined as one atmosphere of barometric pressure. When we look up known values for the melting point of various substances, they're almost always defined at one atmosphere of pressure. Thus, in science, a substance's melting point is the temperature at which the solid becomes a liquid at one atmosphere of pressure.

What Happens During Melting

As a solid substance is heated, or absorbs heat from the environment, the molecules begin to have enough energy to overcome the intermolecular forces, or forces that hold the molecules together in a rigid fashion, like the water molecules in ice, shown here.

Hydrogen bonds between water molecules help maintain the solid structure of ice.
ice structure

The molecules on the outside of a sample start to melt, even as the inside stays cool enough to remain solid. You've seen this before: think about an ice cube. It melts from the outside in, instead of turning into a puddle all at once. So there are really two processes involved in melting: absorbing heat energy, then using that energy to break apart the orderly lattice structure of the solid.

Melting Point Ranges

The short answer to this is human limitations. Measurement of melting ranges requires human observation, and it requires observation of a sample large enough to observe, for most practical applications anyway. A compound's melting range starts with the temperature where the crystals first begin to liquefy to the temperature at which the entire sample is liquid. Most pure compounds have a melting range of 1-2 degrees Celsius. In the lab, a narrow range like that is obtained only when the sample is heated slowly.

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

Register to view this lesson

Are you a student or 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
What teachers are saying about Study.com
Try it risk-free for 30 days

Earning College Credit

Did you know… We have over 200 college courses that prepare you to earn credit by exam that is accepted by over 1,500 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 risk-free for 30 days!
Create an account