Modeling Changes in State: Heat & Energy

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  • 0:01 Phases of Matter
  • 1:37 Melting & Freezing
  • 3:28 Evaporation & Condensation
  • 5:36 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Matter exists in different phases, but it can also change from one phase to another. It's not magic, it's energy! In this lesson, you'll explore how energy can change matter from one state to another and what the names of these phase changes are.

Phases of Matter

Last night after dinner, I went to the kitchen and got a glass of ice water. Right as I was about to take it back with me to the living room, I got a phone call. When that call ended thirty minutes later, I returned to that glass of ice water to find that it no longer contained any ice! Now, either some little kitchen elves came and stole all my ice out of my glass, or the ice melted. As much as I'd like some kitchen elves (who knows, maybe they do dishes too!), I know enough about heat and energy to know that the melting is the more likely option here. In fact, you do too, even if you don't realize it just yet.

You might recall that there are three phases of matter called solid, liquid and gas. You experience these phases all around you all the time. The ground you walk on, the water you drink and the air you breathe, just to name a few examples. But what's really cool is that matter can change from one phase to another with the addition or subtraction of energy. This transfer of energy is also known as heat.

Before we go any further, I need to make one thing really clear. When matter goes through a phase change, we call this a physical change, because there is no change to the chemical composition of the matter. In other words, no bonds are broken, no molecules rearranged, nothing of that nature. This is really important because physical changes can usually be reversed, but chemical changes cannot. For example, you can refreeze your melted water back into ice, but you can't un-burn a piece of wood. Make sense? Great!

Melting & Freezing

Let's go back to our glass of water to see how energy influences the different phases of matter. When in the solid phase of ice, our water has very little energy, but there is some there. Even though it doesn't look like it, the particles of that ice are vibrating ever so slightly. We can't see it because it's happening on a molecular scale and they have so little energy that they only vibrate a tiny bit, but not enough to move around. So we are left with a solid piece of ice that doesn't do anything but just sit there and be ice. But if we add some energy to this ice cube, something awesome happens. It melts into liquid water. Melting is just the change from solid to liquid and it occurs when we add energy (heat) to a solid. By adding energy to the ice, we add energy to the particles within that ice and they can now move around a bit more.

You can try this at home. Put an ice cube in a glass and try to move it around. The whole ice cube will just bump around the glass in one piece. But if you do this with liquid water, it will flow around the glass like any good liquid does. The particles in the liquid are not held together as tightly as the particles within a solid so, unlike a solid, a liquid will spread out to take the shape of its container up to the surface of the liquid.

Now, what do you think will happen if we take back our energy to get liquid water? That's right, we'll end up with solid ice again. This change from liquid to solid is called freezing and you can think of it as the reverse of melting. In fact, while the melting and freezing points of substances are different from each other, they are the same for that one substance. What I mean by this is that, while water and mercury melt at different temperatures, water both melts and freezes at 0% Celsius and mercury both melts and freezes at -39% Celsius. Cool, huh?

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