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The Processes of Melting & Freezing

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  • 0:01 Physical Changes
  • 0:58 Melting: Solid to Liquid
  • 2:24 Freezing: Liquid to Solid
  • 3:28 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.

Substances can go through physical changes that don't change their chemical makeup. Two important phase changes are melting and freezing. You might think of these as being very different, but in reality they are very similar in several ways.

Physical Changes

I want you to try a little experiment. Take an empty glass and put a few ice cubes in it. Then wait a while, and come back to the glass. After a long enough time goes by, you'll come back to find no ice cubes in the glass, just water! Now try something else. Pour that liquid water into an ice cube tray and stick that tray in the freezer. Come back to it a few hours later, and you've got ice again!

What you've done here is changed the phase of the water from solid to liquid and back to solid again. Each of these transformations is a physical change because all we did to the water was change the physical state but not the chemical composition. In other words, we've simply changed the physical form of the water, not the chemical structure that makes water, water.

There are several different phase changes that substances can go through, all of which require that you either add or remove heat. The two we've just experimented with are melting and freezing. Let's look at each of them more closely.

Melting: Solid to Liquid

In our first activity, we took the solid form of water and left it out until it was in a liquid form. This process of turning a solid into a liquid is called melting. In order to melt a solid substance, you have to add heat to it. When you do this, the molecules in the substance start getting excited and begin to move around more. This moving around makes it difficult for the particles in the substance to hold on to each other as tightly as they do in the solid phase, so they break apart, and the substance turns to liquid.

Heat can be added gradually or quickly to a solid to make it melt. For our ice cubes, we simply took them from a cold environment and put them in a warmer one. This added heat to the frozen water, causing it to slowly melt in the glass. If we had put the ice cubes and the glass in the fridge, it would have taken longer for them to melt because it is not as warm in the fridge as it is out on the kitchen counter. If we had put those ice cubes and the glass in a hot oven, though, the ice cubes would have melted much quicker, because there is a lot more heat in the oven than the air in the kitchen.

While the process of melting is the same for substances, the temperature at which each substance melts is not. For example, the melting point of water is 32°F, or 0°C. But, the melting point of mercury is -37.89°F, or -38.83°C. That's a big difference!

Freezing: Liquid to Solid

If you thought melting was cool, you'll love freezing because it's just the opposite! That's right, freezing is the process of turning a liquid into a solid. With freezing, instead of adding heat we remove it, taking away that energy that made it so difficult for the particles to hold on to each other before.

We did this in our second activity when we put the liquid water back into the cold environment of the freezer. We removed the heat, taking away the energy, and slowing those particles down to the point where the substance returned to its solid state.

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