I live in a high desert. The best part of high deserts is that the winters are mostly mild. We periodically get a little snow. But, the temperature stays in the thirties or forties. So, it's not so bad. The biggest challenge is that the summers are hot! Every July, we have a week or two with multiple hundred-degree days. So, I drink a lot of ice water. And, sometimes, I eat more ice cream than I should. At this point, I think I'm an expert in melting. I've watched a lot of ice become liquid. Let's study what's going on at a molecular level!
The Phases of Matter
Let's do a quick refresher on Van der Waal's forces. Remember, there are a few different types. Every atom and molecule experiences London dispersion forces. What causes them? The random movement of electrons. Sometimes, the electrons don't evenly distribute. So, there is a temporary dipole. It creates an electrostatic attraction between neighboring particles. But, dispersion forces are weak. So, the attraction is minimal. Polar molecules have much stronger dipoles. The electronegativity difference between the atoms creates a partial charge. So, they experience dipole-dipole attractions. The most robust dipole-dipole interaction is hydrogen bonding.
Now, let's get back to the phases of matter. What causes something to be a solid, liquid, or gas? The balance between the kinetic energy and Van der Waal's forces. Remember, the temperature is our measure of the average kinetic energy. At high temperatures, atoms and molecules move unbelievably quickly. At low temperatures, they start to slow down. In solids, there is very little kinetic energy. So, Van der Waal's forces hold the particles together. The molecules can rotate and vibrate, but they don't translate. In a gas, the kinetic energy is much greater than Van der Waal's attraction. So, the particles freely move. The liquid phase is an in-between situation. The molecules have enough kinetic energy that they can translate. But, Van der Waal's attraction keeps them close together. They slide past one another. But they can't break free.
Melting
So, what happens when we melt a solid? We're adding heat. At first, the particles start to vibrate and rotate a little faster. Then, a few have enough energy and begin sliding past each other. As the temperature rises, more particles can get past each other. But, they are still attracted to each other. So, they are moving randomly around. But, they can't break free from one another. We have a liquid!
This process can happen backwards where a liquid can become a solid, which happens at the freezing point. Similarly, by decreasing the temperature, the particles start moving slower and lose energy, eventually forming a solid.
