Blue Sky Science: What determines the melting or boiling point of a substance?

Jenna Jereb


What determines the melting or boiling point of a substance?

Before we can answer that question, we need to know first: What is a solid? What’s a liquid? What’s a gas?

These three materials behave in terms of the tiny particles that they’re made up of. Solids, liquids and gases are all made up of molecules.

In a liquid the particles don’t stick together as strongly as they do in a solid, so the particles can move freely past each other. That is what allows you to pour a liquid from one container to another.

In a gas the molecules are moving around constantly. The molecules are usually not in contact with each other, except for brief encounters.

You can imagine molecules gas moving, but molecules in liquids and solids also move. The molecules in solids are vibrating, not moving very far, but vibrating constantly.

This is where the question of temperature comes in. Temperature represents is the speed at which the molecules are moving. The higher the temperature, the faster the molecules move, or the faster they vibrate.

In a solid, as the molecules vibrate more strongly, at some temperature they will have enough speed and energy to break from the rigid structure and move around past each other. This is when they become a liquid.

So the melting point is the temperature at which molecules in a solid can move past each other and form a liquid.

The boiling point, on the other hand, involves liquids and gases. As liquid molecules are moving around, some molecules at the surface of the liquid are escaping. They don’t get very far because of air at the top that pushes the molecules back into the liquid.

As temperature is increased, the liquid molecules move faster and faster and more of them escape as gas. The boiling point is the temperature at which the gas from the liquid is pushing the air with the same force the air is pushing back.