States of matter and changes of state

Last time

Before we start: last time, in Diffusion and gases, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Ice, water, steam

Ice, water and steam are the same substance, H₂O, in three different states. What changes is how the particles are arranged and how they move. Once you can describe that, you can explain every change of state.

The three states of matter

In a solid, the particles are packed very close together, touching, in a regular pattern. In a liquid, the particles are still touching, but they are arranged randomly. In a gas, the particles are far apart and arranged randomly, with lots of empty space between them.

Watch the particles

In the solid, each particle only vibrates about a fixed position. In the liquid, the particles slide past each other, so a liquid flows and takes the shape of its container.

Watch the particles

In a gas the particles rush about quickly in all directions, so a gas spreads out to fill any container it is put in.

Movement and energy

Solid particles vibrate about fixed positions. They cannot move from place to place. Liquid particles move around each other, so a liquid can flow. Gas particles move quickly and randomly. They have the most kinetic energy. And only a gas can be squashed into a smaller volume, because it is mostly empty space.

Six changes of state

A solid turns into a liquid by melting, and a liquid turns back into a solid by freezing. A liquid turns into a gas by boiling, and a gas turns back into a liquid by condensing. A solid can turn straight into a gas by subliming, without becoming a liquid first. Dry ice does this. Evaporation is different from boiling. It happens only at the surface, and at any temperature below the boiling point.

Why the temperature stops rising

Heating a solid gives its particles more kinetic energy, so they vibrate more and the temperature rises. At the melting point, the energy goes into overcoming the forces between the particles, not into speeding them up. So the temperature stays constant until all of the solid has melted. Boiling works in the same way.

The heating curve

Here is the heating curve. The temperature rises as the solid is heated. The flat part at the melting point is the solid melting. The temperature rises again for the liquid, and goes flat again at the boiling point while it boils.

Evaporation or boiling?

Boiling happens throughout the liquid, at one fixed temperature, the boiling point, and bubbles form inside. Evaporation happens only at the surface, at any temperature below the boiling point, and there are no bubbles. In evaporation, the fastest particles at the surface have enough energy to escape, so the liquid that is left cools.

Physical state from melting and boiling points

To find the state of a substance, compare the temperature with its melting point and boiling point. Below the melting point, it is a solid. Between the melting point and the boiling point, it is a liquid. Above the boiling point, it is a gas.