Packed, sliding, flying

About this video

A solid keeps its shape, a liquid takes the shape of its container but keeps its amount, and a gas spreads out to fill any container. But why? The particle model answers it: all matter is made of tiny particles, far too small to see, that never stop moving. What makes the three states different is how close the particles are, how tightly they hold on to each other, and how fast they move.

In a solid, the particles are packed close together in tidy rows and hold on to each other tightly. They can only shake on the spot. Tip a solid and it moves as one piece, which is why it keeps its own shape.

In a liquid, the particles are still close together, but they are not in rows. They hold on more loosely, so they can slide past one another. That is why a liquid flows and spreads to the shape of its container. But the particles stay close, so the same number of them fills the same amount of space: a liquid keeps its amount.

In a gas, the particles are far apart and hardly hold on to each other at all. They fly about fast in every direction until they hit something. That is why a gas spreads out to fill any container, and why you can squeeze it: there is lots of empty space between its particles to squeeze out.

Heating makes the particles move faster. That is what shakes them loose from their rows when ice melts, and sends them flying off when water boils.

Transcript

1. 0:00Why does ice keep its shape, while air fills a whole room? Look at the particles.
2. 0:08In a solid, the particles are packed in rows. They hold on tightly and only shake. So it keeps its shape.
3. 0:18In a liquid, they are close, but not in rows. They slide past each other. So it flows, but keeps its amount.
4. 0:28In a gas, they are far apart and fly about. There is empty space between them, so you can squeeze it.
5. 0:38Packed and shaking. Close and sliding. Far apart and flying.
6. 0:46The state is how the particles are arranged and move.