Heat and phase transitions

You've heard about heat before. But what is it exactly? Let's see what heat from this stove does to cold water. The heat from the stove moves into the pot of cold water. Let's zoom in to the molecules as they heat up.

See them vibrating? That's caused by the heat. When they vibrate faster, they start moving further and further apart. See? Some of the molecules start to break away from one another, flying away.

And that's when they turn into water vapour. When we say something is getting hotter, it means the molecules are gaining energy and moving faster than before. Without added heat from the stove, the water vapour cools down as the molecules slow down. But what about the molecules that flew away? With less energy, the water molecules move slower and get closer to each other.

And when they are really close together again they turn to liquid. Let's see what happens when we cool it down even more. The more we cool down the water, the closer the molecules come together. Until it changes into a solid mass instead of liquid. The water has now turned into ice.

When a substance changes between different forms like ice, liquid and gas we say that it's changing phase. The three most common phases are: the solid phase, the liquid phase and the gaseous phase. Heating up or cooling down a substance causes it to change phase. Okay, Philip! Let's make this water change phase into water vapour!

The water vapour takes up more space but has the same amount of water molecules that the liquid had. So adding heat increased the volume of the water but didn't change the amount of water. Aha! We can use this knowledge when measuring temperature. This substance is called mercury and it's a metal that is liquid at room temperature.

When we heat up the tube, the liquid's volume increases, taking up more space. When it cools down, it takes up less space. By adding some numbers along the side, we can measure temperature. That's how thermometers work! Thermometers used to have mercury in them but nowadays we use a different liquid with similar properties since mercury is poisonous for humans.

So heat is movement; the more something moves, the hotter it is.