Changes of State: Why the Temperature Stops Rising

0:00Does heating something always raise its temperature?

0:04We heat ice steadily at sea level, starting at minus 20 degrees Celsius.

0:09Kinetic molecular theory says matter is made of tiny moving particles. In ice, attractions hold them, so they only vibrate in place.

0:20Temperature measures the particles' average kinetic energy, their energy of motion.

0:26As heat flows in, they vibrate faster and the ice warms up to 0 degrees.

0:32Then the ice melts into a liquid, and though we keep heating, the temperature stops rising.

0:38The energy loosens the attractions instead of speeding the particles up, so they can slide past each other as a liquid.

0:47Then the particles speed up again, and the water warms up to 100 degrees.

0:52At 100 degrees the water boils, and the temperature holds steady again.

0:58The energy pulls the particles far apart, into a gas called water vapour.

1:04Liquid to gas is called evaporation; boiling is evaporation all through the liquid.

1:10Only then does the water vapour heat up.

1:14Cooling runs it backwards: condensation, gas to liquid, at 100 degrees, then freezing at 0 degrees.

1:24Dry ice, solid carbon dioxide, skips the liquid and turns straight into a gas: sublimation.

1:32The reverse, gas straight to solid, is deposition, which is how frost forms.

1:38During a change of state, energy changes the attractions, not the particles' speed, so the temperature stays the same.