What is Heat?

Heat is a form of energy. It comes from the movements of atoms and molecules, which are the smallest particles that make up all matter? The faster these particles move, the hotter they are. The temperature of an object tells us how fast its atoms or molecules are moving. Cold is a lack of heat energy. In cold objects, the atoms and molecules do not move very fast. The slower they are, the colder they become. Eventually they stop altogether.

When something’s hot, it has a lot of heat energy; when it’s cold, it has less. But even things that seem cold (such as polar bears and icebergs) have rather more heat energy than you might suppose.

What happens when something has no heat at all?

Now suppose we try the opposite trick. Let’s take a jug of water and put it in the refrigerator to cool it down. A refrigerator works by systematically removing heat energy from food. Put water inside a refrigerator and it immediately starts to lose heat energy. The more heat it loses, the more kinetic energy its molecules lose, the more slowly they move, and the closer they get. Soon or later, they get close enough to lock together in crystals; the liquid turns to solid; and you find yourself with a jug of ice!

But what if you have a super-amazing refrigerator that keeps on cooling the water so it gets colder… and colder… and colder. A home freezer, if you have one, can take the temperature down to somewhere between −10°C and −20°C (14°F to −4°F). But what if you keep on cooling lower than that, taking away even more heat energy? Eventually, you’ll reach a temperature where the water molecules pretty much stop moving altogether because they have absolutely no kinetic energy left. For reasons we won’t go into here, this magic temperature is −273.15 °C (−459.67°F) and we refer to it as absolute zero.