Why things get warm or cold, and how heat moves around.
What it is and why it matters — intuition first, little or no math.
The boiling point is the temperature at which a liquid bubbles into a gas, and it depends on how hard the air presses on the liquid. High on a mountain, water boils below 100 °C; in a pressure cooker, it boils above 100 °C.
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Evaporation is a liquid slowly turning into a gas, and it soaks up heat — that is why wet skin feels cold. Condensation is the opposite, a gas turning back into a liquid, and it gives that heat back.
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Heat is energy that flows from something warmer to something colder. By itself it always flows from hot to cold, never the other way — that is why a hot drink cools down in a cool room.
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Air conditioners and heat pumps don’t make heat or cold. They move heat from one place to another, using a special liquid that soaks up heat when it evaporates and gives it back when it condenses.
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A refrigerant is the special fluid inside fridges, air conditioners and heat pumps. It boils at very low temperatures, so it can soak up heat even from cold air and carry it somewhere else.
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Temperature tells how hot or cold something is. Everything is made of tiny particles that are always jiggling — the faster they jiggle, the higher the temperature.
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