How a Fridge Keeps Food Cold

You can't make cold — you can only move heat

Here is a surprising truth: a refrigerator does not make coldness. In physics there is no such thing as "cold" to create — there is only heat, and "cold" just means a place that has less of it. So what a fridge really does is move heat out of the food compartment and dump it somewhere else. That is why, if you reach behind a working fridge, the back feels warm. The heat that left your milk and vegetables is right there, being released into the kitchen. Understanding how a fridge pulls off this trick is a wonderful tour through everyday thermodynamics.

The big idea: evaporation absorbs heat

The whole machine is built on one simple effect you have felt yourself. When you step out of a swimming pool, you shiver — even on a warm day. As the water on your skin evaporates (turns from liquid into gas), it takes heat energy from your skin to do it, leaving you cooler.

Turning a liquid into a gas always needs energy. That energy has to come from somewhere — usually as heat absorbed from the surroundings. So:

Evaporating liquid = surroundings cooled down.

The reverse is also true. When a gas condenses back into a liquid, it releases that heat again. A fridge uses both halves of this idea: it makes a liquid evaporate inside the fridge to absorb heat, and condense outside to release it. This builds on the ideas in changing states: melting, freezing, evaporating.

The refrigerant cycle: a heat-carrying loop

A fridge contains a special fluid called a refrigerant that boils at a very low temperature. It is pumped endlessly round a sealed loop of pipes. Follow it around:

1. Evaporator (inside the fridge). The refrigerant arrives as a cold liquid. Inside the cold pipes behind the fridge wall, it evaporates, absorbing heat from the food and air in the compartment. The inside gets colder; the refrigerant becomes a cool gas carrying that heat away.

1. Compressor (the motor). This cool gas is sucked into the compressor, which squeezes it. Squeezing a gas raises its pressure and makes it hot — now it is a hot, high-pressure gas. This is the part that uses electricity, and the hum you hear is the compressor running.

1. Condenser (the warm coils at the back). The hot gas flows through coils on the back or underside of the fridge. Here it is hotter than the kitchen, so it releases heat to the room and condenses back into a liquid. This is why the back feels warm.

1. Expansion valve. The warm liquid is squeezed through a tiny narrow valve. As it expands on the other side, its pressure drops and it becomes very cold again — ready to go back inside and absorb more heat. The loop repeats.

Each lap of the loop carries a parcel of heat from inside to outside. Run it continuously and the inside stays cold.

Why the back is warm: energy is conserved

A fridge is a heat pump — a machine that pumps heat from a cooler place to a warmer place. This doesn't happen on its own (heat naturally flows the other way), so it needs energy from electricity to force it. Because of the conservation of energy, all that energy has to go somewhere. Nothing is destroyed, an idea explored in the conservation of energy.

So the heat released at the back equals the heat removed from the food plus the electrical energy used by the motor.

Worked example: counting the energy

A fridge removes 300 J of heat from the food inside. To do this, its compressor uses 100 J of electrical energy. How much heat is released at the back?

Heat out = heat from food + electrical energy used Heat out = 300 J + 100 J = 400 J

This is why you can never cool a room by leaving the fridge door open: it pumps 300 J out of the cabinet but releases 400 J into the room, so the room gets warmer overall. The extra 100 J comes from the motor.

Where you meet this physics every day

The same refrigerant cycle runs in:

• Air conditioners, which pump heat from inside a room to the outside air.
• Freezers, which run the same cycle but keep the inside well below 0 °C.
• Heat pumps for heating homes, which run the cycle in reverse to pump heat into a house from the cold outside air — a very efficient way to heat.

All of them rely on the same physics: evaporation absorbs heat, condensation releases it, and a pump moves the heat where you want it.

Try it yourself! 🧪

Feel evaporation cooling — the heart of refrigeration.

You need a little rubbing alcohol (or hand sanitiser) and some water. Do this with an adult, away from flames, since alcohol is flammable.

1. Dab a small amount of water on the back of one hand and a small amount of alcohol on the back of the other.
2. Gently wave both hands in the air so the air flows over them.
3. Notice that the alcohol hand feels much colder. Alcohol evaporates faster than water, so it absorbs heat from your skin more quickly.
4. Blow gently on each patch — feel the cooling increase as faster-moving air carries the vapour away and speeds up evaporation.

You have just felt the exact effect a fridge uses, scaled down to your skin: an evaporating liquid pulls heat out of whatever it touches. Wash your hands afterwards and keep alcohol away from any flame.