Kitchen science · Heat and transformation

Boiling point and pressure: tea on a mountain, chickpeas in a pressure cooker

Does water boil at 100 °C everywhere? Find the boiling point in Ankara, in Erzurum and on Everest; discover how a pressure cooker raises the pressure to take water up to 120 °C, why salt makes hardly any difference, and the rules for using one safely.

18 min
2 simulations
Mini quiz

Updated: 22 October 2026

Preview

The question

Water is being heated in identical pans in two kitchens: one by the sea in Izmir, on the Aegean coast, and one in Erzurum, high up in eastern Türkiye. Both pans are bubbling away. Dip a thermometer into the water, and in Izmir it reads 100 °C, but in Erzurum it shows something around 93–94 °C.

Why doesn't boiling water have the same temperature everywhere? What does this have to do with dried beans taking longer to cook in Erzurum? And how does a pressure cooker turn this rule on its head?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Boiling point

The climber's kettle

Change the altitude with the slider or the arrow keys. Press Heat to watch the bubbles; the air pressure and the boiling point are shown on screen. When you've found the value closest to the target, press Check.

Predict first

What do you think happens to the boiling point of water as you climb above sea level?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Boiling point

120 °C in a pressure cooker

Change the extra pressure (kPa) with the slider or the arrow keys. The total pressure and the boiling point are shown on screen. When you've found the value closest to the target, press Check.

Predict first

In a pan with a tightly closed lid, what do you think happens to the boiling point of water if the pressure rises?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Lab

The refrigeration cycle of a fridge

Open full page in the Lab →

Fig. 1The refrigeration cycle of a fridge

Tap the door, the thermostat knob or the compressor, or take the fridge apart

Inside 5.2 °C · room 22.0 °C · compressor waiting: the fridge is cold enough

Parts and hidden mechanism

Separation0%

In the opened-up fridge, hover over a part, tap it, or pick it from the list.

Pick an assembly: what it does, and why a real fridge is built that way, appears here.

Inside the fridge

5.2 °C

Room

22.0 °C

Thermostat setting4.0 °CThe compressor starts at 5.0 °C and stops at 3.0 °C

Compressor: waiting: the fridge is cold enough

Time passed: 0 min (1 second on screen = 1 minute)

Heat budget (right now)

Heat taken from inside0 W
Electrical power of the compressor0 W
Heat given to the room0 W

While the compressor rests, the refrigerant does not circulate and no heat is carried.

Enter your own values

Thermostat

Room temperature

Compressor power

Points on the cycle

Compressor outlet · hot vapourpressure 1.85 bar · temperature 22 °C
Condenser outlet · warm liquidpressure 1.85 bar · temperature 22 °C
Capillary outlet · cold liquid + vapourpressure 1.85 bar · temperature 5 °C
Evaporator outlet · cold vapourpressure 1.85 bar · temperature 5 °C

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

On a hot day, if you leave the fridge door open and keep it running, what happens to the room?

Gold tube: hot refrigerant · Purple tube: cold refrigerantSolid line: liquid · Dots: vapourThe numbers come from one simple model: the refrigerant is isobutane (R600a, common in household fridges), and pressures are absolute, in bar, from a curve close to this refrigerant’s real saturation curve. The default values are a 4 °C thermostat, a 22 °C room and a 60 W compressor; you can type your own values in the boxes. A stronger compressor cools the evaporator further and removes more heat, but the electricity it uses grows faster. The fridge and the room are each treated as one body at a single temperature, and heat stored in the tubes is ignored. In a real fridge the values depend on the model, the refrigerant and the surroundings.

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Frequently asked

5 questions

Is boiling water always 100 °C?

No. Water boils when its vapour pressure equals the outside pressure; 100 °C belongs to the air pressure at sea level. Pressure drops as you go higher: in Ankara water boils at about 97 °C, in Erzurum at about 93.5 °C.

Does salted water boil hotter and cook food faster?

No, the effect is tiny. A tablespoon of salt in 2 litres of water raises the boiling point by only about 0.15 °C. The water foams when you add salt not because it gets hotter, but because the rough surfaces of the salt crystals make it easier for bubbles to form.

Does boiling water get hotter if I turn up the heat?

Not in an open pot; the pressure sets the temperature of boiling water. A fierce flame only makes more steam and burns more fuel. Turning the heat down once the water boils hardly changes the cooking speed, as long as it keeps boiling.

Why does a pressure cooker cook food faster?

In the sealed pot steam cannot escape, so the pressure rises and so does the boiling temperature. In home pressure cookers, 0.5–1 bar of extra pressure makes water boil at about 110–120 °C. Hotter water softens foods such as chickpeas, dried beans or meat on the bone in much less time.

Why shouldn't I open a pressure cooker before the pressure drops?

If the pressure drops suddenly, the water inside, which is above 100 °C, boils instantly, and steam and food can spray out. Release the pressure the way the manual describes, and do not open the lid until the indicator or pin goes down. Children should not use a pressure cooker on their own.