Watch a pot of water on the stove. First, tiny bubbles cling to the bottom. Then big bubbles rush up, and the water rolls and splashes: it boils. A thermometer in the water would show about 100 degrees Celsius (100 °C).
But water does not always boil at 100 °C. On a high mountain, it boils while it is much cooler. In a pressure cooker, it has to get much hotter first. Let’s find out why.
What happens when water boils?
Water is made of tiny particles that jiggle all the time. The hotter the water, the faster they jiggle. That is what temperature tells us.
Even in cold water, a few fast particles escape from the surface into the air. They become water vapour, an invisible gas. This slow escape is called evaporation.
As the water heats up, more and more particles escape. At last, water starts turning into vapour even deep inside the pot. The vapour gathers into bubbles, which rise and burst at the top. This is boiling, and the temperature at which it happens is the boiling point.
The tiny bubbles you see at the start are different. They are air that was dissolved in the water, now coming out.
Once water boils, it stops getting hotter. Turning the stove up only makes it boil faster. The extra heat goes into turning water into vapour.
Why does the air matter?
Air may feel like nothing, but it has weight. A huge stack of air, many kilometres high, rests on everything around you, including the water in the pot. It presses down on the water. This push is called air pressure.
A bubble of vapour inside the water has to push back against that pressure. If its push is too weak, the bubble gets squashed before it can grow. The vapour pushes as hard as the air only when the water is hot enough. Then bubbles can grow, and the water boils.
Boiling on a mountain
The higher you climb, the less air there is above you. So up high, the air presses more lightly on the water. The vapour needs less push to make bubbles, and the water boils at a lower temperature.
On top of Gerlachovský štít, the highest mountain in Slovakia (2,655 metres), water boils at about 91 °C. On Mount Everest, the highest mountain in the world (8,849 metres), it boils at only about 70 °C.
Water at 70 °C can still burn you badly. But it is not hot enough to cook food well, so pasta and eggs take much longer. People who live or climb high in the mountains often cook with a pressure cooker.
The pressure cooker
A pressure cooker works the other way round. Its lid closes tightly, so the steam cannot escape. The steam builds up inside and presses on the water — about twice as hard as the air outside.
Now the water must reach about 120 °C before it can boil. Hotter water cooks food faster, so beans, soups and stews are ready much sooner.
Every liquid has its own boiling point
Water is not the only liquid that boils. Every liquid has its own boiling point, even with the same air pressing on it.
| Liquid | Boils at (normal air pressure) |
|---|---|
| Water | 100 °C |
| Alcohol (ethanol) | about 78 °C |
| Propane, a gas for camping stoves and barbecues | about −42 °C |
| Liquid nitrogen | about −196 °C |
Some liquids boil far below 0 °C, the temperature at which water freezes. Liquid propane boils even on a freezing winter day, soaking up heat from the cold air around it.
Liquids like this are called refrigerants. They flow in the pipes of fridges and heat pumps. Squeeze a refrigerant hard, and its boiling point goes up, just like water in a pressure cooker. So a heat pump can make its refrigerant boil while it is cold and turn back into a liquid while it is hot.
Go deeper: Vapour pressure: when exactly does a liquid boil? for grown-ups and the extra curious
Particles escape from a liquid all the time, and some of them fly back in. In a closed jar, the vapour above the liquid builds up until as many particles return as escape. The push of that vapour is called the liquid’s vapour pressure. It grows very fast as the liquid gets hotter.
A liquid boils when its vapour pressure reaches the pressure around it. Then bubbles of pure vapour can form inside the liquid and survive.
| Water temperature | Vapour pressure of water | Where water boils at this temperature |
|---|---|---|
| 20 °C | about 2.3 kPa | in a jar with almost all the air pumped out |
| 70 °C | about 31 kPa | around the top of Mount Everest |
| 91 °C | about 73 kPa | on top of Gerlachovský štít |
| 100 °C | 101.3 kPa | at sea level |
| 120 °C | about 200 kPa | in a pressure cooker |
A kilopascal (kPa) is a unit of pressure. Normal air pressure at sea level is about 101 kPa.
Strictly speaking, a bubble at the bottom of a pot must also push against the water above it, and against the pull that holds the surface of the bubble together (surface tension). Bubbles therefore start on tiny scratches and specks of dirt on the bottom of the pot.
Go deeper: Where the door picture is too simple for grown-ups and the extra curious
- The air does not lean on the water only from above. Pressure pushes in all directions, and the water passes it on to every bubble inside, even at the bottom of the pot.
- Very clean water in a very smooth cup can get hotter than its boiling point without bubbling. This can happen in a microwave oven. A small shake can then make it boil over all at once, and it can scald. So heat water in a microwave only as long as needed, and take it out carefully.
Check yourself
Why does pasta take longer to cook high up on a mountain?
Show the answer
Water boils at a lower temperature there, so it is not as hot — Up high, the air presses less on the water, so it boils at a lower temperature. Cooler water cooks food more slowly.
Up high, the air presses less on the water, so it boils at a lower temperature. Cooler water cooks food more slowly.
Check yourself
What is inside the big bubbles in boiling water?
Show the answer
Water vapour — The big bubbles are water that has turned into a gas: water vapour. Only the first tiny bubbles are air that was dissolved in the water.
The big bubbles are water that has turned into a gas: water vapour. Only the first tiny bubbles are air that was dissolved in the water.