Water Does Not Boil At 100°C

We’ve all learned it in school: water boils at 100 degrees Celsius. But that’s not always true. In fact, it’s rarely true unless you’re at sea level. So, if you’ve ever tried boiling water on a mountain and wondered why your pasta won’t cook right, the answer doesn't lie within the temperature, but in the air pressure.

At its core, boiling is when a liquid turns into vapor, and that only happens when the pressure inside the liquid equals the pressure outside it. At sea level, atmospheric pressure is about 101.3 kilopascals (kPa), and that’s when water boils at exactly 100°C. But climb a mountain, and the air gets thinner, meaning less pressure. With less pressure pushing down, water molecules need less energy to escape into vapor, and voilà, water boils at a lower temperature.

In places like Denver, Colorado (the “Mile High City”), water boils at around 95°C. On Mount Everest, closer to 70°C. That’s not enough to brew proper coffee or cook rice without a struggle. Which brings us to a clever culinary hack: pressure cookers.

As you know, a pressure cooker is a sealed pot with no escape hatch (except for the valve), as heat turns water into steam, the trapped vapor builds up, increasing the pressure inside. This added pressure acts like an invisible lid pressing down on the water, forcing it to stay liquid until it’s much hotter; boiling at around 120°C instead of 100°C. That higher temperature cooks food faster and more thoroughly.

Go in the other direction, say, outer space or a vacuum chamber, and you get the opposite effect. In a vacuum, where there’s no atmospheric pressure, water doesn’t boil at 100°C. It boils at room temperature or even lower. If you put water in a vacuum chamber you'll notice it starts boiling on its own with no heat, once the chamber gets to a low enough pressure. And in fact, your water will get cooler boiling in a vacuum. So next time someone says water boils at 100°C, you’ll know the truth: only if the air says so.