Why does water boil?

You put a pot on the heat and, after a few minutes, the water starts to stir, bubbles appear, and steam rises. We see it every day, but we rarely ask what’s going on in there. Boiling water is a precise physical phenomenon, and it hides a surprise or two (like the fact that it doesn’t always happen at 100 degrees).

From liquid to gas

Water can exist as a solid (ice), a liquid, and a gas (vapor). When you heat it, its molecules move faster and faster. Boiling happens when that motion is so intense that the liquid starts turning into vapor not just at the surface, but throughout its interior, forming bubbles.

Those bubbles that rise and burst are not air: they are pure water vapor, water in a gaseous state making its way upward.

The key: pressure

Here’s the central concept. Water boils when its vapor pressure equals the atmospheric pressure around it. While the air pressure pushes down on the surface of the liquid, the liquid needs to heat up until its own vapor pressure is enough to overcome it and release bubbles.

As Britannica explains, that’s why at sea level water boils at 100 °C: it’s the temperature at which its vapor pressure equals normal atmospheric pressure.

Why it boils sooner up a mountain

And here comes the surprise: 100 °C is not a fixed rule. At altitude, atmospheric pressure is lower, so the water needs less temperature to match it. As the USGS notes, high up a mountain water can boil at 90 °C or less.

This has a curious practical consequence: because the water boils at a lower temperature, it’s harder to cook food at altitude (an egg or a pot of lentils takes longer), since the water never gets as hot as it does at sea level.

Boiling isn’t the same as evaporating

It’s easy to confuse them, but they’re different:

• Evaporation happens only at the surface and at any temperature (that’s why a puddle dries in the sun without boiling).
• Boiling happens throughout the entire mass of the liquid and only when it reaches the boiling point.

The pressure cooker trick

The same principle explains why the pressure cooker cooks faster. When sealed, it traps the steam and raises the pressure inside; with more pressure, the water needs more temperature to boil, so it goes above 100 °C. That hotter water cooks food in far less time.

An everyday phenomenon full of physics

Boiling water seems about as simple as it gets, but behind it lies a precise dance between temperature, pressure, and change of state. Understanding it explains things as varied as why mountain coffee tastes different or why the pressure cooker is so handy. The next time you wait for it to come to a boil, you’ll know exactly what those bubbles are doing.