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Explainer #19 · Home & Kitchen

Why does boiling water bubble?

When you heat a pot of water, the first tiny bubbles are air that was hiding in the water. Warm water can't hold as much air, so it comes out. The big bubbles of a real boil are water turned into steam. They start in tiny scratches in the pot, and at 100 °C they reach the top and escape. One spoon of water makes about 1,700 spoons' worth of steam. On a mountain, less air presses on the water, so it boils sooner: about 70 °C on top of Mount Everest. A pressure cooker does the opposite and boils at about 120 °C.

The principle

A pot of water on the stove. Heat a pot of water and watch closely: the bubbles change on the way. First come bubbles of air, then bubbles of steam.

Step 1: Heat comes from below. The flame heats the bottom of the pot first. Warm water rises and cooler water sinks to take its place, round and round, like the blobs in a lava lamp. That way the whole pot slowly warms up.

Step 2: The first tiny bubbles. Long before the water boils, tiny bubbles appear on the bottom and sides. They are not steam, they are air. Cold tap water has a little air mixed into it, and warm water can’t hold as much, so the air comes out, a bit like the fizz in a soda.

Step 3: Steam starts in scratches. The bottom of the pot is the hottest place, and there some water turns into steam. It starts in tiny scratches and dents in the pot, where a bubble has a little corner to grow from. At first the water higher up is cooler, so the steam bubbles shrink and pop on the way up, at around 90 °C. That is the humming and ticking you hear just before a kettle boils.

Step 4: A full boil at 100 °C. Once all the water is hot enough, 100 °C at sea level, the steam bubbles make it all the way to the top and escape. Steam takes up a lot of room: one spoon of water turns into about 1,700 spoons’ worth of steam. Turning the heat up now doesn’t make the water any hotter, it just makes more steam, faster.

Step 5: Air pressure sets the number. The air above the pot presses down on the water and holds the bubbles back. On a mountain there is less air pressing down, so bubbles form sooner: on top of Mount Everest water boils at about 70 °C, too cool to cook pasta properly. A pressure cooker traps the steam so it presses down harder, and the water boils at about 120 °C, so food cooks faster.

Common myth

Boiling bubbles are just air

Only the first tiny bubbles are air. The big rolling bubbles of a real boil are the water itself, turned into steam. You can test it: boil the water, let it cool, and heat it again. The tiny air bubbles barely come back, because the air has already left.

How this video was made: Rendered 3D animation, simplified and drawn as a pot cut in half and seen from the side. Small white dots are air bubbles, bigger bubbles are steam, and the temperature on screen is rounded; sizes and speeds are shown roughly, not to scale.

Where the facts come from

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