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Water turning to gas inside itself

What Are the Bubbles When Water Boils?

Boiling bubbles are water vapour, formed throughout the liquid rather than just at the top.

What people assume Trapped air escapingWhat it is Water vapour — wateritself, as gas
The bubbles are made of water.

  1. Step 1 of 5

    Water always evaporates at the surface.

    Water surface Fast moleculesescaping

    Fast molecules escape from the top at any temperature. That is evaporation, and it is quiet and gradual.

  2. Step 2 of 5

    Boiling happens throughout the liquid.

    Evaporation Surface only, anytemperatureBoiling Throughout, at 100°C

    At 100°C molecules anywhere in the water have enough energy to become vapour, so bubbles form deep inside, not just on top.

  3. Step 3 of 5

    Each bubble is pure water vapour.

    A rising bubble Water molecules in gasform

    Not air. It is water in gas form, which is why the bubbles vanish if they rise into cooler water — they condense back.

  4. Step 4 of 5

    The temperature stops climbing.

    Boiling waterHeat going inAll spentturning waterto steam

    Keep heating boiling water and it stays at 100°C. All the extra energy goes into converting liquid to gas, not raising temperature.

  5. Step 5 of 5

    Pressure changes the boiling point.

    Sea levelboils at 100°CHigh mountainboils at about90°C

    Less air pressing down means molecules escape more easily. On a high mountain water boils near 90°C, which is why food takes longer to cook.

    A pressure cooker raises the pressure so water boils above 100°C — hotter water, faster cooking.

The short version

Boiling bubbles are water vapour forming throughout the liquid, and the temperature holds steady while it boils.

Try it yourself

Watch a pan carefully. Tiny bubbles appear first — that is dissolved air. Real boiling bubbles are much bigger.

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