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How does water boil?

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Quick answer

Water boils when it is heated to its boiling point, the temperature at which its molecules have enough energy to break free as a gas. At normal sea-level pressure, this happens at 100°C (212°F). Bubbles of water vapour form throughout the liquid and rise to the surface, releasing steam.

Water boils when its molecules gain enough heat energy to overcome the attractive forces holding them together as a liquid, turning into a gas called water vapour. This happens at a specific temperature, called the boiling point, which depends on the surrounding pressure.

Step by step

  1. Heat is applied to liquid water, increasing the kinetic energy of its molecules.
  2. The molecules move faster and collide more forcefully, pushing against the attractive forces between them.
  3. When the temperature reaches the boiling point, molecules throughout the liquid have enough energy to overcome those forces.
  4. Bubbles of water vapour form at nucleation sites, such as scratches or impurities in the container.
  5. The bubbles rise to the surface and burst, releasing water vapour into the air.

Key things to understand

  1. Heat energy makes water molecules move faster and faster.
  2. At the boiling point, molecules have enough energy to break free from the liquid's surface and from each other.
  3. Bubbles of water vapour form throughout the liquid, not just at the surface.
  4. The boiling point of water at normal sea-level pressure is 100°C (212°F).
  5. If the surrounding pressure is lower, water boils at a lower temperature; if higher, it boils at a higher temperature.

Labelled diagram

  1. Heating

    Heat energy makes water molecules move faster and faster.

  2. Bubble formation

    Molecules with enough energy form vapour bubbles inside the liquid.

  3. Bubbles rise

    The bubbles are less dense than liquid water, so they float upward.

  4. Surface burst

    At the surface, bubbles pop and release water vapour into the air.

How water boils: from liquid to gas

Real-world examples

  • On Mount Everest, at about 8,848 metres, water boils at around 70°C because the air pressure is much lower.
  • In a pressure cooker, the sealed environment raises the pressure, so water boils above 100°C, cooking food faster.
  • A pot of water on a stove at sea level boils at 100°C and stays at that temperature until all the water evaporates.
  • In a vacuum chamber, water can be made to boil at room temperature by removing the air pressure above it.

Common mistake

✗ Myth: Water always boils at exactly 100°C.

✓ Fact: Water boils at 100°C only at standard atmospheric pressure at sea level. At higher altitudes, lower pressure makes water boil at lower temperatures, and in a pressure cooker, higher pressure raises the boiling point above 100°C.

People also ask

What temperature does water boil at?
At normal sea-level pressure, water boils at 100°C (212°F). However, the boiling point changes with altitude: on a high mountain, it can be much lower, while in a pressure cooker, it can be higher. The key factor is the surrounding air pressure.
Why does water boil faster with a lid on?
A lid traps heat and steam inside the pot, so less energy escapes. The trapped steam also increases the pressure slightly, which can raise the boiling point. Both effects make the water heat up and boil sooner.
Can water boil and freeze at the same time?
Yes, under a special condition called the triple point, where the temperature and pressure are just right for water to exist as solid, liquid, and gas all at once. This occurs at 0.01°C and a very low pressure.
Why does water boil at a lower temperature on a mountain?
At high altitudes, the air pressure is lower, so water molecules need less energy to escape as gas. That means water boils below 100°C on a mountain, and food may take longer to cook because the water is not as hot.
What is the difference between boiling and evaporation?
Evaporation happens at any temperature when molecules at the surface escape into the air. Boiling happens at a specific temperature when bubbles of vapour form throughout the liquid and rise to the surface.
Can water boil without being hot?
Yes, if the pressure is low enough. In a vacuum, water can boil at room temperature because there is almost no air pressure pushing down on the liquid, so molecules escape easily.
Why does boiling water sometimes stop boiling when you add salt?
Adding salt raises the boiling point slightly, so the water may briefly stop boiling until it heats up a bit more. The effect is small for normal cooking amounts.

Part of the guide: Everyday Science: Light, Heat & Matter →