# When water is about to boil

Have ever noticed? When water is about to boil, no matters the kettle, there is some sound I have no idea where it comes from, sometimes long before it boils.

Is there any explanation for this phenomena?

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The water near the heating element turns into water vapor. This vapor then rises up to the surface but as it meets colder water upwards it turns back into water.

As the water/steam transition is not smooth (e.g. the volume changes rather rapidly during phase change), the constant transition between vapor and liquid states produces noise. As the water gets to +100C (boiling point), the vapor bubble will not turn back into liquid before reaching the surface thus making less noise.

For more on this click this.

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It can even be seen, when you have a translucent electric kettle. –  mbq May 1 '11 at 10:20