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A whistling kettle will start to whistle when the water boils and turns into a jet of steam which then exits the small aperture in the spout.

But why doesn't this happen much earlier - when the air molecules in the kettle get heated up enough, shouldn't they also (due to the increased pressure) forcefully escape through the aperture? (Assumee that the kettle was half filled with water, and so the upper half was filled with air). This should happen long before boiling (which is when the water molecules gain enough momentum to escape the water surface). Yet in practice, the whistling only starts at the time the water boils - how come?

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There is a very recent journal article about why kettles whistle: "The aeroacoustics of a steam kettle" Phys. Fluids 25, 107101 (2013)… – DavePhD Apr 14 '14 at 0:37
up vote 10 down vote accepted

Let's assume a one litre $1000{\,\rm W}$ electric kettle, filled with $0.5$ kilograms of water at $20^\circ \mathrm{C}$:

It takes 4.2 joules to warm one gram of water one degree Celsius.

So, to warm the $500$ grams of water $80$ degrees from $20$ to $100$ takes $168,000$ joules. The kettle will supply $1000$ joules per second, so it'll take $168$ seconds for the kettle to come to a boil.

During this time, the $0.5$ litres of air will expand by a factor of $\frac{373}{293}$, to a volume of $0.637$ litres. So in the almost three minutes of heating, only $0.137$ litres of air will be forced out through the whistle spout.

Now we're at the boiling point. It takes $2,280$ joules to vaporize $1$ gram of water. So the kilowatt heater will vaporize $0.439$ grams of water each second!

Those $0.439$ grams of water vapor will occupy around $0.750$ litres at $100^\circ$ Celsius. So this much gas will be forced out the spout each second...

$0.137$ litres of air in three minutes, vs $0.750$ litres of steam each second.. That explains the difference...

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Before the water boils, the rate that the water becomes water vapor is slow compared to after it is boiling. The pressure inside the kettle does not increase significantly until after the water is boiling. Only after the water is boiling, is there enough pressure inside the kettle to send a stream of air through the whistle.

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