0

Let's say you can't get to the kettle for a few minutes after the kettle starts to whistle.

Presumably the temperature of the water inside the kettle is still rising, past boiling point If there wasn't the little opening to let the steam escape at some point it'd get hot enough in there for the kettle to burst (if the lid was un-removable somehow.)

So is it all steam inside the kettle at that point, all super-excited colliding with the kettle walls at high speed? If you open the lid right then does ALL the steam rush out at the same time? (don't want to try it, I think my skin would be burnt off.)

To go reductio ad absurdum, what if it's a HUGE kettle with a swimming pool's worth of water, is all the water boiling evenly when the kettle whistles and doesn't it become thousands of degrees before it has a chance to make its way out through the tiny whistle hole?

Duke Leto
  • 111

3 Answers3

3

If you seal the pot nearly completely, then you are simply describing a pressure cooker, a method of cooking at high speed that's been around for many years. As more water is turned into steam the pressure inside the vessel rises dramatically and the temperature rises well above the boiling point. You can cook a roast in 1/3 the normal time because of the high temperature. If you release the lid, then the whole thing will blow all over with scalding hot liquid. (My brother once released the lid of a pot of pea soup and OH, what a mess it made) Search on pressure cooker for more info. Normal pressure cookers have a pressure release valve or else eventually the pot would explode from the building pressure inside. Other practical applications of water pressure include the steam engine. It's all the same science.

foolishmuse
  • 5,016
3

As the water starts to boil the pressure inside the kettle rises.

As the pressure rises, so does the rate at which steam is expelled through the whistle. If the whistle is sensiblly sized then an equlibrium should quickly be reached where the rate at which steam is produced by the boiling water equals the rate at which it is expelled through the whistle. This equlibrium is maintained as long as significant water remains in the kettle.

Once the water boils dry the kettle can get much hotter, depending on the details of the heat source and the material the kettle is made from this may severely damage the kettle.

With your reductio-ad-absurdium case the real question is how much heat are you putting into this mega kettle. If the heat input is the same as a regular kettle it may not boil at all or if it does it will do so slowly and the whistle valve will keep up.

OTOH if you scale the heat input with the size of the kettle, keep the whistle valve the same and seal the kettle against any other leaks you will get a much larger pressure building up. The likely result being rapid unscheduled dissasembly of your mega-kettle.

Peter Green
  • 1,181
2

So is it all steam inside the kettle at that point?

The gas in the kettle was mostly air when you first put water in. By the time the kettle is whistling, there will be gas generated inside (pure steam), and there will be gas escaping (at first, a mixture of air and steam), and there will be no gas entering (because of the higher pressure inside the kettle.)

So, you've got air coming out, and no possibility of air going in, and there wasn't much air in there to begin with. Somebody probably could work up a differential equation that relates the amount of air in the kettle to the elapsed time, but I think it safe to say that after a minute or two of whistling, the gas inside the kettle is, for all practical purposes, pure steam.

all super-excited colliding with the kettle walls at high speed?

That's a fancy way of saying, "hot." Yes. When the kettle is whistling, the steam inside will be hot.

Pure steam at atmospheric pressure must have a temperature of 100C because that's how vapor-pressure works. At higher pressure, it must be even hotter. Alternately, if you turned the heat off, and sealed the opening so that no air could get in, then the pressure inside would become quite low as the kettle cooled. Around 2.3% of atmospheric at room temperature. The kettle might be crushed by the ambient pressure.

Solomon Slow
  • 17,057