How does a whistling kettle work

The word work in the question above could confuse. In common parlance, we say a kettle works or does not work, meaning it either functions as a kettle or is useless. But following the example in the previous section, we now realize how the word work has a carefully defined thermodynamic meaning. Operate would be a better choice in this context. In fact, a kettle does not perform any work at all, since it has no moving parts and does not itself move. 

In a modem, automatic kettle, an electric heater warms the water inside the kettle - we call it the element . The electric circuit stops when the water reaches 100 °C because a temperature-sensing bimetallic strip is triggered. But the energy for a more old-fashioned, whistling kettle comes from a gas or a coal hob. The water boils on heating and converts to form copious amounts of gas (steam), which passes through a small valve in the kettle lid to form a shrill note, much like in a football referee s whistle. 

The whistle functions because boiling is accompanied by a change in volume, so the steam has to leave the kettle. And the volume change is large the volume per mole of liquid water is 18 cm3 (about the size of a small plum) but the volume of a mole of gaseous water (steam) is huge. 

A volume of 0.031 m3 corresponds to 31 dm3, so the water increases its volume by a factor of almost 2000 when boiled to generate steam. This staggering result helps us realize just how great the increase in pressure is inside the kettle when water boils. 

The volume inside a typical kettle is no more than 2 dm3. To avoid a rapid build up of pressure within the kettle (which could cause an explosion), the steam seeks to leave the kettle, exiting through the small aperture in the whistle. All the vapour passes through this valve just like a referee blowing time after a game. And the 

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