Recently our lovely soft water, which came from a loch high in the Cairngorm mountains, has been replaced by slightly hard water which comes from an underground borehole. Our kettle and our hot water tanks now pop and crackle loudly when the water is heating, rather than offering just a gentle hiss as before. Can anybody explain the reason for this odd effect when we are heating water with a higher mineral content?
• When an electric kettle is switched on, the element heats the water near it by convection. After that, small bubbles of vapour form on the element at nucleation points – areas that are slightly rougher than the rest of the element and which facilitate the change of state from liquid to gas.
As the small bubbles rise through the colder water, they collapse – or condense implosively – before they reach the surface, and release latent heat which raises the water temperature. This implosive condensation is called cavitation, and it is responsible for the soft susurration or hiss that you can hear.
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As the water gets hotter, the vapour bubbles rise higher and grow bigger. These larger bubbles produce louder noises when they collapse, which is why a kettle gets louder as the water temperature rises. When the water reaches boiling point, the bubbles break on the surface, so there is no cavitation and the sound of the kettle alters entirely.
When hard water is heated, minerals precipitate out of the water, mainly in the form of calcium carbonate, and form on the element. This layer prevents an even heat transfer from the element to the water, allowing large vapour bubbles to form early in the heating process in cracks and crevices in the limescale.
As these large bubbles implode, bangs and bumps are heard above the normal hiss. This is even more noticeable in central heating systems, where it is known, unsurprisingly, as kettling. When this effect happens in the boiler, shock waves can be felt right through the system.
Limescale can be removed from a kettle element by using a proprietary descaler or vinegar, but leave heating systems to the professionals.
If you want to see these principles in action, first watch water heating in a flat-bottomed pot. Then repeat this adding a layer of sand a few centimetres thick in the bottom of the pot. This will demonstrate the knocking.
David Muir, Science department, Portobello High School