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Why do water droplets remain on plastic items after they’ve gone in the dishwasher? © Getty Images

Why do water droplets stay on plastic items after they’ve gone in the dishwasher?

Published: 24th February, 2022 at 04:00
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Have you ever wondered why glass items come out sparkling, while plastic items still look wet?

Dishwashers work by spraying jets of water and detergent at up to 68°C – much hotter than you would use for washing up by hand. At the end of the washing cycle, the hot, clean crockery dries itself just by evaporation. But although everything inside the dishwasher starts off at the same temperature, plastic items cool down much quicker. This is because they have lower ‘thermal mass’.

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The plastic of a food storage container is thinner than a cup or plate, and plastic is a lower density material, so for a given temperature, its molecules hold less heat energy. As the water evaporates from the surface, it quickly cools the plastic down and this slows the rate of evaporation.

Heavy plates and pans hold their heat much longer and are still quite warm even after all the water has been evaporated away. China, glass and metal are also much better heat conductors than plastic. This means that all the heat in the vertical sides of an upturned bowl or cup can flow quickly to supply heat to the horizontal surfaces, to evaporate any water that has puddled.

The heat in a plastic container spreads more slowly so small water droplets remain on the surfaces.

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Asked by: Koby Shomron

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Authors

luis villazon
Luis VillazonQ&A expert

Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.

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