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’.
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.
Read more:
- Why does bolognese sauce stain my plastic containers?
- Why does tea taste wrong out of a plastic cup?
- Why are ceramics cold to touch and plastics warm?
- Is it possible to recycle plastics an infinite number of times?
Asked by: Koby Shomron
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- This article first appeared inissue 371ofBBC Science Focus Magazine–find out how to subscribe here
