Why plastic, but not china plates stay wet in the dishwasher

28 January 2007




  • A dishwasher
  • Lots of different plates, bowls, mugs and cutlery made of plastic, ceramics, metals and whatever else you have in the kitchen




  1. Load your dishwasher.
  2. Turn it on and wash all your dirty kitchen stuff.
  3. Open the washing machine up and have a look at what's dry and what's still wet.



The plastic stuff is still wet but the ceramic plates are dry.


Everything was wet to start with and got more or less the same amount of water on it. The thing about plastic is that it's lighter, and we generally make plastic things thinner. So there's less material there, which means less stored heat. When a dishwasher gets hot at the end of its cycle, it relies on the fact that the plates retain heat, and that heat gradually evaporates the water. Although everything winds up at the same temperature at the end of the cycle, a plate or a mug might have ten times more heat stored in it than a plastic cup. So that's heat capacity, but there's another element to this experiment called heat conductivity. There's a lot of heat stores down the sides of the upside down cup. In a china cup, the heat will conduct quite well up the walls of the cup and up to where the water is pooled in the base. So all the heat that's stored in the walls of the cup can also be used to evaporate the water. But in a plastic cup, the heat stored in the walls won't conduct its way up the walls to where the water is. So there's only a very small amount of hot plastic right where the water is to do the evaporating.

Another experiment to look at heat conduction is if you put a piece of bread on a plate in the oven. They're both the same temperature, but you can pick the bread up easily. You certainly wouldn't want to touch the plate though! This is because the ceramic plate is a good conductor and will conduct the heat straight out of the plate and into your hand. The bread is a terrible conductor so doesn't easily transfer its heat into your hand.

The thermal tiles on a space shuttle are very light and porous and have low thermal conductivity just like bread. So when a space shuttle re-enters the atmosphere, its surface can get really really hot but the inside is cool enough to touch.


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