Why do glass and plates dry up in the dishwasher but plastics don’t?


You know how plastic tupperware cups always have drops on them when you take them out of the dishwasher? The plates and glass have long since airdried. What is the properties of plastic that does that?

In: Chemistry

Glass and ceramic have a much higher heat capacity, so they retain more heat when put in the dishwasher. The heat transfers to the water on them which helps it evaporate quickly. Meanwhile the plastic cools quickly and doesn’t heat up the water as much.

Mor original comment was auto removed because it was too short…..now I have to write a longer reply about heat retention and how that makes the left over liquid steam away…..

Ok. So plastic doesn’t hold heat very well and as an added bonus is a lot more porous than porcelain, as you can see if you heat tomato sauce in a plastic container.

So while your dishes get washed they heat up and hold some of that temperature. When the washing stops, the residual water “steams” away- think hot asphalt, sprinkle water on it…dries quickly. Put tarp over it, sprinkle it, fries but slower.

Technically it is the opposite. Metal and ceramic objects in the dishwasher get wet but the plastic object is dry with water drops on them. The water will evaporate faster from the wet object where it is spread out and not in larger drops.

Wet is now not exactly what you think it is, an object with distinct water drops on it is not expected wet, a object with water all over it is wet even if the layer is quite thin

Wetting is when a liquid on an object spread out on it and does not form drops you can see. Compare how it looks if you put a drop of water on plastic and a paper towel, the paper towel gets wet but the drop on the plastic does not spread out so it is in fact dryer.

Wikipedia has a [good illustration](https://en.wikipedia.org/wiki/Wetting#/media/File:Surface_tension.svg) where A have very little wetting and S have the most.


The result is that ceramics and metal are wet when you open the dishwasher. the water is spread out on a thin film on their surface and can evaporate quite quickly.

The plastic object and not as wet and the water from drops evaporate a lot slower. The formation of water droplets also makes it possible for more water to collect on the plastic objects.

The heat capacity that the other post mention comes into effect too and results in faster evaporation from metal and ceramics. But is not the complete explanation.

Try to put a bit of metal foil in the dishwater because it has less heat capacity than the normal plastic object you use but will be a dryer as long as it does not have a shape so water collects on it. A mug that is not upside down will contain a lot of water regardless of material, That happens with other stuff so be careful with the foil if you test it so that is not a relevant effect.

In the podcast https://aproblemsquared.libsyn.com/wetness-trees-and-pet-mysteries there is an explanation of it at 39 minutes

Dishes get dry in the dishwasher by getting hot and making the water evaporate from it.

If you’ve ever hand washed plastics with hot water (like Tupperware), you’ll notice it doesn’t get hot. Even if you have boiling water rolling over the plastic, you can touch it seconds later and it still won’t be hot.

Because plastics don’t retain heat.

So since they don’t get hot in the dishwasher, the water evaporates much slower off of it than things that do get hot.