Why do plastic cups coming out of the dishwasher usually have water left on them but the glass cups are dry?
In: Chemistry
There are 2 factors at play here:
1. The amount of heat that plastic vs. glass can retain (heat capacity)
2. The way in which water spreads itself on plastic vs. glass (adhesion)
For the first point, glass can retain heat for much longer than plastic. As you know, heat will cause water to evaporate at a faster rate, so as the water stays on the glass it absorbs much more heat from the glass which allows it to evaporate at an increased rate. Similarly, water stays on plastic but the plastic cools down much faster than the glass, and it doesn’t absorb nearly the same amount of heat so more of it will remain in liquid form and not evaporate.
For the second point, water has a tendency to ”stick” to itself because water (H2O) molecules have a special type of intermolecular force called ”hydrogen bonding”, it basically means that the forces between each water molecule is strong and they stick together because of this (this is the reason that water is found in the shape of ”beads” on a leaf for example, water prefers to stick to itself in a sense). Not all molecules have the special property of hydrogen bonding. If the material that water is on also is made of molecules with strong bonds, then the water doesn’t have to adhere to itself, like how it does on a leaf in the form of water ”beads”. Instead, the water will spread itself across the surface into a film of water because it is attracted to the molecules that make up the glass. Glass is made up of these strong bonds (silicon dioxide), so water spreads itself across the surface of glass. Similarly, plastic inst made of very strong bonds, so the water molecules are not attracted to the molecules that make up plastic and therefore the water will remain as ”beads” on the plastic, minimizing surface area.
The point is that surface area is also factor for how fast a liquid can evaporate. Since water spreads itself on glass and doesn’t spread itself on plastic, the water covers more surface area on the glass and will therefore evaporate faster as a result of that.
In conclusion, its a mixture of both factors. Water absorbs more heat and has more surface area on glass, therefore evaporating at a fast rate. Water absorbs less heat from plastic and doesn’t spread itself, which minimizes the surface area it covers, therefore not evaporating as fast as if it were on glass.
Note: I know that’s a very butchered way of explaining polarity.
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