I understand that there is a thin layer of wax on paper cups that prevents water through seeping through. But there aren’t any (apparent) wax coatings. Since a styrofoam (molecule?) is only so big, why does the water not flow through the cracks?
Since wax molecules are only so big, why does the water not flow through the cracks?
The answer is that the cracks are too small to let water molecules pass through. If you are talking about the bead-shaped bits that styrofoam breaks into when you squish it, the answer is that the space between these is too small to pass water. Styrofoam cups are made by expanding the little beady things until they touch each other everywhere (and stick together), so there are no gaps in a well made cup.
I imagine it has a lot to do with the surface tension of water: the “cracks” in a styrofoam cup are too small to let water seep through.
Styrofoam cups are quite porous but as douglips sez, surface tension is a big part of it. Put some greasy hot soup or stew in a styrofoam drink cup and watch it seep through.
You guys must use some cheap assed Styrofoam cups! I serve soup in them all the time, and they don’t leak. Coffee, same thing. (By the way, I hate the things, and use ceramic cups whenever I am not out in the world.)
But, anyway, the little beads are not molecules, just beads. The Molecule of Polystyrene is fairly large, actually, as molecules go. Certainly many times larger than the molecule of water. That has very little to do with the permeability of the finished product, though. Wax molecules are smaller.
Anyway, get better cups.
Tris
They use wax to coat paper cups because wax is “hydrophobic.” That means that it repels water because water molecules are much more strongly attracted to one another than they are to wax. (This is part of what gives rise to surface tension.) Paper, however, is “hydrophilic,” which means that water molecules are strongly attracted to the cellulose molecules that make up paper. So paper by itself doesn’t repel water and it can soak through by seeping through the spaces between molecules. Polystyrene, as you might have guessed, is hydrophobic, so it doesn’t need a wax coating.