Re: When you put a hot glass into cold water why does it crack?

Glass and ice cubes crack due to three major effects:

1) Hot things generally are larger than cold things.

2) Stresses in solids move only as fast as the speed of sound.

3) Large temperature differences create large stresses (partly because of #1 above).

Now I will discuss these three effects:

Things, including glass and ice, are made up of atoms. Temperature is a measure of how much jostling (vibrating) the atoms are doing. The more vibrating the atoms do the farther apart they tend to be from each other. Think of an example: Put 100 students in a room, having each student connected to the nearest 6 students with a strong rubber band. Have them get as close together as they can. These are "cold" students. Now ask the students to run around as fast as they can. These running students are "hot", and they have more space between them than when they were "cold"! They stay more or less close together because of the rubber bands. It's like this with the atoms in solids. The atoms are constrained to stay near each other (by interatomic forces), but when they are moving with more energy (hotter) they are farther apart on average. So hotter solids are larger than the same solid that is cooler, because there is more distance between the atoms.

The atoms in a solid arrange themselves so that they are as close together as possible at the temperature at which they are. If the temperature changes, the atoms rearrange so that they are a different distance from each other, and if the temperature change is sufficiently slow the atoms can rearrange in an orderly manner. The rearrangement ripples through the solid at the speed of sound for that solid. This is because the speed of sound in a solid depends on the average distance between atoms and the speed at which they can move.

Large and fast temperature changes make the atoms rearrange so fast that the other atoms in the solid can't keep up with the rearrangement, and so a crack occurs. So it takes both a large temperature change and a fast temperature change to make a solid crack.

I hope this helps!

John Link, MadSci Physicist