eli5: Why is it that glass shatters when it goes from extremely hot to extremely cold temperatures and vice versa? Moreover, what does that say about the glass manufacturing process if anything? Thank you!



I just saw a video of people shattering Pyrex glass by pouring ice cold water within the measuring cup after heating it with a blowtorch, and I am curious as to what causes the shattering!

Here’s a link to the video if anyone is interested:

Thanks in advance to anyone responding!

In: Chemistry

All materials expand and contract with temperature. If you heat the bottom of a glass but not the top, then the bottom will expand and the top will not. Since glass isn’t very flexible, it instead just breaks.

This means that the whole temperature must be virtually the same as it heats or cools.

A combination of things.

First of, as stated by u/TheJeeronian, with heat things expands and with cold things contract. As a result heating and cooling will force the object to move. With material that are not naturally flexible, this movement can lead to the object shattering.

Second, rapid heating and cooling can weaken the bond between atoms. Objects flexibility can then change again. Making it harder or softer depending on how it is cooled. That can also be seen in the different cooling method for forged metals. Some are used to make the metal softer and malleable while other to make them hard, but more brittle.

Glass follows the same pattern, but the rapid heating heating and cooling ends up making the glass less flexible while forcing movement within it. Eventally, the glass ca no longer handle the movement and break.

I’ll add something to the others’ good explanations. It usually isn’t an issue when an object expands or contracts due to temperature changes. The issue arises when these deformations are constrained in some way. If you have a very hot glass slab and quickly cool down one of its faces than that face will want to contract, but it will be constrained by the other face which is still hot. At some point within the thickness of the slab the atoms in the cold part will want to move closer but they can’t because they’re still bonded to the “hot” atoms which are fine being as far apart as they are: this generates a stress inside the material. The same happens to the “hot” atoms which are pulled closer together by being bonded to the “cold” ones, however this generates a compressive stress which is usually less dangerous. Long story short, glass being very brittle, the stress might exceed a critical point and a crack will form and propagate leading to fracture by thermal shock. It isn’t actually the temperature itself causing it, but the temperature difference between two regions in the same material. Glasses and ceramics are particularly prone to this, while other materials are more resistant. For example it’s much harder for thermal shock to happen in metals, as they’re ductile (they tend to deform instead of cracking) and also very good heat conductors (so any temperature differences within the material quickly disappear).

The first point to note is that glass itself is not a solid or a liquid – it’s a different state of matter. Most materials cannot become a glass (they cannot undergo the glass transition) because of they do not have an alternative setup for their atoms. But materials that do have a glass transition state will not heat from solid into liquid (like most materials like ice do), but will instead heat from solid into glass into liquid. Depending on your particular chosen definition, you could still consider the glass state to be a liquid, but glass is certainly not a solid for thermodynamics reasons.

Describing the state of atoms in glass is a bit tricky. We can sort of imagine it as a group of people who are holding each others arms. The length of their bonds (arms and where on the arm they grab) as well as the angle of their bonds (angles between groups of people) can vary. Some bonds will be stronger or weaker. Some people will be standing nearer or further away from others (this is important because it highlights the difference between glass and solid – for a solid, every single person would be forced into a particular position with a particular bond length and bond angle, without room for variation). For a reasonably large piece of glass (e.g. larger than 3cm by 3cm), every single person (atom) needs to form part of the same global unit. The bonds (people’s grip on each other) are extremely strong here. If we lose/ break any of these bonds in a localised region, then that part of the glass will be split off from the rest. e.g. if 50 people lose their connections to the main group, they will essentially drop out of that part of the glass, forming their own splinter of glass.

Understanding what temperature does to bonds then tells us why these bonds might break – either by dropping or raising the temp very fast. The temperature can be understood to be the amount of movement energy and variation (in exact position) within each person (atom) and their arms (bonds). A very high temperature would be the equivalent of a massive sugar rush. The person would easily have enough energy to stop caring about holding onto any arms, and could leave their “place” in the glass. A very low temperature would do the opposite – basically force the person in place, and would also force them to make tighter and stronger bonds (with less variation in length and angle). Now we need to imagine what happens when we force everyone in the group to do this. If we force them all to be cold, bound and lock their bonds and then suddenly inject a huge amount of energy in, it’s very likely that certain sub-groups will lose all contact with the main part of the glass (shatter off). Likewise if everyone has a lot of energy and is bustling, then suddenly told to freeze and follow stricter bonds – it’s very likely that certain groups will no longer be able to bond with the main part of the glass.

An excellent follow up question, then is why doesn’t this happen with a traditional solid (like a metal, plastic or rubber)? And the reason is highlighted again in the difference in positions for the atoms of a solid vs a glass. For a solid, everyone is following the same rules. They are all 1m away with 90 degree bonds (for example). This means when we heat/cool them as a group, their global behaviours will be very similar. They are either almost all going to lose permanent contact with each other (and melt into a liquid), or they are all going to keep their bonding (and remain a solid). It isn’t really possible for 25% of the group to lose bonding like they could in a glass, since they all have the same localised rules. In a glass, it’s entirely possible (likely, even) that 25% of the atoms in the glass have pretty weak bonds and aren’t very close to other atoms, while the other 75% have stronger bonds, are close to other atoms, and have the heat-resistance of a normal solid.

So heating things cause then to expand and cooling them makes them shrink. Using a blowtorch is applying a lot of heat to a small area very fast. This causes the hot area to expand but the cold area doesn’t so the glass has no where to go and shatters. As long as the glass is heated evenly it won’t shatter so you can use a glass dish in the oven but not on a stove top.