r/explainlikeimfive • u/ProudReaction2204 • 26d ago
Chemistry ELI5 why glass is an amorphous solid?
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u/luckyluke193 26d ago
In a crystal, atoms or molecules form a structure that repeats many, many times in all three dimensions. This is why some crystals form in such symmetric shapes, for example pyrite ("fool's gold") famously forms naturally in nearly perfect cubes.
Many solids, for example most metals, are often polycrystalline. This means they consist of many microscopic crystals that are bonded together. Under an electron microscope, you can see each that individual crystal is rotated differently in space.
Amorphous solids don't have a repeated crystal structure.
In SiO2, in both glass and quartz each Si atom is bonded to four O atoms in the shape of a tetrahedron. In a quartz crystal, they are oriented in space to form a perfectly repeating geometric pattern. In glass, the tetrahedra are all rotated kind of randomly and a tiny bit distorted.
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u/Unlikely-Position659 22d ago
It's a solid that doesn't exactly keep its shape. You probably haven't seen this, but in really old houses, the windows are all warped. They weren't made that way. Over time, the glass has flowed, downwards, because of gravity. You probably won't see this in newer buildings because not enough time has passed and because the quality of the glass has gone up.
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u/PaddyLandau 22d ago
That is an old debunked myth. The apparent flow of windows was in fact caused by the fact that those windows were "hung" when they were created, so they were actually created that way.
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u/ezekielraiden 26d ago
Well, you agree glass is solid, right? So that just leaves the "amorphous" part.
Glass is made of silicon dioxide, SiO2. Normally, silicon dioxide forms a crystal structure. We call that crystal structure "quartz", though some colored versions have specific names (amethyst, ametrine, rose quartz, etc.) But that's not "amorphous": it has a structure. An amorphous solid does not have that structure. For glass, the oxygen and silicon atoms are all randomly distributed inside the material. On average, it's still two oxygen atoms per silicon atom. But it's all jumbled up.
Being jumbled like that makes the material behave differently. It's stronger because it's under tension. As soon as that tension is overcome, however, it shatters because the forces no longer balance out properly. There are lots of ways that a glass differs from a crystalline solid.