Is glass a solid... or a liquid?

It's a very interesting question.

The answer is both—and neither! There are widely differing opinions on how to refer to materials such as glass that seem to be a bit like liquids in some ways and a bit like solids in others.

In schools and in books, we tend to learn that solids all have a fixed structure of atoms.

In fact, there are different kinds of solids that have very different structures and not everything we describe as "solid" behaves in exactly the same way. Think of a lump of iron and a lump of rubber. Quite clearly they are both solids, and yet the rubber is very different from the iron. Inside, rubber and iron have their atoms (in the case of iron) and molecules (in the case of rubber) arranged in totally different ways. Iron has a regular or crystalline structure (like a climbing frame with atoms at the corners), while rubber is a polymer (made from long chains of molecules loosely connected together). Or think of water. As you may have discovered, water is an almost unique solid because it expands to begin with when it freezes. In short, not everything fits neatly into our ideas of solid, liquid, and gas and not all solids, liquids, and gases behave in a nice, neat, easy-to-explain way. The exceptions are the things that make science really interesting!

Amorphous solids

Let's return to glass. Peer through a microscope inside some glass and you'll find the molecules from which it's made are arranged in an irregular pattern. That's why glass is sometimes referred to as an amorphous solid (a solid without the regular crystalline structure that something like a metal would have). You may also see glass described as a "frozen supercooled liquid". This is another way of saying "glass is a liquid that has never set", which is the puzzling statement you'll sometimes find in science books. We could say glass is a bit like a liquid and a bit like a solid. It has an internal structure that is somewhere between the structure of a liquid and a solid, with some of the order of a solid and some of the randomness of a liquid.

Glass is by no means the only amorphous solid. It's possible to make a type of water called amorphous ice that could be described as in-between solid (water) and liquid (ice). You do this by cooling water very quickly. The ice forms so fast that it doesn't have time to build up its normal, crystalline structure. So what you get looks like ice but behaves in some ways like liquid water. Other substances can be made into amorphous solids too. Solar cells are often made from something called amorphous silicon.