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Why sand turns to glass when heated

How and why does sand turn to glass when heated? What physically happens to the sand?

Here’s some sand under a microscope

As you can see, it looks just like tiny bits of glass. That’s because it is. Sand and glass are primarily silicon dioxide, although you often find impurities that give it color. Quartz is also silicon dioxide, but the difference isn’t chemical, it’s structural. Sand and glass have disorganized molecules which point every which way, while quartz is made up of regular crystals.

When you melt sand, it becomes “amorphous” like in the right hand diagram. Quartz has to develop under particular conditions, like diamonds do. A lot of sand is ground quartz and when you heat it to melting, it becomes amorphous. Once you melt it, the silicon and oxygen atoms start attaching to each other so, when you cool them, they stick together instead of staying in discrete pieces. Imagine taking a bunch of ice cubes, melting them, then putting the whole container back in the freezer - you wind up with one big ice cube.

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