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Interactive video lesson plan for: Is Glass Really A Liquid?

Activity overview:

You may have heard (or even read in a textbook) that glass is a liquid. Or that it's a solid. The truth is a little more complicated, but Lauren is here to explain.

Whether the topic is popcorn or particle physics, you can count on the HowStuffWorks team to explore - and explain - the everyday science in the world around us on BrainStuff.

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Hi. I’m Lauren, this is BrainStuff, and here’s today’s question: “Is glass a liquid or a solid?” If you’ve ever looked at the windowpanes in an old building, you may have noticed that the glass was rippley, and thicker towards the bottom of the pane. And you might’ve lept, gazelle-like, to the logical-sounding conclusion that the glass had flowed into that shape very slowly over a couple of centuries. You might’ve even read it in a textbook.

The truth is that the glass has always been that way. OK, so, up through the 1800s, panes of glass were made by hand – glassblowers used what’s called the crown process. They’d take a flattened bubble of very hot glass and rotate it fast so that the centrifugal force would spin it out into a large, mostly flat disc. The disc would be thicker at the edges, and each pane cut from it was bound to be a little lumpy. And workers tended to install them with the thicker side down – probably because the slightly larger edge provided better balance.

So the glass in those old panes isn’t flowing – at least, not that researchers can discern. They’ve looked at samples of glass from 2000 years ago and haven’t found telltale evidence of flow.
Scientifically speaking, glass is considered an amorphous solid – that means its atoms and molecules are locked into place like in a solid – like in ceramics, really. But those molecules are arranged more randomly than in most solids – more similar to a liquid.

If you wanna get into semantics, you could sorta call glass a supercooled liquid – that’s a liquid cooled to below its melting point carefully so that it doesn’t crystalize. And that’s part of making glass – but at that stage, it’s still hundreds of degrees above room temperature. It’s then cooled until it transitions into the rigid amorphous solid that we know and love. So you might say glass is its own state of matter, neither a liquid nor a solid.


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