Glass
Education can be really tough, from both sides of the learning divide. When people try to explain something without taking into account how you learn or what your background in a subject is, it becomes difficult not only to learn, but to even pay attention. Conversely, if a learner in your charge has preconceived notions about the topic you’re teaching, it can take a lot of brute force to dislodge their ideas. In truth, the real process of teaching is subtle and interactive, done as much by feel as according to rules or guidelines, and should always involve a back-and-forth of ideas.
That said, I’m still going to rant about something that I’ve heard a lot of people get wrong without particularly opening the floor to debate. If you think I’m wrong, or just want a little more convincing, my contact details are on my home page.
As you might be able to guess from the title, my topic of choice is glass, and the physical state that it is in. I’d expect most people would say that glass is solid, though some people (those who, like me, have a love of pub-quiz standard trivia) might point out that glass is actually a liquid, and flows over time. These people will generally then go on to say, with a knowing smile, that the evidence for this is in the windows of old buildings, where the panes of glass are visibly thicker at the bottom than the top.
This is a tricky area: where do we draw the line between solids and liquids? For that matter, what’s the difference between a liquid and a gas? And are there other states that we need to worry about?
I’ll work backwards.
Yes, there are other states of matter. Many other states. The thing to remember here is that the science you were taught at school was subject to three rules:
- it had to be largely uncontentious,
- it had to fit into about 200 hours of teaching per year, and
- it had to be comprehensible to hormonal teenagers.
As a result of these constraints, we gloss over more esoteric states of matter like liquid crystals, supercritical fluids and Bose-Einstein condensates. Take a look at one of those links and tell me that isn’t a good thing. What isn’t a good thing, I suspect, is that we also never really get into what makes a gas a gas or a liquid a liquid.
The distinction between a gas and a liquid is actually fairly arbitrary, and is arguably what leads to the existence of supercritical fluids: gases are tenuous, and liquids are dense. More specifically, liquids have a density high enough to suggest that the atoms or molecules from which they are formed are effectively touching one another. In practice, this means that liquids are hard to squeeze. This is simple to test: find an empty plastic bottle and give it a squeeze. Not hard, right? Now fill it all the way up, to the very top, with water, put the top back on and try to squeeze it again. More or less impossible.
Now we have to get down to brass tacks, though. What makes a solid solid? The answer is wrapped up in the idea of a shear force. Pick up a solid object, put one hand on top and the other underneath, and then move your hands in opposite directions, parallel to the floor. If you push the top of your object to the right and the bottom to the left, the object doesn’t let you change its shape: it doesn’t flow. If it does, you didn’t pick up a solid object.
Now flow is quite difficult to pin down. Do we count bending? If we do, steel isn’t a solid any more: bending without breaking is the reason we use steel as much as we do. To separate out exactly what we mean, let’s place a condition on it. A liquid is a material which takes on the shape of its container (that actually defines a fluid, but it’ll do). Not all liquids do this right away: treacle will happily pile up while you pour it, but eventually it flattens on top and ends up the same shape as the jar it’s in. How slow can something flow and still be a liquid, then?
Fortunately, the world’s longest running experiment has the goal of demonstrating a slowly moving liquid. In 1927, at the University of Queensland, Professor Thomas Parnell set up the pitch drop experiment. There’s lots of information and a live feed on that site, but the gist is that a funnel was filled with (apparently solid) bitumen and left in a case at room temperature for what has so far been eighty three years. In that time, eight drops of pitch have fallen from the funnel, none of them observed. Since the pitch is flowing, it must therefore be a liquid.
And as for the physical state of glass? Follow that link, and look at what the funnel is made of.
This is from the