Glass: Solid or Liquid?

Glass is a solid, right?  Obviously.  You touch it, your finger doesn’t go through.  You stand it upright, it doesn’t collapse.  You whack a baseball through it, and it shatters into a million pieces. 

You live your whole life based on knowing glass is a solid.  You install it in your walls to let the sun shine in, you place it in front of your eyes to help you see, and you keep it in your pocket (with some electronics behind it) so that you can stay on the grid.

But wait!  There’s more to glass than what you may think…

Glass isn’t a solid.  But it’s not a liquid either.  It’s something in the middle – an amorphous solid more on that later).

Why don’t we just call glass a plain old “solid?” Glass sure looks and feels like it’s in a solid state of matter.  But in fact, at the molecular level, glass looks a little bit like a liquid. Not enough to act like one, but not enough make iti stick out from the solid category. To put it plainly, glass is stuck in limbo between a solid state and a liquid state.

Come to think of it, how do we define solids, liquids, and gasses at the molecular level?

To explain the differences amng the three states of matter, let’s take as an example a chemical that we often see in all three states of matter in our everyday lives: dihydrogen monoxide (H₂O).  In its solid state, dihydrogen monoxide floats in our drinks as ice.  In its liquid state, it fills our oceans and lakes as water, and in its gaseous state, it fills the air as water vapor.

The difference between ice, water, and water vapor has nothing to do with the chemical composition of H₂O. I mean, H₂O is H₂O. Rather, it has everything to do with how densely packed the H₂O molecules in the substance are, which directly affects how orderly they arrange themselves.

In water vapor, H₂O molecules float around like the balls in a lottery machine: they fly through the air and occasionally bump into each other as they pass, but overall, they don’t make and permanent connections.  On the other side of the spectrum, in ice, the molecules are very densely packed.  They are arranged in a very orderly matrix called a crystal where electrical forces hold them together, preventing them from moving.  In the middle, we have water, where the molecules spend most of their time in contact with each other, but they can move and rearrange — or flow — very easily.

Quartz is another good example of a crystalline solid. Quartz is made of pure silicon dioxide (SiO₂), and in pieces of quartz, the SiO₂ is arranged in a stable, orderly crystal, like ice.

Your standard glass is also mostly made of SiO₂. But it is not quartz because of how the SiO₂ molecules are arranged.  Unlike in quartz, the SiO₂ in glass is arranged in a random, amorphous cluster (see below).

This random structure makes it hard to classify glass as a solid. Rather, glass is technically an “amorphous” solid, which means it is not structured like a solid but acts like a solid anyway.  It doesn’t have a melting point (meaning the SiO₂ cannot break apart to form a true liquid). On the other hand, the SiO₂ molecules in it cannot arrange themselves into a strong structure to make a true solid.  How is it possible to be stuck in such a confusing state of matter, you ask?

It all has to do with how glass is made.

Making sheets of glass involves three components: a former, a flux, and a stabilizer.  The “former” is the base material for the glass.  In normal sheet glass, manufacturers use SiO₂ as the former. Sand is a great source of SiO₂, so manufacturers start the process of making glass with sand.

To make glass, the glass-maker has to take the sand and break down the solid SiO₂ within it.  But sand is a normal solid, so the molecules are arranged in a very rigid structure, making them hard to break apart.  To facilitate the process, the glass-maker has to heat up the sand.

Heat destabilizes and eventually breaks connections between molecules, hence why sticking in a warm glass of water causes it to melt.  However, melting sand is not that easy.  Silica, which is where the SiO₂ comes from, has a melting point of about 2500 degrees Celsius (5600 degrees Fahrenheit), not a temperature you see often on Earth.

This is where the second component of glass comes in – the “flux.”  The purpose of the flux is to reduce the melting point of the silica so that the glass-maker doesn’t have to turn the temperature up to such a high, energy-intensive, dangerous level.  The flux can reduce the melting point by as much as 1700 degrees Celsius, which makes the whole glass-making process easier, safer, and cheaper.

After the former is heated up, the glass-maker cools it very quickly.  He or she does this so that the SiO₂ starts solidifying but doesn’t have time to arrange itself neatly in structured formation.  To further prevent the SiO₂ from stabilizing during the cooling process, glass-makers introduce the third component – the aptly named “stabilizer.”  Stabilizers are relatively large atoms (like calcium and magnesium) that are put in the mixture to hinder the formation of crystals.  In other words, these big atoms get in the way of the silicon and oxygen and prevent it from making solid, orderly connections.

So, in the end, you have a rigid piece of glass.  At the same time, however, since the SiO₂ was never able to form a crystal, this glass is technically considered “amorphous” in structure.

Glass still confuses scientists for the same reason it confuses the rest of us: it looks like a half-solid/half-liquid because the SiO₂ molecules are arranged in a random order, yet they don’t flow.  This begs the question: is it possible to make a kind of glass that is a complete, unquestionable crystalline solid?

Funny you ask. In 2015, scientists at the University of Chicago created an entirely new type of glass whose molecules actually have an orderly arrangement.  Aha!  Finally, solid glass!

How did this happen?  By accident, of course.

Professor Juan de Pablo was simply trying to test the properties of light using glass that he created, but when he analyzed his data, he found strange results where there shouldn’t have been.  At first, he thought his calculations were wrong.  But then he realized that his unexpected results were caused by the unusual properties of the glass he was using. He had accidentally created a type of glass that is unlike what anyone has ever seen: it was as solid as quartz.

de Pablo created this glass by vaporizing organic matter and cooling it in such a way that it would solidify on a surface one layer at a time.  By using this process, de Pablo accidentally exploited an unusual characteristic of liquids.

One odd thing about liquids is that the molecules on the surface , which touch the air, tend to organize themselves and form a more stable layer (this gives liquids “surface tension.”)  Because de Pablo formed his glass by building one layer at a time, each layer made contact with air and was able to organize before the next layer was applied.  By the time he was done, the entire piece of glass consisted of an organized array of molecules.

Only time will tell how we can use this new glass.  de Pablo thinks we might be able to use it in solar cells, LEDs, and optical fibers.  And just think, this all started because of an accidental discovery!

Fun fact: Just because glass is technically not a true sold doesn’t mean it flows like liquids.  In fact, scientists have gone to great lengths to prove that glass doesn’t flow.  Some scientists even looked at 20 million year old amber to see it flowed at all over its lifetime – and they found out that it didn’t.  If you see rippling in old windows, it is the result of old manufacturing practices, not the result of flow.

Sources:

Glass – Chemistry Encyclopedia – structure, reaction, water, uses, elements, metal, property

Chemistry of Glass

All About Glass | Corning Museum of Glass