The ISC Blog

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 put it in your walls to let the sun shine in, you put it in front of your eyes to help you see, and you put 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.  It is actually a liquid.  Well…more specifically, it’s a super-cooled liquid (but more on that later).

How can glass be a liquid?  Glass sure looks and feels like as solid.  But in fact, glass has enough of the properties that define liquids to categorize it as one.  To put it plainly, at the molecular level, 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?

Properties of the three states of matter
Properties of solids, liquids, and gasses. From the Fuse School

To explain the differences between 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 is ice.  In its liquid state, it fills our oceans and lakes as water, and in its gaseous state, it floats in the air as water vapor.  The difference between ice, water, and water vapor is 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 numbered balls in those lottery machines: 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, because of electrical forces holding them together, the molecules can’t move relative to each other.  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 very easily.

Quartz and glass
The crystalline structure of quartz versus the random structure of glass. From Feral Big Ten.

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.

Your standard glass is also mostly made of SiO2, yet it is classified as a liquid because of how the SiO2 molecules are arranged.  Unlike in quartz, the SiO2 in glass is arranged in a random, amorphous cluster (for example, look at the picture above to see how the SiO2 molecules in glass are arranged, compared to how they are arranged in solid quartz.)

Solids typically have an orderly structure, which is why glass is not considered a solid.  Glass is technically a super-cooled liquid, 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), and also, the SiO2 molecules in it cannot arrange themselves into a strong structure to make a true solid.  How is that possible, you ask?

It all has to do with how glass is made.  (I am just going to talk about how sheet glass is made.  Blown glass is made a bit differently.)

Making glass involves three components: a former, a flux, and a stabilizer.  The “former” is the base material for the glass.  In normal sheet glass, SiO2 is used 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 break down the solid SiO2 in the sand.  Remember, in solids like sand, the molecules are arranged in a very rigid structure.  To break down the solid SiO2, the glass-maker heats up the sand. Heat always destabilizes and eventually breaks connections between molecules (this is why ice turns into water at room temperature and why water turns into water vapor if you put a flame under it).  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!)

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 and 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 SiO2 and prevent it from making solid, orderly connections with its SiO2 neighbors.  Think of magnesium as the dog that always gets in the way when you’re trying to shake someone’s hand.



The periodic table of elements
Bulky atoms such as sodium, magnesium, and calcium (in the blue boxes) are used to keep SiO2, which is made up of the relatively svelte silicon and oxygen atoms (in the red boxes), from forming stable connections. You can imagine how these molecules get in the way by comparing their sizes in the period table above.

So, in the end, you have a rigid piece of glass.  However, at the same time, since the SiO2 was never able to to form a crystal, this glass is technically considered a liquid.

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

Just a few months ago, 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.

Researchers at the University of Chicago created this new type of glass, which has an orderly structure.
Researchers at the University of Chicago       created this new type of glass, which has an orderly structure.

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 a liquid doesn’t mean it flows like other 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, its is the result of the manufacturing process, not the result of flow.

Ben Marcus is a recent graduate of the University of Chicago, where he received his Ph.D. in neurobiology. He is a co-editor of the ISC blog and a moderator of Ask A Scientist. You can follow Ben on Twitter @bmarcus128.


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

Chemistry of Glass

All About Glass | Corning Museum of Glass

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