States of Matter: Solid and Its Common Classes


By Robert Hazen, Ph.D.George Mason University

Solid is one of the states of matter, and it’s basically an everyday manifestation of the submicroscopic organization of atoms and molecules. Larger molecules are more likely to be condensed into solids or perhaps liquids. Well, the behavior of our planet, indeed the presence of life itself, depends critically on the coexistence of various states of matter: solid, liquid, and gas.

An image of three-dimensional solid figures
Solids have a more or less fixed shape. (Image: Besjunior/Shutterstock)

Properties of Solids

The most immediately obvious property of a material is its state. Now, the state is sort of a vague term. It relates primarily to a substance’s volume and its shape. Solids have a more or less fixed shape as a consequence of a three-dimensional arrangement of relatively strong and rigid directional bonds. Solids include all the hard objects like rocks, wood. They also include a variety of flexible materials, things such as aluminum foil and cloth. 

Plastic wraps is another example of a solid that can easily change its shape. Now, you can think of the structure of a solid something like a ball and a stick model, like Tinker Toys, in which the sticks may be somewhat flexible, but the arrangement of the balls relative to each other is fixed because they’ve been linked together in some sort of permanent long-term way.

There are three different classes of common solids in everyday use, and these are crystals, glass, and polymers. And these three different types depending on the regularity of the bonding of those atoms.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.


In crystals, the atoms occur in a regularly repeating pattern—the same atoms over and over again. And to think about a crystal, you could imagine a box, maybe something like a shoebox, which can stack top to bottom, side to side, front to back, as you might find in the storeroom of a shoe store. Now imagine that in each of these boxes, all exactly the same size and shape, are the exact same arrangement of a few atoms. And that’s sort of what a crystal is like. 

An image of salt crystals on a black background.
Atoms in crystals are repeated to form cubes. (Image: posteriori/Shutterstock)

For example, in table salt—the common mineral halite—you have an arrangement that’s a cube, the tiny box is a cube. And the atoms are one sodium for every chlorine. So each box contains a few sodiums and a few chlorines, and those cubes repeat over and over and over again, and they form beautiful cubic crystals.

By the same token, we have the beautiful mineral fluorite. Often it comes in purple colors or otherwise colored, and here the box again is a cube. In each box, you have one calcium and two fluorines to form CaF2. And, again, the crystal structures are very commonly perfect cubes that reflect that internal structure.

Learn more about phase transformations and chemical reactions.


The second kind of solid is glass, and glass is a general term for a solid that doesn’t have a regular crystal structure. In most glasses, you have an immediate local environment that’s quite predictable. For example, most glasses have silicon with those atoms surrounded by four oxygens. And virtually, any time you find a silicon atom in a glass, you’re going to find four oxygens in the immediate vicinity. 

But, if you go a couple of atoms away from there, it’s very difficult to predict what you’re going to find because of the irregularity of the glass structure. Most common glasses are predominantly silicon and oxygen, and most glasses also have a small number of other elements. And those other elements can drastically change the property of the glass. 

Some elements make the glass brightly colored. Other elements—for example, lead—make the glass bend light in a more dramatic fashion. And so, most cut glass or crystal chandeliers have leaded glass to give you that very beautiful display of colors in a prismatic effect. 


An image of colorful plastic bottles
Plastics can be flexible. (Image: DidiPho/Shutterstock)

The third kind of solid is plastics, and they’re made from polymers. Plastics are formed from intertwined chains of carbon-based atoms cross-linked by hydrogen bonds. Now, as a result, plastics have a very predictable structure in only one direction, along that polymer chain. If you have one small molecule linked to another, and another, and another, you can pretty much predict the structure along the chain. But side to side, you can’t predict how the fibers are going to be interwoven. So it’s kind of a one-dimensional structure.

Plastics can be quite flexible, and indeed they can be all different sorts of materials. You’ve seen plastic shopping bags that are incredibly strong and thin. Plastics make virtually all the materials that go into modern sneakers, from the rubber-type soles to the fibers that make up the cloth of the sides and the laces. And we have a wonderful variety of plastics that go into the various tapes and the other kinds of adhesive materials that we use in our everyday life.

Learn more about polymers, the chemical building blocks of plastics.

When All Classes of Solid Are Combined

Another important class of solid materials, which combines glasses and plastics, and crystals, are called composite materials. Now, composite materials constitute a growing class of solids that combine two or more different materials together to give enhanced properties.  

One of the most common type is plywood. The plywood is made of very thin layers of wood that have been glued together, and the trick here is that they’ve been glued together, so the grains run at right angles. 

If you take a piece of poplar, just a pure piece of wood, place it between two bricks, and just give it a slight tap, it splits quite easily. But the plywood, the exact same size and shape, even though it’s made of softer woods, you can hit it as hard as you want to; nothing happens. That’s because plywood is a strong, composite material, a material that enhances the property of wood simply by placing the pieces of wood with crossed grains.

Common Questions about Solid and Its Common Classes

Q: What are the properties of solids?

As a state of matter, solids have a certain shape and volume. This is due to the three-dimensional arrangement of intermolecular bonds.

Q: What are the different classes of solids?

Solids, a state of matter, have several different classes. Crystals, glass, and plastics are three different groups of solids. The combination of these three groups leads to the production of another solid called composite.

Q: What is a crystal?

Crystal is one of the classes of solids. Table salt and fluorite are examples of crystals formed in the shape of small cubes. The atoms in the crystal have a definite pattern and are repeated over and over again.

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