Topic 3.2: Properties of Solids

Properties of solids: the waterfall made of rock

This looks like a waterfall, and it pours like a waterfall, but what’s falling here is rock, melted rock hotter than a thousand degrees Celsius, pouring off the edge of a cliff. This is the south coast of Kīlauea, in Hawaiʻi, in January of 2017, where a tunnel of lava had broken open in the face of a sea cliff. For weeks, a stream of molten rock a meter or two wide poured out and fell about twenty meters, straight into the Pacific.

And look at what happens the moment it hits the water. The sea boils, bits of lava go flying, and the glowing rock goes dark in seconds and shatters into black sand. If you picked up a grain of that sand, you’d find out it’s glass, real volcanic glass, made by the ocean in a few seconds. And the seawater the lava boils away rises up as this huge white cloud, the thing everybody calls steam.

So in one shot liquid rock turns into solid glass, and that glass shatters instead of bending, and the white in that cloud isn’t a gas at all, so where’s the gas? Some solids shatter like that, while a copper wire would just bend, so what makes the difference? It comes down to what a solid’s particles are, and what’s holding them together.

Those two questions work for almost any solid, from a grain of salt to a copper wire. Answer them, and you can predict whether it shatters or bends, and whether it carries a current. Then we’ll come back to this coast. We’ll zoom into every part of this picture and draw what its particles are doing. That includes the gas, which is hiding right where you can’t see it.

Here’s a guess to hold on to. Salt is packed with electrically charged particles, so can a solid crystal of salt carry an electric current? So let’s ask those two questions about a solid you probably have in your bag right now, the gray stuff in a pencil.

Same atoms, different solid

Pencil lead is mostly graphite, which is pure carbon, and two physicists in Manchester, England, peeled it apart with ordinary sticky tape, again and again, until they had sheets of carbon just one atom thick, called graphene, and their experiments with it won them a Nobel Prize. Here’s what they were peeling. Graphite is made of flat sheets of carbon atoms locked into hexagons, stacked up like the pages of an enormous book.

Inside one sheet, every carbon atom is covalently bonded to three neighbors, and the sheet goes on like that in every direction. But between one sheet and the next there are no bonds at all, only weak London dispersion forces. So there are two very different ways to split graphite. I’d like you to try both and see which one gives way.

Grab the two handles and try to pull the top sheet apart. Then try the tab on its edge. So graphite is soft and slippery because whole sheets slide over each other, and that’s how a pencil writes, smearing sheets of carbon onto the paper.

Almost everybody slips here. They say graphite is soft, so its bonds must be weak. That answer costs points on the AP exam. The bonds inside a graphite sheet are actually shorter and stronger than the bonds in diamond, so graphite is soft only because of the gaps between its sheets.

Diamond is the same carbon arranged differently, with every atom bonded to four neighbors that reach out in all three dimensions. Now it’s your turn. Try to slide a layer of diamond the way you slid graphite.

Push it as hard as you like. Then watch what the bonds do. There’s nowhere for it to go, because every layer is stitched to the next by covalent bonds at fixed angles, so the network is rigid, and that’s why diamond is so hard.

Chemists call both of these covalent network solids, because covalent bonds run all the way through. Quartz and silicon carbide are network solids too, and network solids form only from nonmetals and metalloids. And since melting one means breaking covalent bonds, they all stay solid to very high temperatures.

And that black glass at Kamokuna is built on a web of strong silicon–oxygen bonds, like quartz, with nothing free to slide, so it cracks instead of bending. Graphite next to diamond is a classic AP question, and the answer always comes down to how the atoms are connected, since the atoms themselves are identical.

Salt shatters, and carries a current only when it melts

Now let’s take our questions to a forge. There we can hit a solid, melt it and run a current through it. Here’s a crystal of rock salt, sodium chloride, and with one good hit it doesn’t bend at all, it shatters into smaller blocks with flat faces.

Inside are positive sodium ions and negative chloride ions, each surrounded by six of the opposite charge, with no separate salt molecules. The hammer shoves one layer over by a single ion, so positive sits on positive and negative on negative, they repel, and the crystal splits along that plane.

That’s what brittle means for an ionic solid, because sliding a layer lines up like charges, and the repulsion breaks it apart. Every ion strongly attracts all of its neighbors, so ionic solids melt and boil at high temperatures and barely evaporate at all, and salt melts at eight hundred and one degrees, still cooler than that lava.

So let’s test your guess from the coast. Touch the probes to solid salt and the bulb stays dark, even though it’s packed with charged particles, because each one is locked in place, so it can shake but can’t travel. So let’s melt some salt. We’ll put it in a crucible, and the crucible goes right in the coals.

Solid salt left the bulb dark, so will molten salt light it? Make your guess, then move the probes into the crucible yourself. Salt dissolved in water works too, because the ions are floating free, each one wrapped in water molecules.

So in the solid the ions are locked in a grid, in the melt the same ions slide past each other, and in solution they’re spread out among water molecules. People often say that melting frees up electrons, but the electrons stay put, and the ions themselves do the moving.

Metal bends, and conducts as a solid

Now give a copper bar the same hit, and it just flattens. A smith could hammer it into a sheet, or draw it out into a wire. In a metal the particles are positive metal ions, and their outer electrons wander through the whole piece, a sea of electrons that the ions sit in.

When the hammer pushes rows of ions past each other, every ion is still surrounded by the same sea of electrons, so the metal just rearranges and holds together. That sea is also why metals conduct, because the electrons move right through the solid, so the bulb lights, even when the wire bends. Those same electrons carry heat, so heat races along a copper rod.

One picture explains all of it: metal ions, each with a plus charge, sitting in a sea of electrons that can wander anywhere. Free electrons carry charge and heat. The ions slide into new places, and nothing breaks. Metals are malleable and ductile. You can hammer them flat or draw them into wire.

Pure iron is fairly soft too, but carbon atoms are small enough to squeeze into the gaps between iron ions, and iron with carbon in its gaps is steel. Make the iron harder to bend by dropping carbon atoms into the gaps, where they’ll stop the rows from sliding. Chemists call that an interstitial alloy, because the extra atoms sit in the gaps, making it more rigid, less malleable and less ductile, but it still conducts.

Molecules: strong inside, weak between

The last kind of solid is the one you’ve held most often, because ice, sugar and candle wax are molecular solids, made of separate molecules. Inside each molecule there are strong covalent bonds, but between molecules there are only weak intermolecular forces, so molecular solids tend to melt at low temperatures, like ice at zero degrees.

It’s tempting to think melting ice breaks the water molecules apart, but every H2O stays an H2O, and only the attractions between the molecules change. And molecular solids don’t conduct, because every valence electron is tied up in a bond or a lone pair inside its own molecule.

Intermolecular forces also decide how easily a liquid boils, so let’s compare two nonpolar near-twins, pentane, which boils at thirty-six degrees, and hexane, which boils at sixty-nine. Seal each in a flask at room temperature, and some molecules escape into the space above and push on the walls, which is the vapor pressure, and pentane’s is more than three times hexane’s.

Hexane is the bigger molecule. Its electron cloud is bigger and more polarizable, so its dispersion forces are stronger. Hexane boils higher, and fewer of its molecules escape into the flask. Boiling and evaporating both pull the molecules all the way apart. Stronger forces make that harder.

Stronger forces mean a higher boiling point and a lower vapor pressure. The normal boiling point is just the boiling point at one atmosphere, ordinary air pressure. Your turn. All four liquids are at twenty-five degrees. Which one has the highest normal boiling point?

Pentane has a twin called neopentane. Same formula, C five H twelve, but its carbons bunch into a ball. Two pentane chains can lie side by side and touch all along. Two neopentane balls touch at just one spot. Less contact means weaker dispersion forces. Neopentane boils lower, at nine and a half degrees.

Pentane boils higher, so its forces are stronger. Which one do you think melts at the higher temperature? Boiling points follow the forces between molecules. Melting points usually do too, but how well the molecules pack can flip the order.

Giant molecules, and how the AP exam asks it

Some molecular solids are made of enormous molecules, like the polyethylene in a milk jug, which is long carbon chains held to each other by dispersion forces. Along each chain the bonds are covalent, but from chain to chain the attractions are noncovalent, like the hydrogen bonds that link nylon’s chains. Those weak attractions can also hold one single giant molecule in its shape, like this one, green fluorescent protein, from a jellyfish that glows.

It’s one chain of two hundred thirty-eight amino acids, folded into the shape of a can, and hydrogen bonds between neighboring strands of that same chain hold the can together. The glowing part sits inside, and it only glows while the can is folded around it, so its job depends on its shape, and noncovalent attractions hold that shape. When GFP unfolds, its glow fades, but the chain itself stays in one piece. So what let go?

Here’s how the AP exam asks about all this. In 2021 it said that solid calcium sulfate doesn’t conduct electricity, and asked why. So let’s run our questions. The particles are calcium ions and sulfate ions, held in a lattice by strong attractions, and to conduct, charged particles have to be able to move. In solid calcium sulfate the ions are held in fixed positions in the lattice, so they can’t move to carry charge, and an answer about free electrons misses the ions and earns nothing.

So here’s everything we found, all in one table. Every box comes from our two questions, about the particles and what holds them together. Network solids melt highest, and most are hard. Graphite is the exception: it conducts. Ionic solids shatter.

They conduct only when their ions can move. Metallic bonding, the sea of electrons, holds metals together. They bend, and they conduct even as solids. Molecular solids melt low and never conduct. Every solid in this table keeps its own shape, but lava flows, so now let’s ask how particles are arranged and how they move, which is what makes a solid, a liquid or a gas.