Polymers: recognising one, and why polymers are solids

Why is a carrier bag a solid, when the gas burning on a hob is not? The plastic in the bag and the methane in the gas are both made of carbon and hydrogen only. Every atom in both is held by the same kind of bond, a covalent bond. Yet the bag carries six tins of beans, and the gas stays a gas even in a freezer. Stronger bonds in the bag is the tempting answer, and it is not the reason. Park the bag and the hob for now; the picture starts with chains.

A polymer is a substance made of very large molecules. Each molecule is a long chain of carbon atoms, joined to each other by strong covalent bonds. The plastic in a carrier bag is poly ethene. Each of its chains is thousands of carbon atoms long, with hydrogen atoms attached all the way along. A diagram never shows a whole chain, because it would not fit. You see a short section inside brackets with a small n outside, or a chain trailing off at both ends. Both mean the chain goes on and on. Spotting a polymer from a diagram turns up on Foundation and Higher papers alike, so it is worth a quick test. Two diagrams sit side by side. One is a single molecule of five atoms. The other is a line of carbon atoms that runs off both edges of its box, with a bracket and a small n. Which diagram shows the polymer, and which two features gave it away? The second one. The bracket with the n says the pattern repeats, and the chain running off both edges says it keeps going. The five-atom molecule is a small molecule.

Now the bag and the hob gas, side by side. Methane is a small molecule, one carbon atom with four hydrogen atoms. A poly ethene molecule is a chain thousands of atoms long. Between molecules, both have intermolecular forces, the same kind of attraction. The difference is how much of it there is. A long chain lies alongside its neighbours for its whole length, a bit like two pieces of string tangled together all the way along. There is a lot of contact, so the forces between chains add up. So the intermolecular forces between polymer chains are relatively strong. Relatively means compared with small molecules: stronger than the forces between methane molecules, and still much weaker than a covalent bond. It takes a lot of energy to pull the chains apart, so a polymer is a solid at room temperature. When it does melt, the chains slide apart, and not one covalent bond inside a chain is broken. Now put that into one answer, written as a comparison rather than two separate stories. Pause and explain, as one comparison, why poly ethene is a solid but methane is a gas. Poly ethene is made of long chains, so the intermolecular forces between its molecules are much stronger than those between tiny methane molecules. More energy is needed to separate the chains, so poly ethene is a solid and methane a gas. One examiner's report says this about poly ethene: "Poly(ethene) contains covalent bonds was common but long chain molecules very rare. Few knew the intermolecular forces were between chains of poly(ethene)." The covalent bonds are true but they do not explain the solid. The fix is to make long the first word you write: long chains, so strong forces between the chains.

There is one picture a polymer must never be confused with: a giant covalent structure. In a giant covalent structure, covalent bonds run right through the whole thing, and there are no separate molecules at all. A polymer is different. It is a tangle of separate chains. Each chain is one molecule, and what links one chain to the next is intermolecular forces, not bonds. That gives a ladder of three steps. Small molecules sit at the bottom, polymers in the middle, and giant covalent structures at the top, with the video on diamond and silicon dioxide covering the top step. Separate chains also means a carrier bag is not one giant molecule. It holds an enormous number of very large molecules, tangled together. A student writes: poly ethene is a solid because it is a giant structure, so the covalent bonds have to be broken to melt it. It sounds confident, and the picture underneath it is wrong. What is wrong with the student's picture of poly ethene, and what is separated instead? Poly ethene is not a giant structure; it is separate long molecules. Melting separates the chains by overcoming the forces between them, and no covalent bond is broken. An examiner's report on a question about methane and poly ethene puts it this way: "However, many students did not mention molecules at all, seeming to believe that the polymer is a giant structure." It goes on: "Students often thought intermolecular forces had to be broken in methane but bonds had to be broken in poly(ethene)." The fix is the same rule for both substances. Each is made of molecules, so in each, the forces between molecules are overcome. Only the size of the molecules differs. Now two polymer chains of the same kind, one drawn above the other. The lower chain is twice as long as the upper one. Answer using the word chains, and keep the word molecules out of it. Which of the two needs more energy to melt, and why? The one with the longer chains. Longer chains lie alongside their neighbours for more of their length, so the forces between them are stronger, and more energy is needed to separate them.

The recap runs in your head first now: each question waits for your answer before mine. What on a diagram tells you it shows a polymer? A bracket with a small n outside it, or a chain of carbon atoms trailing off at both ends. Now an unseen polymer: poly propene has chains thousands of atoms long. Solid, liquid or gas at twenty degrees C, and why? A solid. Its long chains give relatively strong forces between molecules, which need a lot of energy to overcome. A different question: is a polymer a giant covalent structure? No. It is separate long molecules with intermolecular forces between them, and those forces are what melting overcomes. One more: why is the carrier bag a solid when the hob gas is not? The bag is made of long chains, so the forces between its molecules add up to relatively strong. Methane molecules are tiny, so the forces between them are weak.

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