🌱 When a polymer melts
The intermolecular forces between the long molecules are overcome. These forces are relatively strong, so polymers are solid at room temperature. The covalent bonds stay intact.
Structure and Properties » Polymers and Giant Covalent Structures
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.2.2.5, 4.2.2.6
- Why polymers are solids at room temperature
- Why giant covalent substances have very high melting points
- How to recognise polymers and giant covalent structures from diagrams
- How to explain the difference between the two in an exam answer
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Unlock This CoursePolymers: very large molecules
Plastic bags are polymers - their huge molecules attract each other strongly enough to be solids at room temperature
In the last lesson you saw that small molecules have weak intermolecular forces, so they melt and boil at low temperatures. Remember, intermolecular forces get stronger as molecules get bigger. Now we take that idea to the extreme.
Polymers have very large molecules. One polymer molecule is a long chain that can contain thousands of atoms. Plastics are the polymers you meet most often, for example in drinks bottles, washing-up bowls and food containers.
Inside each polymer molecule, the atoms are linked to other atoms by strong covalent bonds. These bonds hold the chain together.
Between the separate polymer molecules there are intermolecular forces. Because the molecules are so large, these forces are relatively strong. That means much more energy is needed to pull the molecules apart than for a small molecule like water. So polymers are solids at room temperature.
Key terms:
- Polymer: a substance made of very large molecules, in which the atoms are joined by strong covalent bonds, often in long chains.
Recognising a polymer from a diagram
The exam may show you a diagram and ask what type of structure it is. Look for these clues:
- A section of a chain drawn inside brackets, with a small n after the bracket. The n stands for a very large number, so the part in the brackets is repeated many times.
- Bond lines that pass through the brackets. This shows the chain carries on in both directions.
- Sometimes a picture of many long, tangled lines. Each line is one polymer molecule.
You learned how to find a repeating unit in the lesson on Covalent Bonding. Here the key point is simpler: if the diagram shows a very long chain that keeps going, it is a polymer. How polymers are made is covered later, in Addition Polymerisation.
Common mistakes
"When a polymer melts, its covalent bonds break." No. When a polymer melts, the intermolecular forces between the chains are overcome. The chains stay whole. The covalent bonds inside each chain are not broken.
"Polymers have weak intermolecular forces, like small molecules." No. The spec says the forces between polymer molecules are relatively strong, because the molecules are so large.
Giant covalent structures
Some substances do not form separate molecules at all. Instead, all of the atoms are linked to other atoms by strong covalent bonds, in a network that carries on and on. This is a giant covalent structure.
Key terms:
- Giant covalent structure: a structure in which a huge number of atoms are all joined to other atoms by strong covalent bonds, with no separate molecules.
The spec gives three examples you must know:
💎 Diamond
A form of carbon. Every atom is carbon.
✏ Graphite
Another form of carbon. Every atom is carbon, but they are arranged differently from diamond.
⛰ Silicon dioxide
Also called silica. It is made of silicon and oxygen atoms. Sand is mainly silicon dioxide.
You will look closely at how diamond and graphite are built, and why they behave so differently, in the lesson on Diamond and Graphite.
Why giant covalent substances have very high melting points
Sand is mostly silicon dioxide, a giant covalent structure - you'd need about 1,700 °C to melt it
Substances with giant covalent structures are solids with very high melting points. Here is the explanation, step by step:
- All of the atoms are linked by strong covalent bonds.
- To melt or boil the substance, these strong covalent bonds must be overcome. It is not just forces between molecules being overcome.
- There are a huge number of these strong bonds.
- So a large amount of energy is needed to break them, and the melting point is very high.
For example, silicon dioxide melts at about 1,700 °C. That is far higher than any substance made of small molecules.
🔥 When a giant covalent substance melts
The strong covalent bonds themselves are broken, and there are a huge number of them. This needs much more energy, so the melting point is very high.
Recognising a giant covalent structure from a diagram
Look for these clues:
- Atoms joined by lines (covalent bonds) in a network that carries on and on, either in 3D or in flat layers.
- The pattern reaches the edges of the picture, showing that it carries on. You cannot draw a ring around one separate molecule.
- No charges are shown. If you see + and - signs on the particles, it is ionic. If you see positive ions in a sea of electrons, it is metallic.
- It is often drawn as a 3D picture, so imagine the network going back into the page as well.
Worked example
A student is given four diagrams. Diagram P shows a few separate groups of three atoms. Diagram Q shows a long chain in brackets with an n. Diagram R shows atoms joined by lines in a 3D network reaching every edge, with no charges. Diagram S shows + and - particles in a regular pattern. Which is a giant covalent structure?
Step 1: Rule out charges. S has + and - particles, so it is ionic.
Step 2: Look for separate units. P has separate small molecules. Q is one very long molecule, so it is a polymer.
Step 3: Look for a network that carries on. R has bonds everywhere, no separate molecules and no charges.
Answer: R is a giant covalent structure.
Common mistakes
Writing "intermolecular forces" for diamond or silicon dioxide. Melting a giant covalent substance means breaking strong covalent bonds, not overcoming forces between molecules. Always say covalent bonds are broken. Examiners give no marks for "intermolecular forces" here.
Saying "covalent bonds are weak". Covalent bonds are strong. Small molecules have low melting points because of weak forces between molecules, not weak bonds.
Forgetting "many". Say there are many strong bonds, and that a lot of energy is needed. Both ideas earn marks.
Exam-style question
Silicon dioxide melts at about 1,700 °C. Explain why silicon dioxide has a very high melting point. [3 marks]
Model answer
Silicon dioxide has a giant covalent structure (1). All of its atoms are joined by many strong covalent bonds (1). A lot of energy is needed to overcome these bonds when it melts (1).
Exam tip
Before you explain any melting point, decide the structure first: small molecules, polymer, giant covalent, ionic or metallic. Then name what must be overcome. Small molecules and polymers: intermolecular forces. Giant covalent: covalent bonds.