Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want macromolecules — giant covalent structures and polymers — to become a reliable source of marks instead of a structure-property guessing exercise.
What query it owns: how to understand and revise macromolecules in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the macromolecules revision-guide angle, while Tutopiya’s Macromolecules subtopic page owns the learning resource and the free Macromolecules quiz owns the practice.

Not all covalent substances are small molecules. Macromolecules and giant covalent structures contain huge networks of atoms joined by strong covalent bonds. Cambridge IGCSE Coordinated Science (0654) expects you to compare diamond, graphite and silicon dioxide, explain how structure determines properties, and describe how polymers form from monomers. This guide links each structure to what examiners reward.

Key takeaways

• Giant covalent structures (macromolecules) have atoms joined by strong covalent bonds throughout the lattice.
• Diamond — each carbon bonded to 4 others; hard, high melting point, does not conduct (no free electrons).
• Graphite — each carbon bonded to 3 others in layers; soft, conducts electricity (delocalised electrons between layers).
• Silicon dioxide (SiO₂) — giant covalent lattice; hard, high melting point.
• Polymers — long chains of repeating monomer units joined by covalent bonds; addition polymerisation joins alkene monomers.

What are macromolecules in Cambridge IGCSE Coordinated Science?

Macromolecules are very large molecules or giant covalent networks. In diamond and graphite, every carbon atom is covalently bonded in a continuous 3D or layered structure — breaking the substance means breaking covalent bonds, which requires a lot of energy. Polymers are macromolecules made by joining many small monomer units into long chains. Structure directly determines properties.

You can read the full explanation, structure diagrams and notes on Tutopiya’s Macromolecules subtopic page before you attempt questions.

Diamond vs graphite — same element, different structures

Property	Diamond	Graphite
Bonding	Each C bonded to 4 others (tetrahedral)	Each C bonded to 3 others (hexagonal layers)
Hardness	Very hard	Soft (layers slide over each other)
Melting point	Very high	Very high
Conducts electricity?	No (no free electrons)	Yes (delocalised electrons between layers)
Use	Cutting tools, jewellery	Lubricant, pencil lead, electrodes

Giant covalent structures vs simple molecules

Feature	Giant covalent (e.g. SiO₂)	Simple molecular (e.g. CO₂)
Structure	Continuous network of covalent bonds	Separate small molecules
Melting point	Very high	Low
Bonds broken on melting	Strong covalent bonds	Weak intermolecular forces
Conductivity	Usually none (except graphite)	None

Macromolecules in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical macromolecule stem
Explain why	Link structure to property	”Explain why diamond is hard.”
Compare	Differences between structures	”Compare diamond and graphite.”
State	Give a fact	”State why graphite conducts electricity.”
Describe	Outline polymer formation	”Describe how ethene forms poly(ethene).”

Worked exam-style stems (how to answer the wording)

1. “Explain why diamond has a very high melting point.” Diamond has a giant covalent structure with strong covalent bonds throughout; a large amount of energy is needed to break these bonds. Mark-scheme reward: giant covalent structure and strong bonds.
2. “Explain why graphite conducts electricity but diamond does not.” In graphite, each carbon has one delocalised electron that is free to move between layers and carry charge. In diamond, all four outer electrons are used in bonds — no free electrons. Reward: delocalised electrons in graphite; none in diamond.
3. “Describe how poly(ethene) is formed from ethene.” Many ethene monomer molecules undergo addition polymerisation; the C=C double bond opens and monomers link to form a long chain with single C–C bonds. Reward: addition polymerisation and double bond opening.

Test yourself with the Macromolecules quiz once you can compare structures and explain properties.

How macromolecules connect to the rest of Coordinated Science

Macromolecules extend Molecules And Covalent Bonds to giant structures and link to Polymers and Biological Molecules. The Cambridge IGCSE Coordinated Science resource hub links every subtopic.

Common mistakes students make

• Saying diamond and graphite have different elements (both are carbon — different structure).
• Attributing graphite’s softness to weak covalent bonds (layers slide; bonds within layers are strong).
• Confusing giant covalent structures with simple molecular ones.
• Forgetting graphite conducts because of delocalised electrons, not ions.
• Stating polymers are formed by condensation when the question specifies addition polymerisation of alkenes.

When you need more support

If macromolecule questions keep costing marks, work through the Macromolecules quiz, then get focused help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

Is macromolecules hard in Coordinated Science? The structures are limited — marks are lost when students cannot link bonding arrangement to properties like hardness and conductivity.

Why do diamond and graphite have different properties? Both are carbon, but diamond has a 3D network (each C bonded to 4 others) while graphite has layered hexagonal rings (each C bonded to 3 others) with delocalised electrons.

What is the difference between a monomer and a polymer? A monomer is a small repeating unit; a polymer is a long chain made of many monomers joined together.

How do I revise macromolecules effectively? Draw diamond and graphite structures, compare properties in a table, describe poly(ethene) formation, then take the Macromolecules quiz.

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