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Detailed notes on Atoms, Elements and Compounds for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Macromolecules are very large molecules formed by polymerisation. Both natural (proteins, DNA, starch) and synthetic (polyethene, nylon) macromolecules consist of repeating units called monomers linked by covalent bonds.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Macromolecules form when monomers are repeatedly joined by covalent bonds. Two polymerisation types: addition (alkenes) and condensation (with byproduct).
Macromolecule: a very large molecule with a high relative molecular mass, composed of many repeating structural units (monomers) joined by covalent bonds.
Polymer: the product of polymerisation — a chain of monomers.
Addition polymerisation:
Condensation polymerisation:
Natural macromolecules:
| Macromolecule | Monomer | Bond formed |
|---|---|---|
| Starch/cellulose | Glucose | Glycosidic bond |
| Protein | Amino acids | Peptide bond |
| DNA | Nucleotides | Phosphodiester bond |
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'Name the type of polymerisation that produces polypropene' (1 mark — addition). 'Name the small molecule produced when nylon is formed by condensation polymerisation' (1 mark — water). Given a monomer structure, draw the repeat unit of the polymer (2 marks). Identify whether a polymer is addition or condensation from its monomer structures.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C3); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on macromolecules , written exactly the way a tutor would explain them at the board.
Question
Explain why silicon dioxide (SiO₂) has a very high melting point of approximately 1710°C.
Step-by-step solution
Step 1
Silicon dioxide has a giant covalent structure in which every silicon atom is covalently bonded to four oxygen atoms, and every oxygen atom is bonded to two silicon atoms throughout the structure.
Step 2
There are many thousands of strong covalent bonds throughout the three-dimensional lattice.
Step 3
An enormous amount of energy is needed to break all these strong covalent bonds; hence the melting point is very high.
Answer
Giant covalent structure with many strong Si–O covalent bonds throughout; enormous energy needed to break them → very high melting point.
Examiner tip
Use the phrase 'giant covalent structure' and specify that bonds must be broken (not just weakened) for melting.
Question
Explain, using metallic bonding, why metals are malleable.
Step-by-step solution
Step 1
In a metal, positive ions are arranged in regular layers surrounded by a sea of delocalised electrons.
Step 2
When a force is applied, layers of positive ions can slide over each other without breaking bonds.
Step 3
The delocalised electrons move with the layers and maintain the metallic bonding; the structure is not disrupted.
Step 4
Because layers can slide, the metal changes shape without cracking — it is malleable.
Answer
Layers of positive ions slide over each other; delocalised electrons move with the layers maintaining metallic bonding; shape changes without breaking — malleable.
Examiner tip
Say layers 'slide' not 'break'. The key point is that metallic bonding is maintained throughout the deformation.
Question
State the difference between thermosoftening and thermosetting polymers, and explain which can be recycled.
Step-by-step solution
Step 1
Thermosoftening polymers consist of long polymer chains with only weak intermolecular forces between them. When heated, the chains can move past each other and the polymer softens and melts.
Step 2
Thermosetting polymers have covalent cross-links between polymer chains. These cross-links do not break on heating; instead, the polymer chars or decomposes.
Step 3
Thermosoftening polymers can be recycled because they can be re-melted and remoulded. Thermosetting polymers cannot be recycled.
Answer
Thermosoftening: weak intermolecular forces, softens/melts on heating, recyclable. Thermosetting: covalent cross-links, does not soften, not recyclable.
Question
Explain why metals are good conductors of electricity.
Step-by-step solution
Step 1
In metallic bonding, the outer electrons of metal atoms become delocalised — they are not associated with any single metal ion but are free to move throughout the metallic lattice.
Step 2
When a potential difference (voltage) is applied across the metal, these delocalised electrons flow in one direction through the lattice.
Step 3
The movement of electrons constitutes an electric current; metals therefore conduct electricity.
Answer
Delocalised electrons are free to move through the metallic lattice; when a potential difference is applied, they flow, constituting an electric current.
Definitions to memorise and the exact keywords mark schemes credit for macromolecules answers — sharpened from recent examiner reports for the 2026 0654 sitting.
The strong electrostatic attraction between a lattice of positive metal ions and the surrounding sea of delocalised (free) electrons.
A polymer that softens and melts when heated because only weak intermolecular forces hold the chains together; can be re-moulded and recycled.
Example
Poly(ethene), PVC, poly(propene).
A polymer with covalent cross-links between chains that do not break on heating; the polymer does not soften — it chars or decomposes. Cannot be recycled.
Example
Bakelite, melamine-formaldehyde resin, epoxy resin.
Able to be hammered or rolled into different shapes without breaking; a property of metals due to sliding layers of ions in the metallic lattice.
Covalent bonds that connect adjacent polymer chains laterally, preventing them from sliding past each other; they make thermosetting polymers rigid and infusible.
The traps other students keep falling into on macromolecules questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
Why it happens
Students assume any binary compound (two elements) has ionic bonding.
How to avoid it
SiO₂ has giant covalent structure (Si and O are both in the same region of the periodic table and form covalent bonds). Check: two non-metals → covalent.
Why it happens
Students confuse thermosetting with thermosoftening because they both soften when hot in their minds.
How to avoid it
Thermosetting: cross-links hold chains permanently; heating causes decomposition, NOT melting; cannot be recycled. Only thermosoftening polymers can be remoulded.
Why it happens
Students know metals have positive ions but confuse the charge carrier with the ions themselves.
How to avoid it
Metals conduct via movement of ELECTRONS (negative charge carriers), not ions. Ions are fixed in the lattice (in the solid state).
The things students keep getting wrong in this sub-topic, answered.