Ask two questions. Does the monomer have a C=C double bond, and is a small molecule released at each link? If the answers are yes and no, the reaction is addition. If the monomers have two reactive groups and water is released, it is condensation.
This lesson is part of polymers. The next step is drawing units, in identifying repeating units.
How are the two types different?
The key differences fit on one table.
| Feature | Addition | Condensation |
|---|---|---|
| Monomer feature | C=C double bond | Two reactive groups, one at each end |
| By-product | None | Small molecule, such as water |
| Example | Ethene to poly(ethene) | Diamine and diacid to nylon |
| Polymer mass against monomer mass | Equal | Smaller |
The last row is the useful check. In addition, every atom of the monomers ends up in the polymer.
Worked example: addition
Propene, CH₂=CH–CH₃, is the monomer. The double bond opens, and each molecule links to the next.
The equation shows only the unit: n CH₂=CH–CH₃ → [–CH₂–CH(CH₃)–]ₙ. There is one product, and the polymer contains all the atoms of the propene molecules.
Worked example: condensation, with a mass check
Nylon forms when a monomer with an amine group at each end reacts with a monomer with an acid group at each end. Each time a link forms, one water molecule leaves.
Imagine, as an illustration only, that 100 g of monomers produce 88 g of polymer. The missing 12 g is the water that left, which would be collected separately. The total mass is still 100 g, so conservation of mass holds.
The mistake that costs marks
The slip is to name the reaction from the word “condensation” instead of from the evidence, or to think every polymer is addition.
| Statement | Problem | Better |
|---|---|---|
| “It is condensation because the product is a liquid” | Physical state is not the test | Check for released water |
| “All polymers are made by opening double bonds” | True only for addition | Condensation joins two functional groups |
| “Polymer mass equals the monomers in condensation” | Ignores the by-product | Polymer mass is lower |
Write “double bond opens, no other product” or “water released” as the reason in every answer. You can use the mole and stoichiometry steps tool if you want to check a mass balance.
Check yourself
A student says that ethene joins to form poly(ethene) and releases water. Identify the error.
Answer
Ethene has a C=C double bond, so it undergoes addition polymerisation. The double bond opens and the molecules join with nothing else released.
Water is released only in condensation polymerisation. So the statement should say that the polymer is the only product.
What to study next
Practise drawing the repeating unit in identifying repeating units. You can also go straight to the polymers practice set.
For a teacher to check your classifications, see online one-to-one Chemistry tuition.