Every paper from 2021 to 2025 has a question here. You need to draw a linkage, name the reaction that formed it, and break it again with water. It is worth more marks than mechanisms, naming and pathways put together.
The one idea underneath all of it
Condensation = two molecules join and a small molecule (water) is released. That is the definition examiners want, in those words.
Hydrolysis = the reverse. Water is used to break the linkage back apart.
A condensation polymer needs monomers with two reactive groups each — otherwise the chain stops after one join.
The two condensation polymers you need
Polymer
Monomers
Linkage formed
Example
Polyester
A diol + a dicarboxylic acid
Ester –CO–O–
Dacron / PET (2024, 2025)
Polyamide
A diamine + a dicarboxylic acid
Amide (peptide) –CO–NH–
Nylon, Kevlar (2022)
Polypeptide
Amino acids (each has both groups)
Amide (peptide) –CO–NH–
Proteins (2021, 2022, 2023, 2025)
✓
Drawing a linkage: what actually earns the mark
Circle or draw –CO–NH– for an amide/peptide link and –CO–O– for an ester link. Both contain a C=O; the difference is whether the next atom is N or O. Schedules award the mark for the linkage being correct and the reaction being named — write “condensation” or “hydrolysis” explicitly, every time.
Amino acids
An amino acid has both an amine (–NH₂) and a carboxylic acid (–COOH) group: general structure H₂N–CHR–COOH.
Having both is why one molecule can join to the next at either end, and why they polymerise on their own.
Zwitterion: the acid group donates its proton to the amine group within the same molecule, giving ⁻OOC–CHR–NH₃⁺ — no overall charge, but charged at both ends.
In acid the –COO⁻ picks up H⁺ (net +); in base the –NH₃⁺ loses H⁺ (net −). That is why amino acids buffer.
You will not be asked to recall the common names of amino acids — the 2026 specification says so directly.
Worked example Two amino acids → a dipeptide (2025, Q3a) Two amino acids react to form a dipeptide. Name the reaction and explain what happens. The –COOH of one amino acid reacts with the –NH₂ of the other. The C–OH of the acid and one H of the amine leave together as H₂O. The C and N left behind bond directly, forming the amide (peptide) linkage –CO–NH–. ✓ A condensation reaction: two small amino acids join to form a larger dipeptide, with a water molecule released as the amide (peptide) linkage forms. ℹ
Two amino acids make two different dipeptides
Glycine + alanine can join either way round — whichever one donates the –COOH end comes first. If a question says “draw one dipeptide”, either is fine; if it asks how many are possible, the answer is two (plus the two self-pairs if it does not say the monomers must differ). Worked example Hydrolysing Dacron with NaOH (2025, Q3b) Dacron (a polyester) is heated with NaOH solution. Explain what happens and give the products. Hydrolysis: water breaks the ester linkages in the chain. The C–O side of each ester link gains an –H from water → an alcohol group. The C=O side gains an –OH → a carboxylic acid group. But the conditions are basic, so the –COOH immediately donates its proton to OH⁻ in solution… …leaving the salt –COONa rather than the free acid. ✓ Base hydrolysis of the ester linkages, giving a diol and the sodium salt of the dicarboxylic acid (–COONa, not –COOH). Saying “carboxylic acid” alone loses the last mark. ✕
The mark students drop here
Hydrolysing an ester or amide under basic conditions gives the salt (–COO⁻Na⁺), not the acid — because any –COOH formed is deprotonated instantly by the OH⁻ around it. Under acidic conditions you do get the free –COOH. Schedules mark these differently, so read the conditions before you write the product.