A condensation reaction joins two molecules together with the formation of…

The idea

A condensation reaction joins two molecules, forms a new chemical bond between them and eliminates one molecule of water. The water comes from the groups being joined: a hydroxyl group from one molecule and a hydrogen atom from the other. The bond formed depends on the molecules involved - a glycosidic bond between two monosaccharides, a peptide bond between two amino acids, an ester bond between glycerol and a fatty acid, a phosphodiester bond between two nucleotides - so an answer that only says 'a bond forms' usually loses the mark. Count carefully, because one water molecule leaves per bond formed: joining n amino acids into one polypeptide forms n-1 peptide bonds and releases n-1 water molecules, and building a single triglyceride releases three.

What earns the marks

• Award credit for stating that two molecules are joined and a named chemical bond is formed.
• Award credit for stating that a molecule of water is released, eliminated or removed.
• Award credit for naming the correct bond for the molecules given, such as peptide, glycosidic or ester.
• Award credit for stating that one water molecule is released per bond formed, so n monomers release n-1 water molecules.
• Award credit for identifying condensation as the reaction that builds polymers, and hydrolysis as its reverse.

Common mistakes and how to put them right

• Saying water is added or used, which describes hydrolysis rather than condensation.

  Put it right: Condensation eliminates water, hydrolysis adds it.

• Writing that 'hydrogen and oxygen are removed' instead of one molecule of water.

  Put it right: State that one water molecule is eliminated.

• Describing the reaction without naming the bond formed.

  Put it right: Name the specific bond, such as peptide or glycosidic.

• Stating that one water molecule is released when three fatty acids join a glycerol, instead of three.

  Put it right: One water molecule is released per bond, so three bonds release three water molecules.

• Assuming condensation only happens in digestion or only outside cells.

  Put it right: Condensation builds polymers in cells, while hydrolysis breaks them down in digestion.

Exam tips

• Write condensation answers as a three-part sentence: what joins, which bond forms, and that one water molecule is released.
• If a question gives a number of monomers, expect to calculate bonds and water molecules as one less than that number.
• Name the bond every time, even when the question does not ask for it - it is often the difference between one mark and two.