IB Biology A1.1.2 Hydrogen bonds in water
Water molecules are polar, so partial charges attract between neighbouring molecules to form hydrogen bonds that explain water’s distinctive physical properties.
- Syllabus
- First assessment 2025
- Objective
- ib_null_biology_001_001_002
- Level
- All levels
Quick summary
A water molecule contains two polar covalent O–H bonds arranged in a bent shape. Oxygen attracts the shared electrons more strongly, giving it a partial negative charge, while each hydrogen has a partial positive charge. The attraction between a partially positive hydrogen on one molecule and a partially negative oxygen on another is a hydrogen bond. Hydrogen bonds are intermolecular; they must not be confused with the covalent bonds inside a water molecule. Individually they are weak, but together they strongly influence water’s boiling point, specific heat capacity and latent heat of vaporization. Energy is required to disrupt these attractions, so water changes temperature slowly and evaporation removes substantial heat. Exams frequently ask students to identify, draw or explain these interactions and connect them causally to thermal properties and cooling.
Concept visual
One diagram for the relationship students need to remember.
Common mistakes
Drawing a hydrogen bond between atoms within the same water molecule.
Reversing the partial charges on oxygen and hydrogen.
Calling the O–H bond a hydrogen bond instead of a polar covalent bond.
Stating a thermal property without explaining that energy is needed to disrupt intermolecular attractions.
Key takeaways
- Unequal electron sharing makes each O–H bond polar.
- Oxygen is partially negative and hydrogen is partially positive.
- Hydrogen bonds form between different water molecules.
- Disrupting hydrogen bonds requires energy.
- Hydrogen bonding explains important thermal properties of water.
