2.4 Water
- Syllabus
- 9700–2028–2029
- Topic
- 2.4
- Level
- AS
Learning objectives
Water polarity creates hydrogen bonds between molecules
Water is a covalent but polar molecule. Oxygen attracts the shared electrons more strongly than hydrogen, so the oxygen atom has a slight negative charge (delta minus) and each hydrogen atom has a slight positive charge (delta plus); the whole molecule remains electrically neutral.
- Within one water molecule, oxygen and hydrogen are joined by polar covalent bonds.
- Between neighbouring molecules, a delta-plus hydrogen is attracted to a delta-minus oxygen.
- This intermolecular attraction is a hydrogen bond.
- Individual hydrogen bonds continually break and reform, while many together affect how water responds to solutes, heating and evaporation.
Unequal electron sharing creates partial charges; opposite partial charges on neighbouring molecules attract; those intermolecular attractions create the hydrogen-bond network used to explain water's syllabus-limited biological properties in the next card.
Do not draw full ionic charges, call water an ion, or place a hydrogen bond inside one molecule. The covalent O-H bond is intramolecular; the hydrogen bond is an attraction between separate water molecules.
Three water properties support transport and temperature control
For this syllabus objective, relate water to living organisms through exactly three properties: solvent action, high specific heat capacity and high latent heat of vaporisation.
| Property | Molecular explanation | Role in living organisms |
|---|---|---|
| solvent action | water's partial charges surround and separate many ions and interact with polar molecules | dissolved substances can be transported and can meet for metabolic reactions |
| high specific heat capacity | much energy is absorbed in disrupting hydrogen-bond attractions before molecular kinetic energy and temperature rise substantially | aqueous cells, tissues and habitats resist rapid temperature change, supporting stable conditions for enzymes |
| high latent heat of vaporisation | a large energy input is required for molecules to overcome intermolecular attractions and leave as vapour | evaporation from sweat or transpiring leaves removes thermal energy and cools the organism |
Specific heat capacity concerns energy needed to change the temperature of liquid water. Latent heat of vaporisation concerns energy needed for liquid water to become vapour without a temperature increase during the phase change. Both involve intermolecular attraction, but they describe different processes.
Water does not dissolve every substance: non-polar substances dissolve poorly. Do not confuse resistance to temperature change with energy required for evaporation, and do not add other water properties to this objective because the official outcome is explicitly limited to these three.