Key Idea: Water is the medium for life — most of a cell is water, and life's reactions happen in it. Almost every special property of water comes from one fact: a water molecule is polar, so its molecules stick to each other by hydrogen bonds. This topic is a regular on Paper 1A (a quick property/bonding MCQ) and on Paper 2 (drawing the molecule, or linking a property to its use in organisms).
💧 The polar water molecule
Oxygen and the two hydrogens share electrons in polar covalent bonds, but oxygen pulls them closer (it is more electronegative). So oxygen is δ− and each hydrogen is δ+, and the bent shape stops these charges cancelling — the molecule is polar.
The polar water molecule: δ− oxygen, δ+ hydrogens, bent shape.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Use 'polar' for the whole molecule, and 'polar covalent' for the O–H bond.
🔗 Hydrogen bonding
Because water is polar, the δ+ hydrogen of one molecule is attracted to the δ− oxygen of another. This weak attraction is a hydrogen bond. One hydrogen bond is weak, but each molecule makes several, so together they are strong enough to give water all its unusual properties.
Cohesion, high boiling point, high heat capacity and surface tension all trace back to hydrogen bonding between water molecules.
🌊 Cohesion, adhesion & surface tension
| Property | What it means | Why it matters to organisms |
|---|---|---|
| Cohesion | water molecules stick to each other | unbroken water columns are pulled up the xylem in plants (transpiration) |
| Adhesion | water sticks to other polar/charged surfaces | water creeps up narrow tubes (capillary action) and wets cell walls |
| Surface tension | cohesion makes the surface act like a skin | small animals (e.g. pond skaters) can walk on water |
Cohesion = water-to-water. Adhesion = water-to-other surfaces.
🌡️ Thermal properties
| Property | Consequence | Use in organisms / habitats |
|---|---|---|
| High specific heat capacity | water heats and cools slowly | stable temperatures inside cells and in lakes/oceans as habitats |
| High latent heat of vaporisation | evaporating water removes a lot of heat | sweating / panting / transpiration cool the body |
| High thermal conductivity & buoyancy | water carries heat away and supports bodies | aquatic habitats: support, but fast heat loss → insulation (e.g. blubber) |
🧪 Water as a solvent
Being polar, water dissolves other polar and charged (ionic) substances — sugars, amino acids, ions. It is therefore the medium for metabolism (reactions happen in solution) and for transport (blood plasma, xylem and phloem sap). Non-polar substances (e.g. fats) do not dissolve in water — useful for membranes and for transporting lipids in special ways.
Water dissolves polar/charged solutes well, but non-polar ones (lipids) poorly.
✍️ Worked examples
IB-style question — draw the molecule
Draw a labelled diagram of a water molecule, showing the bonds and the partial charges. [2]
Model answer:
Draw a bent molecule: one O joined to two H by single (polar covalent) bonds.
Label δ− on the oxygen and δ+ on each hydrogen.
A bent H–O–H molecule, O–H bonds shown, δ− on O and δ+ on each H.
IB-style question — transport in plants
Explain how the properties of water allow a continuous column of water to be pulled up the xylem of a tall tree. [3]
Model answer:
Water molecules are cohesive — they hydrogen-bond to each other, so they hold together as a continuous column.
As water evaporates from the leaves (transpiration), the cohesive column is pulled upward.
Adhesion to the xylem walls also helps the water move up the narrow vessels.
Cohesion keeps the water column unbroken, transpiration pulls it up, and adhesion to the xylem walls assists.
IB-style question — staying cool
Explain how sweating helps a mammal lose heat, using a property of water. [2]
Model answer:
Water has a high latent heat of vaporisation — a lot of heat energy is needed to evaporate it.
When sweat evaporates from the skin it takes this heat from the body, cooling the animal.
Evaporating sweat absorbs a lot of heat (high latent heat of vaporisation), cooling the body.
✅ Quick self-check
Tap each card to check yourself.
Why is a water molecule polar? Oxygen is more electronegative, so it pulls the shared electrons closer (O = δ−, H = δ+); the bent shape stops the charges cancelling.
What is a hydrogen bond in water? A weak attraction between the δ+ hydrogen of one water molecule and the δ− oxygen of another.
Cohesion vs adhesion? Cohesion = water sticking to water; adhesion = water sticking to other (polar/charged) surfaces.
Why does water make a good coolant? High specific heat capacity (heats/cools slowly) and high latent heat of vaporisation (evaporation removes lots of heat).
Why is water a good solvent / transport medium? Being polar, it dissolves polar and charged solutes, so reactions and transport (blood, xylem, phloem) happen in solution.
Why can pond skaters walk on water? Cohesion produces surface tension — the surface behaves like a thin skin that supports their weight.
Exam Tips
- Trace every property back to the same root: polarity → hydrogen bonding → the property.
- Polar molecule, but polar COVALENT bond — keep the wording precise.
- Cohesion = water-to-water; adhesion = water-to-other-surfaces. Don't swap them.
- Link a property to its USE in an organism — Paper 2 wants the consequence, not just the property.
- Sweating/transpiration cooling = high latent heat of vaporisation (not 'high heat capacity').
- Water dissolves polar/ionic solutes; lipids are non-polar and don't dissolve.
- Draw the molecule BENT — a straight drawing implies the charges cancel.