Water is a tiny molecule that should be a gas at room temperature. Hydrogen bonds hold it together as a liquid, let it dissolve the substances cells need, keep temperatures steady and make sweating such an effective way to cool down.
Oxygen attracts the shared electrons in each O-H bond more strongly than hydrogen, so a water molecule is polar: the O is slightly negative (δ−) and each H slightly positive (δ+). A hydrogen bond is the attraction between a δ+ hydrogen of one molecule and a lone pair on the δ− oxygen of another. Because water is polar it is an excellent solvent for ions and polar molecules. Because hydrogen bonds must be broken first, water has a high specific heat capacity (its temperature changes slowly) and a high latent heat of vaporisation (evaporation removes a lot of heat).
Rotate the cluster of water molecules and follow the dashed hydrogen bonds. Then switch to the solvent view and see which way the molecules face each ion.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the hydrogen bonding view, which partial charges are shown on a water molecule?
A hydrogen bond in water forms between:
In the solvent view, which part of the water molecules faces the Na+ ion?
What is the maximum number of hydrogen bonds one water molecule can form?
For each property, give the molecular reason and then the biological role.
| Property | Because | Role in organisms |
|---|---|---|
| solvent | δ− O surrounds cations, δ+ H surrounds anions; polar molecules such as glucose form hydrogen bonds with water | transport in blood plasma, xylem and phloem; metabolic reactions happen in solution in the cytoplasm; removal of urea |
| high specific heat capacity (4.2 J g−1 °C−1) | energy goes into breaking hydrogen bonds before molecules can move faster | body temperature and the temperature inside cells change slowly; lakes and seas are stable habitats |
| high latent heat of vaporisation (about 2.4 kJ g−1) | a molecule must break all its hydrogen bonds to escape as vapour | evaporation of sweat and transpiration from leaves remove a lot of heat for little water lost |
Do not mix up the two thermal properties. Specific heat capacity is about how much energy it takes to raise the temperature, so it explains stable temperatures. Latent heat of vaporisation is about the energy needed to evaporate water, so it explains cooling by sweating and transpiration.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Why does the temperature of a lake change much less than the temperature of the air above it during a day?
Why is sweating an effective way for a mammal to lose heat?
Why does glucose dissolve in water but a triglyceride does not?
Why does water have a high specific heat capacity?
This skill is now recorded on your profile. Keep the chain going.