AP Biology · Unit 1 · Topic 1.1
Water is polar because oxygen pulls harder on electrons and because the molecule is bent. Every property of water on the exam is one of those two facts in a different situation.
Learning objective 1.1.A Explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.
A water molecule is two hydrogens bonded to one oxygen. Neither fact below is enough on its own — a molecule needs both to be polar, and everything else on this page needs it to be polar.
Oxygen is more electronegative than carbon, so each C=O bond in CO₂ is polar — more so than an O–H bond in water. But CO₂ is linear, so its two bond dipoles point in exactly opposite directions and cancel, leaving no net dipole and no hydrogen bonding. Water's 104.5° angle is what keeps its two dipoles from cancelling.
This is worth holding on to, because it is the difference between "has polar bonds" and "is a polar molecule", and the exam asks about it in both directions.
The O–H bonds inside one water molecule are covalent: shared electrons, several hundred kilojoules per mole to break. A hydrogen bond is an attraction between molecules, and it is roughly a hundred times weaker. Every drawing on this page uses a solid line for the first and a dashed one for the second.
That weakness is the point. A bond strong enough to be permanent would make water a solid; one much weaker would let it evaporate away. Hydrogen bonds constantly break and reform, which is what a liquid that holds together is.
A water molecule can hydrogen bond to four neighbors — it has two hydrogens to donate and two lone pairs to accept. Heating the water does not remove them all at once. Each bond is either intact or broken at any instant, and raising the temperature only shifts the odds.
| Temperature | Thermal energy RT | Bonds per molecule, simulated | Predicted | Fraction intact |
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The same model run with a weaker attraction between molecules. Nothing else is changed — same four possible neighbors, same temperatures.
| Attraction, as a share of water's | Bonds at 25 °C | Bonds at 75 °C | Share lost on heating |
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Each of these is a property of water you are expected to know, and each one is the same fact — molecules stick to each other — showing up in a different situation. Pick one.
If water's behavior came from its molecular mass, hydrogen sulfide should behave more extremely, not less. It has nearly twice the mass. These are measured values, not model output.
| Substance | Molecular mass | Hydrogen bonds? | Boils at | Specific heat, J/g/°C |
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| Substance | Specific heat | Temperature rise on the same energy |
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Answer these in your own words before you open the answers.
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