Chapter 12 · Liquids, Solids, and Intermolecular Forces
12.8Water: A Remarkable Molecule
9 min · two checks
Predict
Which two properties of water come directly from hydrogen bonding?
The idea
- Tie water’s odd properties to hydrogen bonding.
- Explain why water dissolves salt.
Water’s two O–H bonds and two lone pairs let every molecule donate and accept hydrogen bonds. The network is strong for a molecule that weighs only 18 u, so the boiling point is 100 °C instead of something far below room temperature, where H₂S lives. In ice the network is open and regular, which is why ice floats. In the liquid the bonds constantly break and reform, which soaks up energy and gives water its large specific heat and large heat of vaporization.
Water dissolves many salts because the partial charges on H and O stabilize cations and anions. The oxygen end points at positive ions, the hydrogen ends at negative ions. This is not a covalent reaction with every salt; it is solvation. Water is a poor solvent for nonpolar oil, as the polarity section predicted. Those two facts — great for ions, poor for grease — organize a great deal of biology and cleaning.
Keep these
- Hydrogen bonding explains the high boiling point, high specific heat, and open ice structure.
- Ice floats because the solid network is roomy.
- Water solvates ions with its partial charges; it does not mix with oil.