Hydrogen Bonds: Why Water Boils at 100 °C

0:00Why does water boil at 100 degrees Celsius, when heavier H2S molecules boil at about minus 60?

0:08Inside water, oxygen shares an electron pair with each hydrogen: each shared pair is a covalent bond.

0:16Oxygen is very electronegative: it pulls the shared electrons toward itself.

0:23So oxygen is slightly negative, delta minus, and each hydrogen slightly positive, delta plus.

0:31Oxygen also has two lone pairs: outer electron pairs not used in bonding.

0:37The delta plus hydrogen of a second molecule is attracted to this lone pair: a hydrogen bond.

0:44It needs a hydrogen bonded to nitrogen, oxygen or fluorine, and a lone pair on a nearby nitrogen, oxygen or fluorine.

0:54A hydrogen bond is not a covalent bond inside a molecule. It is a weaker attraction between molecules.

1:01In liquid water, each molecule makes hydrogen bonds with its neighbours.

1:07Boiling must break these hydrogen bonds. That takes a lot of energy, so water boils at 100 degrees Celsius.

1:15The covalent bonds do not break: steam is still H2O.

1:21Sulfur is not electronegative enough for hydrogen bonds, so only weaker forces hold H2S molecules together.

1:29In ice, hydrogen bonds hold the molecules in an open pattern of hexagons.

1:35This open pattern leaves empty space, so ice is less dense than liquid water and floats.

1:42Hydrogen bonds also hold the two strands of DNA together.

1:47Hydrogen bonds between water molecules are why water boils at 100 degrees Celsius.