Hydrogen bonds at work

Section 4 of 4

Hydrogen bonds make ice float, and they hold the two strands of your DNA together.

Ice floats

In liquid water, hydrogen bonds break and form all the time as the molecules tumble past each other, and the molecules pack close together. When water freezes, each molecule locks into four hydrogen bonds with its neighbours, in a regular pattern of six-sided rings. The rings have space in the middle, so the molecules end up further apart than in the liquid.

Liquid water and ice Left, liquid water: nine water molecules packed close together in a jumble. Right, ice: six water molecules held in a ring by dashed hydrogen bonds, each molecule's hydrogen pointing at the next molecule's oxygen, with an empty space in the middle of the ring. Each molecule's other hydrogen points outward, with a dashed hydrogen bond to the next ring. liquid water packed close ice open rings, more space
In liquid water the molecules pack close, and their hydrogen bonds keep breaking and re-forming. In ice, hydrogen bonds (dashed) hold them in six-sided rings with space in the middle; each molecule's other hydrogen bonds to the next ring.

The same amount of water takes up about 9% more space as ice, so ice is less dense than water and floats. For most substances, the solid is denser than the liquid and sinks.

This matters for life on Earth. A pond freezes from the top down, and the layer of ice on top helps keep the water below from freezing, so fish live through the winter. Water that expands as it freezes also bursts pipes and splits rocks apart. The six-sided rings even show in snowflakes, which grow as six-sided crystals.

DNA's base pairs

You have already met DNA's base pairs: A pairs with T, held by 2 hydrogen bonds, and G pairs with C, held by 3. These are the same hydrogen bonds as in water. A δ+ hydrogen on an N–⁠H group of one base reaches a lone pair on a δ− oxygen or nitrogen atom of the base opposite.

The A-T base pair, with its partial charges Adenine, two joined rings, faces thymine, one ring. Two dashed hydrogen bonds join them. In the top one, a hydrogen marked delta plus on adenine's N-H group points at an oxygen marked delta minus on thymine. In the lower one, a hydrogen marked delta plus on thymine's N-H group points at a nitrogen marked delta minus on adenine. NNNNNHH NNOOH δ+ δ− δ− δ+ A T adenine thymine
Adenine and thymine, joined by two hydrogen bonds (dashed). Each joins a δ+ hydrogen on one base to a δ− atom with a lone pair on the other. The dotted lines lead to the backbone.

Each hydrogen bond is weak, so a cell can unzip the two strands to read or copy them. Millions of them along a DNA molecule hold the strands together firmly. Hydrogen bonds between -⁠OH groups also hold cellulose chains side by side in plant fibres, as you have seen, and they help hold each protein's chain folded the right way.

Quick check

Why does ice float on water?

That's the end of lesson 12

In this lesson you found out:

  • The pulls between molecules, and how they set boiling points
  • What a hydrogen bond is, and why water boils at 100 °C
  • Why oil and water separate, and what dissolves in what
  • Why ice floats, and how hydrogen bonds hold DNA's base pairs together
Start Lesson 13: Acids and bases All lessons