How molecules interact: H₂O

We experience water in all of its three states: ice, liquid water, and steam; for example during a soak in the bath, or the familiar clink of ice cubes in a drink.

Three quarters of the earth is covered in water, and it makes up 60% of the human body. But what do we know about this most essential of molecules?

Using steam to melt ice in Kuopio [Image: Jukolanpoika under CC-BY-SA licence]

Water is made up of two hydrogen atoms and one oxygen atom (H₂O).

Each hydrogen atom forms a bond to the central oxygen atom (H-O-H). The hydrogen atom has one electron, whereas oxygen has eight, so the oxygen atom is much larger than the hydrogen.

The water molecule looks a little like Mickey Mouse, because the angle between the two hydrogen atoms where they connect with the oxygen atom is about 104 degrees.

This is called the "bond angle".

Because of this bond angle, the hydrogen atoms are at one end of the molecule, and oxygen is at the other. This gives the molecule polarity, with a more positive area near the hydrogen atoms and a more negative area near the oxygen.

Because of this polarity, water molecules tend to attract one another. In other words, they're "sticky".

This "stickiness" is caused by hydrogen bonding, and holds the molecules together to create the liquid state.

If you'd like to know more about chemical interaction and would like to tackle something a little more challenging than water, the Open University course Chemical Change and Environmental Applications deals with topics from chemical thermodynamics through to electrochemistry.