The search for water on Mars
The search for water on Mars

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

2.1 Why is water so important?

To understand why water is important for life, we need to know about the structure and the chemistry of water.

The two hydrogen atoms are not exactly opposite each other, but are instead at an angle. Video 1 shows a three-dimensional representation of water molecules, with the atoms in a water molecule (two hydrogen atoms and one oxygen atom) represented by spheres. Watch this video now.

Download this video clip.Video player: Video 1
Copy this transcript to the clipboard
Print this transcript
Show transcript|Hide transcript
Video 1 three-dimensional representation of water molecules.
Interactive feature not available in single page view (see it in standard view).

The bonds that join the oxygen and hydrogen atoms in a water molecule are made up of electrons shared between the atoms. However, the electrons are more strongly attracted to the oxygen atom than to the hydrogen atoms. This means that, near to the oxygen atom the molecule has a slight negative charge, but near the hydrogen atom the molecule has a slight positive charge. We call molecules with this feature polar and it is this polarity that makes water special. For example, it can form large networks of water molecules where the positive charge of a hydrogen atom attracts the negative charge of an oxygen atom - this forms a hydrogen bond between water molecules. This is shown on Figure 8, which is a different method of visually representing molecules – a ball and stick model – in which the atoms are shown as spheres and the bonds between them as sticks.

The figure shows three water molecules represented by ball-and-stick models. Each water molecule consists of one red ball with delta minus (δ–) in the middle of it and two smaller white balls, each containing delta plus (δ+). Each white ball (or hydrogen atom) is attached to the red ball (or oxygen atom) by a short straight ‘stick’. The angle between the two sticks is more than a right angle. One of the water molecules is on the left of the figure and the other two on the right, one above the other. The molecule on the left has been rotated so that one of its white balls is above the red ball, and the other is below and to the right of the red ball. The first molecule on the right has been rotated so that its red ball is diagonally aligned with the lower white ball of the molecule above and to its left. There is a dashed blue line between this lower white ball at , and the red ball of the first molecule on the right. One of the white balls attached to this red ball is directly below it, and the other above and to its right. The second molecule on the right has been rotated so that its red ball is below and aligned with the lower white ball of the molecule directly above it. There is a dashed blue line between this white ball and the red ball of the second molecule. The white balls attached to this red ball are below and either side of it. The second dashed blue line is labelled as a hydrogen bond. The red ball of the third molecule is labelled 'oxygen atoms have a slight negative charge' (i.e. δ–). Its right white ball is labelled 'hydrogen atoms have a slight positive charge' (i.e. δ+).
Figure 8 Hydrogen bonding between water molecules, shown by the dashed line.

The three-dimensional shape of hydrogen bonds, and the polarity of water molecules is the main reason for some of the unusual properties of liquid water, such as its high surface tension and the fact that ice floats on water. It is also why water is an extremely good solvent for solids, liquids and gases. These properties have critical roles to play in why water is a key target in the exploration of Mars.

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371