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.

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Transcript: Video 1 three-dimensional representation of water molecules.

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This animation illustrates how water molecules behave under normal conditions, for example, in a glass of pure drinking water. The water molecules are in constant random motion, bumping into each other and bouncing apart.

This way of representing water molecules is called a space-filling model because filled spheres are used to represent the atoms. Each water molecule is represented by three overlapping spheres, a single large red sphere, representing an oxygen atom, which is joined to two small white spheres, the two hydrogen atoms in a water molecule. Oxygen and hydrogen are two of the more than 100 unique chemical elements known to science.

Of course, water molecules are not really made of overlapping red and white spheres. The colours chosen for the atoms are simply a convention in chemistry. Oxygen atoms in molecular models like this one are always coloured red, and hydrogen atoms are white.

A static image makes it easier to describe the structure of an individual water molecule. In the space-filling model of a water molecule, the single oxygen atom is red and is joined to two hydrogen atoms coloured white. Here the hydrogen and oxygen atoms have been labeled with their chemical symbols, capital H for hydrogen and capital O for oxygen. Combining these symbols gives the familiar chemical formula for water, H2O. Water is described as a chemical compound, because the water molecule contains more than one type of atom.

End transcript: Video 1 three-dimensional representation of water molecules.

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Video 1 three-dimensional representation of water molecules.

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.

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Figure 8 Hydrogen bonding between water molecules, shown by the dashed line.

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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.