4. Water as a liquid—and as glass(es)

For a liquid to flow, the molecules must move. So can a liquid have a structure? ‘Water as a liquid—and as glass(es)’ illustrates that it can; under normal temperature and pressure conditions, the structure of liquid is represented by the random network model. Important structural data on water have been obtained from the scattering of neutrons and X-rays: the four-fold tetrahedral motif dominates, but the data also tell us how the real structure differs from the reference random network, both under normal conditions and as temperature and pressure changes. Water’s dynamical aspects when it is heated, put under pressure, becomes supercritical, and is supercooled are considered. Three distinct amorphous ice structures are also discussed.

2. The water molecule and its interactions

‘The water molecule and its interactions’ discusses the structural and electrical properties of the water molecule. A water molecule is made up of two hydrogen atoms connected by covalent bonds to one oxygen atom. Water molecules interact with each other through a type of interaction called hydrogen bonding. A tetrahedral arrangement of four water molecules around a central one is the key to understanding water. It helps to explain the structure of water in its various states, its properties, and how it interacts with other kinds of molecules, allowing exploration of the properties and behaviour of the wide range of chemical, physical, and biological systems in which water is involved.

7. Some past and current controversies

The structure, dynamics, and properties of water are pretty well understood. We have a good idea of its structure, and how that structure explains why its properties are sometimes different from those of a ‘normal’ liquid. A number of issues remain, which suggest there may still be some things about water that we don’t yet understand. ‘Some past and current controversies’ considers the concepts of ‘memory’ of water; polywater (a form of water that was thought to be denser and more stable than ordinary water); and whether surface ordering impacts on the biological role of water. These have largely been resolved, but the idea of two possible liquid structures of water still remains.

3. Water as ice(s)

‘Water as ice(s)’ describes the sixteen known crystalline phases of ice, starting with normal ice, as well as a potential seventeenth phase called cubic ice, for which they may be evidence elsewhere in the universe. The different crystalline structures of ice are shown on a phase diagram that depicts how changes in pressure and temperature affect the structure of ice. The Bernal–Fowler rules of ice structures are also explained. Ordered arrangements of water molecules all show four-coordinated geometry, but water shows great molecular versatility under pressure: hydrogen bond lengths and OOO angles can vary when forming the high pressure ice structures.

6. Water as a biomolecule

Why is water biologically important? ‘Water as a biomolecule’ considers how water influences biological processes and what properties of the water molecule enable it to do so by focusing on the role of water in the structure and operation of enzymes. Water molecules are critical in maintaining the active structures of other biomolecules such as stabilizing proteins. It also asks: how can we be sure that water is essential to life? Could other molecules fill a similar role or roles? Could water be replaced by some other medium in some other life form elsewhere in the universe that can still metabolize and reproduce?

5. The anomalies explained

‘The anomalies explained’ considers why water behaves differently from most other liquids and why this is so important chemically, biologically, and environmentally. Ice contracts on melting; a normal liquid expands. Below 4°C liquid water contracts on heating; a normal liquid expands. Between the melting point and 46°C, water’s compressibility falls as temperature increases; for a normal liquid it increases. Water’s viscosity at or below about 30°C falls as pressure increases from 1 to 1,000 atmospheres; it increases for a normal liquid. The local intermolecular geometry is responsible for these ‘anomalies’. The electrical properties of the water molecule result in other distinct properties: water is a very powerful solvent and it conducts electricity.

1. Water, water everywhere …

Water (H2O) is the most abundant compound on the Earth’s surface and occurs naturally in gas, liquid, and solid forms. It is estimated that there are 1.9 billion billion tonnes of water on our planet with even more in the Earth’s mantle. ‘Water, water everywhere …’ outlines how water arose from the Big Bang, how it got to Earth, and describes how scientists deciphered its molecular structure. But why do we need it? Water maintains our climate, keeps all living things alive, is a major influence in forming the surface topography of the planet, and is also critical in what happens in planetary interiors, as well as in the location and eruptive style of volcanoes.