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Living condition
Living condition

Water remains one of the most mysterious and amazing substances on Earth. For example, water is one of the few substances found in nature in three aggregate states - ice, liquid water and steam. Almost all of the physical and chemical properties of water are exceptional. Here we shall describe some of them.

Water has significantly different boiling and melting points compared to other hydrogen-containing substances. It has one of the highest surface tensions (surpassed only by mercury). Water is a universal solvent. When freezing water becomes less dense in solid state (ice) and accumulates on the surface of a body of water. Warmer water at 4°C is as dense as possible and therefore sinks to the bottom. Only because of this aquatic life is possible in bodies of water even during winters. Another important property of all life on Earth is the anomalously high value of its heat-absorbing capacity. Thanks to this property, our planet's climate is stabilised due to the cooling and heating of surface water over a long period of time as the days and seasons change. The anomalous thermodynamic behaviour of water plays a fundamental role in many chemical, physical, and biological processes in nature.

More than 70 anomalies of the properties of water are known. Scientists have so far struggled to find an explanation for many of them. Moreover, the study of liquid water's molecular structure using modern research methods raises even more questions regarding the structure's complexity. Scientists agree that one of the main aspects differentiating water from other liquids is the abundance of hydrogen bonds in its molecular structure. The familiar chemical formula of water - H2O - can be applicable only to its gaseous state. In liquid water, the molecules are combined into larger groups of (H2O) x by a hydrogen bond, where 'x' is the number of basic molecules. The most energetically favourable is a combination of five water molecules (a pentamer, x = 5), in which the four hydrogen bonds form an equilibrium or 'tetrahedral' angle of 109.47 degrees.

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Pentamer of water molecules.
Source: http://www1.lsbu.ac.uk/water/water_hydrogen_bonding.html

However, the results of modern studies carried out using computer modelling show that at least two types of structures based on hydrogen bonds can exist in water: long-lived, which make up about 80% of all hydrogen bonds in water, and quick-switching less ordered bonds, the lifespan of which is comparatively very short.

These data have been confirmed by the results of small-angle X-ray scattering studies carried out in an aquatic environment. It was shown that liquid water actually contains two different structures. Moreover, liquid water can be categorized as a mix of two liquids — low-density water (LDW) and high-density water (HDW). Both types of water transition into each other when temperature or pressure shift. Anomalous properties appear due to an increase in structural oscillations between the two liquids. The range of oscillation is approximately from - 42°C to + 47°C.

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Phase diagram of water.
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4686860/

This means that water is abnormal in the temperature range conductive to life. Most important processes in nature occur under such conditions.

There is another theory that explains the anomalous temperature-dependent properties of water. It postulates that several water molecule pentamers are combined into polyhedra of a more complex structure under specific temperatures. With an increase in temperature there are two competing processes: the elongation of hydrogen bonds (leading to an increase in the volume of water), and the prevailing process - a decrease in the volume of polyhedra cavities. This eventually leads to an observable compression of water as temperature increases. It has been found that each water molecule is tightly connected with three other molecules, weakly connected with a fourth and even more weakly with a fifth molecule. However, some molecules can be the fifth 'participants' of more than one tetrahedra. In this case, several pentamers become 'connected' with each other, causing an increase in the density of water relative to ice.

Thus, scientific debate regarding the molecular arrangement of water continues, and there is no consensus.

As for the aggregate states of water, there are others that have been discovered besides the usual three of steam, liquid and ice. For example, an international group of scientists drew up a phase diagram of non-crystalline (liquid and amorphous) water.

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Phase diagram of non-crystalline water.
Source: https://pubs.acs.org/doi/10.1021/acs.chemrev.5b00750

In the above diagram, stable water is marked purple, and just below it is supercooled water (cooled to a temperature below its freezing point at 0°C, but remaining in a liquid state). Black indicates the area in which only crystalline ice has been observed so far. The two unstable or 'metastable' forms of amorphous water: low-density water (blue zone) and high-density (pink zone) turn into ultra-viscous low and high-density waters (the green and purple areas in the diagram) during melting. However, many possible metastable states of water have been described only in theory or in computer simulations. It is rather difficult to achieve the necessary conditions in a real-life experiment. Most notable are the recent successes in experimental confirmation of existing low-density liquid water and superionic ice production. Superionic ice is a special form of ice in which oxygen ions form a rigid crystal lattice, and hydrogen ions have the ability to move freely along it. This form of ice was theoretically predicted 30 years ago, but this was the first time it was produced in an experiment.

Usually water forms ice with a hexagonal structure. However, other crystalline forms of ice exist, for example cubic. It is metastable within a rather narrow temperature range and turns into its hexagonal form when heated. In reality, both of these structures coexist, and no one has yet succeeded in obtaining ice consisting entirely of a cubic structure. The highest proportion of cubic ice which scientists have been able to produce is 80%. In addition, ice exists in various forms (amorphous and crystalline), and by changing external conditions, ice can be transformed from one to another. What is more, these changes can be carried out at different speeds. If it is done slowly enough, stable states form. If done quickly, the molecules seem to not have enough time to 'adapt' to the new conditions, and are forced to form unstable or 'metastable' phases. Currently 17 different phases of ice are known.

Studies of water still have many complex problems. A great number of scientists around the world are working on solving them. The use of modern experimental and computational methods increases their chances, and thanks to this, surprising discoveries regularly occur. Despite this, water still remains one of the most mysterious substances on Earth and further successes in the study of water can greatly affect science as a whole.