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that are bound by a hydrogen bond (represented by a broken line) as well as free water molecules. The solid lines represent covalent bonds.

This shows the complexity of water’s behavior and its paradoxical reactions in the physical world resulting from its unique structure and properties. Let us remember that of all the elements, water is the only one that can exist in the solid, liquid, or gaseous state in the conditions that are found on the earth’s surface.

2.2.2 The Main Physical, Chemical, and Biological Properties of Water

Physical Properties

We have already noted that water possesses a number of distinct properties due to both the presence of covalent bonds within the water molecule and hydrogen bonds between water molecules.. Beyond its molecular aspects, one of the main physical properties of water is obviously its mobility and its ability to flow freely, to spread out, and to easily fill any container. The fluidity of water is fundamental, and observation of water and its movements provided the basis for the development of fluid mechanics, whether those fluids are gases or liquids. The free-flowing property of water is basically due the presence of the hydrogen bonds between the water molecules, even though some researchers are not in agreement about the underlying physical processes.

The fluidity of water is sometimes explained by the continuous rupturing and re-forming of the hydrogen bonds, or sometimes by the possible deformation of the bonds without complete rupture.

Another physical property of water, mentioned previously, is the fact that the density of ice is lower than that of water. The maximum density of water is reached at a temperature of 3,984°C. The density of water follows a non-linear curve as the temperature goes from zero to 16°C, as illustrated in Figure 2.5.

Furthermore, with the exception of ammonia NH₃, water has the highest specific heat. This explains why it is a bad thermal conductor and also why it has such enormous regulating capacity in terms of climate. This regulating capacity is reinforced by its very high values for latent heat of thawing and for boiling point. This means that water requires a large quantity of heat to increase it temperature and to change it from liquid to gas.

Finally, let us remember that water, in nature, is odorless, and colorless when it is shallow, but appears greenish-blue when deep.

O 2-H⁺ H⁺

O 2-H⁺

H⁺

O 2-H⁺ H⁺

O

2-H⁺ H⁺

O 2-H⁺ H⁺

O

2-H⁺

H⁺

O 2-H⁺

H⁺

O 2-H⁺ H⁺

O

2-H⁺

H⁺ O

2-H⁺

H⁺

Fig. 2.4 : The inter-molecular structure of water and illustration of the hydrogen bond (based on CNRS, 2000).

Table 2.1 shows the main physical characteristics of water.

Chemical Properties

Water is an excellent solvent. Not only can it dissolve more substances than any other liquid, it has the ability to dissolve gasses as well. This explains why water is a favorable environment for the development of life, because it contains so many of the primary elements essential for life. Likewise, its ability to dissolve gases means that fish, for example, are able to breathe by extracting dissolved oxygen. The salinity of sea water is also a result of water’s power as a solvent. This solvent ability is a result of water’s high dielectric constant, which is defined as the relationship between the intensity of an electric field in a vacuum and its intensity in the substance under consideration.

Table 2.1 : Main characteristics of Water.

Properties Value

Molar Mass 18.0153 g/mol

Volumic Mass 18.0182 cm³

Density (solid) 917 kg/m³

Density (liquid) 998 kg/m ³

Melting Point 0 ^oC

Boiling Point 100 ^oC

Latent Heat of Fusion 3.3˜10⁵j/kg Latent Heat of Vaporization 23˜10⁵j/kg Specific Heat Capacity (solid) 2.06˜10³ j/kg/K Specific Heat Capacity (liquid) 4.18˜10³ j/kg/K

999.00 999.10 999.20 999.30 999.40 999.50 999.60 999.70 999.80 999.90 1000.00

0 2 4 6 8 10 12 14 16

Temperature (°C)

Density (kg/m3)

Fig. 2.5 : Density of Water between 0 °C and 16 °C.

For example, the dielectric constant of water at room temperature is 80, which means that any two opposite electric charges in the water will attract each other with a force 80 times weaker than their force of attraction in a vacuum. This explains why salts such as NaCl, for example, will separate easily in water to form the ions Na⁺ and Cl^- (Pauling, 1960).

The second main chemical characteristic of water is its amphiprotic character: it is capable of reacting as a base (by releasing OH^- ions) and as an acid (by releasing H⁺ ions). The dehydratation of the hydronium H₃O⁺ ion, which is an acid, suggests that water is its conjugated base, while the chemical dissociation of water suggests that the OH^- ion could be considered as the conjugated base of water, thus making water an acid. This chemical property is illustrated in the following two equations:

Biological Properties

It is beyond the scope of this book to discuss in detail the role of water in the different mechanisms related to the arrival and maintenance of life on earth, but it is nevertheless important to stress how imporant water is to the living world. It is generally acknowledged that the primitive atmosphere of the earth was composed of a mixture of hydrogen, oxygen, nitrogen, and carbon. This composition allowed for the formation of very stable molecules such as methane, ammonia and water – the basic components of life. Water also plays a fundamental part in the reactions that lead to the formation of amino acids. For example, the combination of water and methane produces the formaldehyde molecule which, combined with hydrocyanic acid (a mixture of methane and ammonia), leads to the formation of a simple amino acid, glycine. Another example: the addition of five formaldehyde molecules produces the amino acid called ribose, and the addition of five molecules of hydrocyanic acid forms the amino acid called adenine.

Another example of the “biological” role of water lies in the mechanism of converting solar energy into chemical energy, which is an essential reaction in any biotic environment. This mechanism is known as photosynthesis; it uses carbon dioxide and water to form the molecule glucose. In this way, water is a source of electrons but also a source of the oxygen gas necessary for breathing.

Isotopes of Water

Water is a mixture of various combinations of oxygen and hydrogen isotopes that differ from each other depending on the number of neutrons associated with the protons in the nucleus. The hydrogen atom has two stable isotopes and one unstable isotope: in addition to the most common form, ¹H (formed of a nucleus containing a proton around which an electron revolves), are deuterium ²H and tritium ³H. Unlike deuterium, tritium is radioactive and has a half-life of 12.26 years. It can be found in

the atmosphere following nuclear reactions (nuclear tests account for the creation of most tritium), but is also produced as a result of interstellar radiation of nitrogen.

However tritium is rare and the ratio of ³H to ¹H in rainwater is roughly 10^-18. Oxygen has three stable isotopes, ¹⁶O, ¹⁷O and ¹⁸O (Table 2.2), as well as three unstable isotopes with masses of 14, 15 and 19.

Given the number of hydrogen and oxygen isotopes, a great many combination can be formed (18 have been found). The most significant of these are deuterium oxide D₂O and deuterium hydroxide HOD. Deuterium oxide or heavy water is used to slow down neutrons during nuclear reactions. This heavy water is physically similar to

“light” water (H₂O) but its melting and boiling points are 3.79 ^°C and 101.42 ^°C, respectively.

Table 2.2 : Relative proportion of stable isotopes for oxygen and hydrogen.