The water molecule – Environmental Encyclopedia

A water molecule is made up of two hydrogen atoms and one oxygen atom joined by a covalent bond.. That is, the two atoms of hydrogen and that of oxygen are joined by sharing electrons. Its formula is H2O.

Through spectroscopic and X-ray analysis, the bond angle between hydrogen and oxygen has been determined, which is 104.5°, and the average distance between hydrogen and oxygen atoms , which is 96.5 pm or, which is identical, 9.65 10-8 mm.


The arrangement of electrons in the water molecule gives it an electrical asymmetry due to the different electronegativity of hydrogen and oxygen. Electronegativity is the ability of an atom to attract shared electrons into a covalent bond.

Since oxygen is more electronegative than hydrogen, it is more likely that electrons, which have a negative chargeare closer to the oxygen atom than to the hydrogen atom, which causes each hydrogen atom to have a certain positive charge called the partial positive charge, and the oxygen, a negative, since it has the closest electrons. This means that water is a polar molecule, because it has a negative and positive part or pole, although the whole molecule is neutral. Almost all of its physico-chemical and biological properties derive from this polar character.

When two water molecules are very close to each other, an attraction is established between the oxygen of one of the molecules, which has a partial negative charge, and one of the hydrogens of the other. molecule, which has a partial positive charge. Such an interaction is called hydrogen bonding or bonding, and the water molecules are arranged in such a way that each molecule can associate with four others. This interaction is what happens with ice.

These hydrogen bonds form between a partially negatively charged atom and a partially positively charged hydrogen, so they are not unique to water. It also occurs between nitrogen, or fluorine, and hydrogen in other molecules such as proteins or DNA.

Regarding the physicochemical properties of water, we can highlight the large solvent capacity, its high specific heat and its high vaporization heat, its great cohesion and adhesion, its abnormal density and its chemical reagent.

Solubilization of salts in water

Water is capable of dispersing a large number of compounds within it due to its polar nature. Thus, with salts, which are ionic substances, the water molecule orients its poles according to the charges of the ions, opposing the negative pole to the positive ions (salt cations) and the positive pole to the negative ions (salt anions ). With polar substances, such as ethanol, water acts in the same way, opposing one pole to the opposite pole of the substance.

Specific heat is the amount of heat that must be delivered to one gram of water to raise its temperature by 1 ºC.while The heat of vaporization is the amount of heat that must be applied to one gram of liquid for it to turn into one gram of vapor.. Water has a high specific and vaporization heat due to hydrogen bonds, because to raise its temperature, water molecules must increase their vibration and, to do this, break the hydrogen bonds, whereas to pass a gram of steam .

Water has a high specific and vaporization heat due to hydrogen bonds, because to raise its temperature water molecules must increase their vibration and to do so break the hydrogen bonds, while to convert one gram of liquid to vapor, the hydrogen bonds must also be broken.

Vapor pressure and surface tension of water in contact with air
temperature °C Steam pressure mm/Hg Surface tension dynes/cm
0 4.58 75.6
5 6.54 74.9
ten 9.21 74.2
fifteen 12.8 73.5
twenty 17.5 72.8
25 23.8 72.0
30 31.8 71.2
Source: The Encyclopedia of the Urban Environment

Cohesion is the tendency of two molecules to stick together.and water has high cohesion due to its hydrogen bonding. Adhesion is the tendency of two different molecules to stick together.and water has strong adhesion to ionic and polar compounds.

Ice has a lower density than liquid water and therefore floats, which has important biological consequences for the ecology of aquatic organisms. Water can react with other compounds through hydrolysis, hydration, etc. reactions.

In addition, water in solution ionizes to a small extent, producing the following equilibrium:

H2O ‹ – › H3O+ + OH-

This balance will form the basis of the pH scale.

The suitability of the aqueous medium to living beings is a consequence of these physico-chemical properties. For example, compounds react better the more they are disintegrated, which is favored in an aqueous medium for ionic and polar compounds.

Water, due to its high specific heat, allows the temperature of the organism to remain relatively constant even if the ambient temperature varies, while its high heat of vaporization allows vertebrates to have an efficient means of losing heat. heat by evaporation of sweat.

Viscosity and density of water
temperature °C Density grams/cm3 Absolute centipoise viscosity Centistokes of kinematic viscosity temperature °F
0 0.99987 1.7921 1.7923 32.0
2 0.99997 1.6741 1.6741 35.6
4 1.00000 1.5676 1.5676 39.2
6 0.99997 1.4726 1.4726 42.8
8 0.99988 1.3872 1.3874 46.4
ten 0.99973 1.3097 1.3101 50.0
12 0.99952 1.2390 1.2396 53.6
14 0.99927 1.1748 1.1756 57.2
16 0.99897 1.1156 1.1168 60.8
18 0.99862 1.0603 1.0618 64.4
twenty 0.99823 1.0087 1.0105 68.0
22 0.99780 0.9608 0.9629 71.6
24 0.99733 0.9161 0.9186 75.2
26 0.99681 0.8746 0.8774 78.8
28 0.99626 0.8363 0.8394 82.4
30 0.99568 0.8004 0.8039 86.0
Source: International Critical, Tables, 1928 and 1929

Water also has high cohesion and adhesion. This property of water is exploited by higher plants to transport nutrients in solution from the roots to the leaves.

Water is involved, among other things, in hydrolysis reactions, characteristic of proteins in digestive processes, such as the breaking of the peptide bond between amino acids:

Break the peptide bond between amino acids

Wastewater treatment group. Polytechnic University School. University of Seville.

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