“Vasile Alecsandri” High School, Galati, Romania
INTRODUCTION
Water is one of the predominant component of Earth, with a major role in the survival of organisms.
The seas and oceans ocuppy 71% of the globe’s surface and in most cells water is approximately 60% of their weight. She is the principal solvent and the dispersion environment of different substances. Water helps at the substances transport and numerous chemical reactions. In the same time, she is present in the human body in 60%, and man’s daily consumption is 2,5 l in food and liquids.
GENERAL CHARACTERISATION
Melting point: 0 ºC.
Boiling point: 100 ºC.
Maxim density: 1g / cm3.
Density at 25 ºC: 0,99701g / cm3.
NATURAL STATE
WATER STRUCTURE
In liquid state each and every water molecule is asociated with two others water molecules through hydrogen links. These explains why water’s boiling temperature is so high (100 degreeds C at a pressure of 1 atm), comparative with the boiling points of oxygen's neighboured combinations with hydrogen elements in the periodic system.
In liquid state each and every water molecule is asociated with two others water molecules through hydrogen links. These explains why water’s boiling temperature is so high (100 degreeds C at a pressure of 1 atm), comparative with the boiling points of oxygen's neighboured combinations with hydrogen elements in the periodic system.
In solid state (ice) she has almost a double number of hydrogen links than in the liquid state. Here are two hypothetical networks of water molecules. Dark blue represents oxygen and red hydrogen. Magenta bonds are bonds within each water molecule, light blue bonds are hydrogen bonds between molecules. Some bonds extend up or down out of the plane of the diagram. Hydrogen-Oxygen bonds extending up are shown by a hydrogen atom superimposed on the oxygen. If a water molecule is show with only one bond, the other extends down. Hydrogen bonds in and out of the plane of the diagram are not shown.
In all ice structures known, hydrogen is in two-fold coordination with oxygen, and oxygen in fourfold (tetrahedral) coordination with hydrogen. This geometry is dictated by the bond structure of oxygen, not by considerations of ionic radius. In size terms the hydrogen is a mere bump on the oxygen atom. In all known forms of ice, the water molecules retain their identities as distinct molecules and bond to other molecules by hydrogen bonding
Ice is used as a refrigerating agent because it takes more energy to melt it than most other substances, due to strong hydrogen bonds. Melting ice remains at a constant 0°C (32F).
Because ice is less dense than water at 0°C (32F) a mass of ice occupies 9% more volume than an equal mass of water. This is why when water in pipes freezes it can cause the pipes to burst. Another important point about ice being less dense than water is that it floats.
In the following figures there are represented microscopic images of some snow crystals:
PHYSICAL PROPERTIES
Water is an incolor, inodorous and insipid liquid (at normal temperature).
CHEMICAL PROPERTIES
At normal temperature water (gas) reacts with metals and the reaction forms the metal oxide and hydrogen gas.
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