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Water and Its Peculiar Properties
Human Body and Water
Water is one of the most important substances known to man. Man can live for a few weeks without food, but only a few days without water. Almost three-fourths of the earth’s surface is covered by water. It is present on the solid part of the earth’s surface as lakes, streams, waterfalls, and glaciers.
The human body is approximately fifty percent water. Water is essential in digestion, blood circulation, waste elimination, and body temperature regulation. Indeed, the activities of every cell in the body take place in a watery environment.
Substances that may be found in water
All natural waters, even when not polluted by people, are impure since they contain dissolved substances. Rainwater is relatively pure, the chief impurities being dust and dissolved gases. Sea water contains about 3.6% of dissolved solids, principally sodium chloride. The types of impurities in the water on the earth’s surface depend upon the nature of the soil and rocks which the water has passed through. These impurities may be classified as:
- Suspended solids: sand, clay, mud, silt, organic materials (such as bits of leaves), and microorganisms.
- Dissolved gases: oxygen, carbon dioxide, nitrogen, ammonia, nitrogen oxides and hydrogen sulfides.
- Dissolved salts: chlorides, sulfates, and bicarbonates of some metals.
- Dissolved organic substances from the decay of plants and animals.
Water that contains dissolved calcium, magnesium and iron salts, such as bicarbonates, sulfates, chlorides, and nitrates are called hard water. On the other hand, soft water, such as rain water does not contain these salts. When soap solution is added to hard water, insoluble salts of calcium and magnesium are formed to form carboxylates. Lather is not formed until the above metals are all precipitated.
The hardness of water may be temporary or permanent. The presence of bicarbonates of calcium and magnesium in water may cause it to have temporary hardness and may be removed by boiling. When the anion present in water is not bicarbonate but sulfate, nitrate, or chloride, the hardness is known as permanent hardness.
Miscibility and immiscibility
Water is considered as the most important solvent so that when the solvent is not mentioned, it is assumed that water is used. A large number of substances readily dissolve in water. Due to the polar nature of water, substances that are either ionic or polar in character are soluble in it, while nonpolar substances such as gasoline and oil are insoluble.
When water solutions of some soluble compounds are evaporated, the dissolved compounds separate as crystals. Crystals that contain a definite proportion of water as part of their crystalline structure are called hydrates. The water contained in a hydrate is called water of hydration or water of crystallization. When the water of hydration is removed from a hydrate, the resulting compound is called anhydrous, meaning “without water”. Substances that lose their water of hydration on exposure to air are called efflorescent. On the other hand, substances that absorb moisture from the air are hygroscopic. Due to this property, they are used as drying agents. One group of hygroscopic substances, which are called deliquescent, remove moisture from the air and form droplets of saturated solution. Thus, all deliquescent substances are hygroscopic, but not all hygroscopic substances are deliquescent.
The presence of water in a substance is sometimes undesirable. For instance, water in gasoline promotes corrosion of metal engine parts. Hence, methods for detection and removal of water are important both in the laboratory and in the industry.