Three fourth of the surface of the earth is covered with oceans. Water sustains the life on our planet. Salts and many organic compounds like sugars are taken by our bodies in the form of their solutions in water. Water is the medium in the cells of our bodies and salts conduct their business of sending the electrical signals to intended parts of the body from the brain. If you lose the water from your body through perspiration or other processes you have to replenish the deficiency by drinking the water else the solutions inside your body and blood will thicken and equilibrium will be disturbed and you will fall ill.
Since water was there from the beginning, our evolution took place in such a manner so as make use of water. Like the volume of water on this earth, our bodies contains about 75-80% water. We use water in umpteen number of ways. Washing the clothes and our bodies, growing the crops, generating electric power are only a few of them. In fact, life cannot be imagined on this planet without water.
Water exists in many places on the earth from some of which it is easily accessible and others from which special techniques have to be applied for obtaining it. It is available on the surface of earth in the form of rivers, lakes and ponds. It exists in the sandy seams underneath the earth from where wells have to be dug to obtain it. Although the water available on the land is a miniscule fraction of the water in the oceans, the water in the oceans cannot be used as such from drinking because of very high salinity. That is why the adage was coined “Water water everywhere but not a water to drink”. Water is a very powerful solvent. Almost nothing can remain unaffected which comes in contact with it.
Structure of water molecules is very simple. It is also made of atoms like other chemical compounds. It is made up of 2 hydrogen atoms attached to 1 oxygen atom But its geometry, as one would expect is not linear with oxygen and two hydrogen atoms on either side in a straight line. No, it is unique. Even being a very simple molecule, it has abnormal properties because it is not linear but V-shaped. This gives water polarity which enables it to dissolve so many salts.
Water never remains static at one place. It is always in the dynamic state and this process is guided by temperature and pressure variations. Water molecules are ambulatory in nature going places and returning back to ocean ultimately but after different times depending on the place they have visited.
Due to evaporation, some of water molecules escape from ocean, rise up and form the clouds and drift to different areas. When the reach the cooler regions, they precipitate and fall back on the earth. If the temperature is very low then water deposit as ice, otherwise it becomes rain and flows to the lowest levels available on earth. Some of these molecules collect in lakes, some get absorbed in the earth and percolate down into the layers where there are porous spaces available. When the mountain ice melts it forms the rivers and begin the journey seawards. They may reach the ocean back or may not. On it way, river picks of more water from its drainage basins. Some rivers become very mighty. The water on the land again evaporates and some of it reaches ocean through precipitation. The time a water molecule spends a given location is called its resident time. Thus the water which percolates into earth has highest resident time.
The water in the rivers leach salts from the land in its contact and become saline. It continues to do so till it empties itself into the sea. So seawater becomes saline. Over a period of millions of years, equilibrium has been established and a general average composition has been attained by the sea water. There might be areas of short lived variations like the mouth of the river and sea interface or the areas where the weather is still and hot.
The water which percolates through layers of earth into a water reservoir is comparatively cleaner because suspended impurities are filtered off from it. But resources are required to pinpoint it and then dig the well. After the water source is found, a tube has to be thrust inside to prevent the formations from collapsing into the well and closing it. The river water can be used for drinking purposes but not before its purification which involves getting rid of suspended impurities, disease causing microbes. The water cleaning plants has to be set up for this purpose. The river waters have suffered by the pollutants which the industrial waste dumped into them introduces.
Water has some peculiar properties which make it so special. One of these is that water has maximum density at 4 degree Celsius. This means that the water which is at this temperature will be the heaviest. It is due to this property that is crucial for the survival of aquatic species in the extremely cold weathers when water in the ocean freezes. When the temperatures begin to drop, the density of water begins to shrink and it becomes heavier. At 4 degree Celsius, it becomes the heaviest and sinks to bottom and lighter water surfaces up. This again cools and a stage comes when temperature reaches freezing point, the upper crust freezes. But the ice which forms at the surface is bad conductor of heat and does not allow the water below to loose heat and freeze. Thus the water remains in the liquid form below allowing the aquatic species to survive.
Water can exist in three physical states like other substances but it has some anomalous behavior. While most of other substances experience decrease in the volume-shrinkage-the water increases in volume while going from liquid to solid state. Thus the ice is lighter than the water. In fact 1 liter of liquid water yields 10 liter volume of ice.
Another interesting property of water is osmosis. In this process if two water solutions containing different amounts of dissolved salts are separated by semipermeable membrane, the water from the solution with lower salt concentration moves through membrane towards the solution of higher concentration and dilutes it till the concentration of salts in the both solutions becomes identical.
Let us illustrate this with an example. Suppose you take out a fish living in the highly salty sea water and put it in the fresh water. The osmosis phenomena will start. Fresh Water rushes inside the body of fish and fish will bloat. In a reverse case, the fish from fresh water will shrink when put into the seawater. Thus habitats of the fishes determine the properties of the animals living in them. Both are fishes but of different qualities.
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