Water is characterised by certain unique features which make it a marvelous resource:
1. It exists as a liquid over a wide range of temperature, i.e., from 0° to 100°C. Water exist on our planet in three physical states {i.e., state of matter) under the ambient conditions that normally occur. Water can be a liquid (water), a gas vapour (clouds), or a solid (ice).
2. It has the “highest specific heat”, i.e., how much energy is needed to increase the temperature of each gram of substance to one degree and a high “latent heat of fusion”, which is the energy required to melt 1 gram of ice at 0°C.
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These properties make lakes slow to thaw and warm in the spring and slow to cool and freeze in the fall, thus providing exceptionally stable thermal environments for aquatic organisms.
3. It has a high latent heat of vapourization (A change of state from liquid to vapour at constant temperature also requires the input of energy, called the latent heat of vapourisation) hence; it takes a huge amount of energy for getting vaporized. That’s why it produces a cooling effect as it evaporates.
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4. Due to high surface tension and cohesion it can easily raise through great heights trunk even in the tallest of the trees.
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5. It has anomalous expansion behaviour, i.e., as it freezes; it expands instead of contracting and thus becomes lighter.
6. It is an excellent solvent for several nutrients. Thus it can serve as a very good carrier of nutrients, including oxygen, which is essential for life. More substances dissolve in water than in any other liquid.
For this reason, water is often called the “Universal Solvent”. The reason for water’s excellent dissolving capability relates to its polarity; water offers positive and negative charges to which other atoms of molecules can attach.
7. The temperature/density relationship of water is also unique. Most liquids become denser (heavier) as they cool. Water also rapidly becomes denser as its temperature drops, but only to a certain point. Water reaches its maximum density at 39.09 degrees F (3.94 degrees C), and then it decreases slightly in density until it reaches 32degrees F (0 degrees C), the freezing point.
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At this point, ice forms and its density decreases sharply. Ice, therefore, is much lighter than liquid water and thus forms at the surface of lakes rather than at the lake bottom. A second important consequence of the temperature/density relationship of water is the thermal stratification of lakes.
Energy is required to mix fluids of differing densities, and the amount of energy necessary is related to the difference in density. In the case of lakes, this energy is provided primarily by wind.
Therefore, the changes in water density that accompany rapidly decreasing water temperatures in the metalimnion during summer stratification are of great importance. The metalimnetic density gradient provides a strong and effective barrier to lake mixing.