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Water is the most abundant liquid-75% of the earth's surface is full of oceans, lakes and rivers. It plays a vital role in the life processes of plants and animals. Oceans, saline lakes and inland seas hold more than 97.3% of the total global water, rest is fresh water in polar ice and glaciers, lakes, rivers etc. Only 0.003% of the total global water is available for human consumption.

Water is the only chemical species that occurs naturally of the earth in all three states, namely, solid, liquid and gaseous. It is close to being the universal solvent; almost anything dissolves in water, at least to some extent. It expands on freezing, something that very few liquids do. It has a usually high specific heat capacity (4.177 J g-1 K-1) and enthalpy of vaporization. Even the very liquid nature of water is interesting. The other comparable hydrides of nitrogen and fluorine (i.e. ammonia, NH3 and hydrogen fluoride, HF) are gaseous over all, or most, of the range of temperature for which water is a liquid.

When water is cooled, its density first increases and then, below 277K, decreases. When it is frozen to ice its density decreases still further to 0.917 g cm-3 at 273 K. It is this unusual variation in density that makes aquatic life possible in cold waters. When the temperature of the air falls below the freezing point, the water at the surface of water-bodies cools first, and then, being denser than the water below it, sinks. Warmer water rising to the top is cooled and sinks in turn, and so on until all the water is at the temperature of its maximum density. When the surface water is cooled below this temperature, it remains on top (being less dense now) and eventually freezes there. The ice, being less dense than water, stays on the surface freezing the water-body at the surface. Since both ice and water are poor conductors of heat, the water below the ice thereafter loses its heat very slowly and the ice layer thickens very slowly. If the cold water and ice were denser than the warmer water, lakes and rivers would quickly freeze from the bottom up. This would have made survival impossible for aquatic animals and plants.

Physical Properties of Water

Water is a volatile, mobile liquid with many abnormal properties, most of which can be attributed to its structure and the presence of extensive hydrogen bonding.
Some physical properties of H2O

Physical Property


Molecular mass, m/amu


Melting point, T/K


Boiling point, T/K


Temperature (T/K) of maximum density


Maximum density, g cm-3


Density, at 298 K g cm-3


Ionization, constant, K/(mol dm-3) 2

1.008 10-14

Dielectric constant at 298 K


Viscosity, /centipoises


Solubility of NaCl at 298 K, g dm-3


Solubility of BaCl2 at 298 K g dm-3


Enthalpy of formation, Hf kJ mol-1


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