Notes on the Unique thermal properties of water


1. It is a stable compound that does not decompose easily. It remains a liquid between 4º C and 90 º C and life too exists within this range.

2. It does not undergo quick changes of temperature because of its high latent heat of fusion. In fact, it requires 80 calories of heat to change one gram of ice into water, with no change in temperature. It would also mean that liquid water freezes only when a large amount of heat energy is abstracted from it. Thus, it protects organisms from thermal shock by preventing water from freezing inside a living cell even when the exterior is very cold.

3. It possesses high latent heat of evaporation (vaporization). In fact, during evaporation of water from vegetation, water bodies and ice surface nearly 536-596 calories per gram are absorbed. This means that a major portion of solar radiation is dissipated in evaporation of water from ecosystems. Such an energy flow moderates climates and makes life possible in all its diversity. It also means that a lot of heat can be lost with minimum loss of water during evaporation, such as transpira­tion in plants and sweating in mammals, which gives a cooling effect. Thus, it helps in regula­tion of body temperature of most terrestrial organisms.


4. It remains in liquid form on cool­ing with a maximum density at 4°C but expands above and below this temperature. When tempera­ture of deeper water (in sea and deep lakes) decreases below 4°C the water expands, rises up and gets converted into solid form, the ice on the surface. Ice at 0°C is less dense and hence, lighter than liquid water and floats. Thus, water rarely freezes solid in water bod­ies even during the coldest weather. The ice on top insulates the water below and helps survival of the aquatic life.

5. Water has a very high specific heat or heat capacity known for any substance (except liquid NH3, 1.27cal/g). Relatively, large amount of heat is involved in changing the temperature of water. To raise temperature of 1 ml. (1 g.) of water to 1°C (be­tween 15°C and 16°C) 1 cal of heat is needed. Thus, it ensures that the process for water to heat up is very slow, as changes in temperature are inversely proportional to the amount of specific heat. In this way it acts as a heat buffer or heat stabiliser and helps in main­taining the temperature of the living organisms. It also pro­tects the labile cell struc­tures from thermal destruc­tion by local short-lived re­leases of thermal energy.

6. Water has the highest boiling point (boils at 100° C). But for the higher boiling point, water would have been gaseous at normal temperature and it would have been difficult for the life processes to be carried on.

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