September 20, 2020 5:35:05 pm
By Rachna Arora
Life can exist without sunlight, even oxygen, with many organisms on earth which can survive without these resources but without exception, all life forms need water. No wonder, the best bet to finding extraterrestrial life is to search for water.
An incredible compound simply made of two hydrogen atoms (hydrogen is flammable) and one oxygen atom (oxygen supports combustion) can extinguish the fire. This gives a sneak-peek into how wonderfully peculiar water is.
Water can move against the force of gravity as seen in capillary action in plants and our blood vessels, can stick together making its surface act like a stretched membrane by virtue of its property of surface tension enabling small insects to walk on water and also preventing the water to pass through the tiny holes in an umbrella, such are the diverse characteristics of this powerful molecule.
Usually, objects expand on heating and contract on cooling, even the Eiffel Tower becomes taller by 15 cm when temperatures reach 400C in Paris. But water shows anomalous behavior as it expands on cooling becoming less dense while changing from the liquid to the solid-state with maximum density at 4oC. That is why ice floats on water. In winters, while the surface of the water bodies may be frozen, this peculiar property insulates the water below enabling survival of aquatic life underneath.
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Its high heat capacity allows it to absorb and release heat slowly, as evident in its application as a coolant and hot water bottles thereby also making life in coastal areas a pleasant experience.
How uncommon is it to see solid and gaseous oxygen or molten copper and a copper wire within common temperature ranges? It’s not. But we do encounter the three phases of water practically every day. Water cycle ensures the endless change of one state of water to the other. This adds to the importance and versatility of water.
Another noteworthy property of water is that it can coexist as solid, liquid and gas simultaneously. The magic temperature happens at a so-called triple point of water, which is 0.01oC or 273.1 K and at a pressure of 0.006 atm. (This is the average pressure on the surface of Mars or only 0.06 percent of the normal atmospheric pressure). The coexistence of the three states seems improbable but at triple point water melts, evaporates and sublimes simultaneously with arbitrarily minute changes in temperature and pressure.
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We are familiar with the change of state – freezing/ melting, vaporisation / condensation or even sublimation. Pressure and temperature are the factors which determine the existence and coexistence of these phases. As pressure is increased, the temperature at which state change occurs increases and vice versa. The melting and the boiling point, 0oC and 100oC respectively we are familiar with occur at normal atmospheric pressure. That is why using a pressure-cooker speeds up the cooking process as the water boils at higher temperature due to high pressure inside the cooker. It is even possible to boil water at room temperature provided the pressure is suitably reduced.
At the triple point, the molecules of water having high energy get converted to vapour, as the energy is lost by the liquid, it freezes and this remains an ongoing cycle till the temperature and pressure are not changed.
The triple point of water corresponds to the minimum pressure at which water can exist in a liquid state, converts into steam without coming into the liquid state. Till last year, the triple point of water was used to define kelvin, SI unit of temperature and as a reference to define the triple point of other substances.
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The baffling properties of water do not end here; experiments have proved that hot water freezes faster than cold water.
We take water for granted, using it to quench our thirst and for our daily chores, it is anything but an ordinary liquid.
Truly, this wonder molecule is an elixir extraordinaire!
(The writer is PGT- Physics at Shiv Nadar School, Noida)
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