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18 Feb 2010 23:20:24 IST

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what exactly is triple point

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what exactly is triple point

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Guruprasad Raghavan

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20 Feb 2010 23:45:21 IST

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This is the point where an object exists in 3 states simultaneously and all 3 states are in equilibrium with each other.

THe triple point of water is 273.16K

ThIrStY of IIT

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15 Mar 2010 21:59:21 IST

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The triple point of substance is that at that point substance exhibit three of its states.for ex.triple point of water is temp. 0.01 degree C and 0.006 atm

edison

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15 Mar 2010 22:58:15 IST

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In thermodynamics, the triple point of a substance is the temperature and pressure at which three phases (for example, gas, liquid, and solid of that substance coexist in thermodynamic equilibrium. For example, the triple point of mercury occurs at a temperature of −38.8344 °C and a pressure of 0.2 mPa.

In addition to the triple point between solid, liquid, and gas, there can be triple points involving more than one solid phase, for substances with multiple polymorphs. Helium-4 is a special case that presents a triple point involving two different fluid phases . In general, for a system with p possible phases, there are ${p\choose 3} = \frac{p(p-1)(p-2)}{6}$ triple points.

edison

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15 Mar 2010 22:59:58 IST

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Triple point of water

A typical phase diagram. The dotted green line gives the anomalous behaviour of water

The single combination of pressure and temperature at which liquid water, solid ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 K (0.01 °C) and a partial vapour pressure of 611.73 pascals (ca. 6.1173 millibars, 0.0060373057 atm). At that point, it is possible to change all of the substance to ice, water, or vapor by making arbitrarily small changes in pressure and temperature. Note that even if the total pressure of a system is well above triple point of water, provided the partial pressure of the water vapour is 611.73 pascals then the system can still be brought to the triple point of water. Strictly speaking, the surfaces separating the different phases should also be perfectly flat, to abnegate the effects of surface tensions.

Water has an unusual and complex phase diagram, although this does not affect general comments about the triple point. At high temperatures, increasing pressure results first in liquid and then solid water. (Above around 10⁹ Pa a crystalline form of ice forms that is denser than liquid water.) At lower temperatures under compression, the liquid state ceases to appear, and water passes directly from gas to solid.

At constant pressures above the triple point, heating ice causes it to pass from solid to liquid to gas, or steam, also known as water vapor. At pressures below the triple point, such as those that occur in outer space, where the pressure is near zero, liquid water cannot exist. In a process known as sublimation, ice skips the liquid stage and becomes steam when heated.

The triple point pressure of water was used during the Mariner 9 mission to Mars as a reference point to define "sea level". More recent missions uselaser altimetry and gravity measurements instead of pressure to define elevation on Mars

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