From our birth, we are seeing that water like another matter can have three forms i.e. solid,liquid and water.Also we know that these forms of water can't occur simultaneously i.e. at room temperature it is liquid,at 0 degree celsius it is solid(ice) and above 100 degree celsius it is gas.But you know, that water can exist in three forms simultaneously.Yes it is true and can be proven by science also,this is called as Triple Point.

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What is Triple point??

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Triple point of a substance is specific conditions of temperature and pressure at which three forms of a substance i.e. liquid,gas and solid can exist simultaneously with each other in thermodynamic equilibrium.E.g. the triple point of mercury occurs at a temperature of −38.83440 °C and a pressure of 0.2 mPa.

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Triple point of water and how to get it??

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From above definition,we can conclude that triple point of water is a temperature and pressure at which all three forms of water i.e. gas,liquid and solid present simultaneously in thermodynamic equilibrium.It means there are certain conditions of temperature and pressure that we have to achieve in order to get Triple point of water.

In case of water we have to acquire a temperature of 273.16 K and pressure about 0.006atm to get the Triple point of water.

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Above figure showing conditions to acquire Triple point of water.The figure is also showing that Triple point of water is present between of solid i.e. ice and liquid phase.The pressure below Triple point result in the formation of ice and above that will form liquid and gas.
If we see the above graph carefully we are introducing with Critical point.So Critical point is a point on a phase diagram at which both the liquid and gas phases of a substance have the same density, and are therefore indistinguishable.