We know that water boils at 100° C and freezes at 0° C. From personal experience, we also know that steam is hot and ice is cold, and that steam when cooled condenses into water, which when cooled further freezes into ice. In our case, we would need ice-cold steam, which seems like a rather absurd idea.
Now as it happens, water can undergo phase changes at temperatures other than 100° C or 0° C. This means that one could actually have boiling water at 50° C or melting ice at -5° C. The factor determining the phase-change temperatures is pressure.
From experiments, it is observed that with decrease of pressure, the boiling point of water gets lowered, while the freezing point gets slightly raised.
In fact, with a strong reduction of pressure from normal atmospheric pressure of 1 atm down to 0.00603659 atm (high vacuum), the boiling point of water drops from 100° C all the way down to 0.01° C.
At the same time, the freezing point increases slightly from 0° C to 0.01° C.
Thus, at 0.00603659 atm and 0.01° C, we get a system where water can boil and freeze at the same time. It’s weird, but that’s how nature works sometimes.
A pure substance other than water can also exhibit a similar behaviour where all three phases, namely solid, liquid and gas can co-exist in thermodynamic equilibrium. This phenomenon happens at only one specific combination of pressure and temperature. Across all other combinations of temperature and pressure, a substance can exist in either one or two phases.
The specific combination of pressure and temperature at which all three phases of a pure substance can co-exist is called the triple point.
Vid Cred: UCSC Physics
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