Caption: A phase diagram of water on a semi-log plot.

Features:

1. The phase diagram applies to thermodynamic equilibrium: that timeless state where the thermodynamic variables are NOT changing.

2. The horizontal axis is temperature in Celsius degrees (C) on the bottom and kelvins (K) on the top.

3. The vertical axis is pressure in pascals (Pa) on the left-hand side and in bars on the right-hand side: 1 bar = 10**5 pascals (Pa) = 100 kilopascals (kPa) = 0.987 atmospheres (atm) ≅ the air pressure near the surface of the Earth.

4. It helps to understand the plot by considering the horizontal level of ordinary Earth-surface air pressure which is at about 100 kPa = 10**5 Pa.

   As one goes to the right, one passes through the three phases: ice, liquid water, water vapor.

   Water vapor dissolved in air exists in the regions where water is ice or liquid water as we well know.

5. At different pressures, condensation and vaporization happen at different temperatures.

6. Now note that below a certain pressure there is NO liquid phase (called the triple point in general), and above a certain pressure (called the critical point in general), there is NO distinction between liquid phase and gas phase. So the region where the liquid phase exists as distinct phase of matter is rather narrow.

   This conclusion is true for all materials, NOT just water.

   The upshot is that a distinct liquid phase is a rather delicate phase of matter and, in fact, in many astrophysical environments the pressure is too low or too high for it, and consequently many astrophysicists NEVER think of the liquid phase at all for any substance or material.

   However, life as we know it requires liquid water, and so astrobiologists spend all their time thinking of liquid water.

7. For water, the triple point = 61173 Pa, 273.16 K (where all three phases of water can co-exist at once) and the critical point ≅ 22.064 Mpa, 647 K (beyond which the distinction between liquid water and water vapor ceases to exist.

8. Also for water, there are known to be circa 2023 19 ice (sub)phases, but the plot shows only 15 ice (sub)phases numbered in Roman numerals I, II, III, ... , XV.