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• 1 media/floarting ice.jpg 2016-05-09T12:46:04+00:00 Physical states of water 28 image_header 2016-07-25T09:54:49+00:00 We all know that H₂O can exist as water, ice and vapour (steam).  These three states are referred to as physical states of matter.  These states depend on physical parameters such as temperature and pressure. At sea level on the surface of the Earth, regular pure water freezes to form ice at 0°C and boils (turns to vapour) at 100°C.  When water freezes or evaporates, it changes its state.  But the temperature at which these changes of state take place depends on the pressure that is exerted on the water surface by the atmosphere (called atmospheric pressure).  Atmospheric pressure is the weight of the column of air acting on a given surface and it increases with increasing height of the column of air.   At Earth’s sea level the atmospheric pressure is 1 atmosphere (atm), 1.013 bars or 101.3 kilopascals, depending on the units you want to use.  At the summit of Mount Everest, the atmospheric pressure is a mere 0.337 bars (or 33.7 kilopascals).  The boiling temperature of water depends on the pressure of the atmosphere above it.  (One can easily think of this as molecules of water wanting to escape from the liquid into the air to vaporize. Greater air pressure makes it harder for molecules to escape the liquid and become airborne).  If you were on the summit of Mount Everest and wanted to make a cup of tea, you would find that the water would boil at a mere 73°C (Note: If you decide to use the calculator in that link, enter 101.3 and 100C at the top and 33.7 for the bottom pressure).
  
  To fully characterise these changes of state with changing temperature and pressure, a phase diagram is used.  A general phase diagram for water often looks like the small diagram on the right. This diagram shows the range of temperature and pressure conditions at which distinct phases of a substance can exist.  Point T in this diagram is referred to as the "triple point" of water.  It is the a specific temperature and pressure  where three phases of water (solid, liquid, vapour) are stable. The pressure of this point is too low to be relevant to Earth (1/166 atmospheric pressure at sea level), but it is similar to the atmospheric pressure conditions on Mars.  Point C is called the "critical point" of water, where temperature and pressure conditions are great enough that liquid and steam are indistinguishable from one another. The critical point is relevant to some geologic environments on Earth, such as deep sea hydrothermal vents.
  
  The phase diagram at right is actually simplified.  For planetary systems, the real phase diagram for water is more complex.  In outer space, temperatures may extend to as low as absolute zero, and pressures may be at vacuum. So, please consider the customized, slightly quirky phase diagram below at and extended scale. In this diagram we use the more commonly used unit for pressure, a bar.  The difference between atmospheres and bars is minor, 1 atm = 1.01325 bar.
  
  
  
  We'll start with the axes.  You'll notice that the temperature axis goes from -273°C (or 0 K) to 400°C.  The pressure axis uses a logarithmic scale which means that every increment is 10 times more (or less) than the previous. That means that our pressure differential from sea level to the top of Mount Everest (1bar to 0.3bar) barely registers on that scale. But we do need that range if we want to consider conditions in the interior of Frost Ice Giants (Neptune, Uranus).  Let us consider the fields in the interior. We still have the regular three phases.  Everything in shades of blue is the land of the Frost Giants field of solids or ices.The Wall boundary between that land field, which in the simplified diagram seems to continue vertically, now curves and becomes nearly horizontal at approximately 10 Kbar (10 Kbar, or 10,000 bars would be equivalent to the pressure in an approximately 100 km deep ocean).  This means the even at very high temperatures, ices can exist at very high pressures.  The slightly different shades of blue indicate fields for different ices.  Unlike the 32 flavours of ice cream you can purchase in some stores on Earth, these different ices are all composed of the same ingredients, namely H₂O.  What differs is how the atoms are packed into the ice structure. The other thing you may notice is that the boundary between the land of dragons vapour and the liquid is dashed beyond 374°C.  This is is the critical point (point C in previous diagram), where the physical properties of liquid and vapour change and become similar. We have approximated where Earth's conditions are by placing the Earth on the diagram.  Clearly conditions on this planet can vary enough for us to see all three different phases of water.  But if the atmospheric pressure is low (looking at you, Mars), liquid is not stable and at very high pressures, only solids can exist.
  
  If you like more information about how phase diagrams work, we suggest you watch this video.