Liquids and Solids

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Liquids and Solids
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Phase Diagrams

• A phase diagram is a summary of temperatures and
  pressures at which the various solid, liquid, and
  gaseous phases of a pure substance exist.
• Figure shows the phase diagram for water with
  regions marked Liquid, Ice and Vapour.
• The solid lines define the boundaries of the
  regions of pressure and temperature at which each
  phase is the most stable.

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• A phase diagram is a map divided into regions
  that tell us which phase is the most stable under
  the corresponding conditions. At 20C and 760
  Torr the most stable phase of water is liquid.
  Which phase is the most stable at 150C and 750
  Torr?
• Describe the physical states and phase changes of
  water as the pressure on it is increased from 5
  Torr to 800 Torr at 70C.

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• The points on a phase boundary show the
  conditions under which two phases coexist in
  dynamic equilibrium.
• The liquid-vapour phase boundary is the plot of
  the vapour pressure of the liquid, because it
  shows the pressures at which the liquid is in
  equilibrium with its vapour. The same may be
  explained for the liquid-solid and solid-vapour
  phase boundaries.

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• The slope of a line helps us predict how the
  phase change it represents responds to
  compression.
• If we compress a sample of water in equilibrium
  with vapour, the vapour pressure will decrease,
  and higher temperatures will be needed to reach
  the same equilibrium again.
• The solid-liquid phase boundary leans over
  slightly to the left. It means if we increase the
  pressure on a sample of ice just below 0C it
  will eventually melt, i.e. lower temperatures
  will be needed to reach the same equilibrium
  otherwise the ice melts.
• Water freezes at a lower temperature under
  pressure.

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• Most liquids freeze at higher temperatures when
  subjected to pressure because the solid phase is
  denser with molecules packed more closely
  together than in the liquid phase, and the
  pressure helps to hold the molecules together.
• Water is an exception to this general rule. Ice
  melts under pressure because water is denser than
  ice many of the hydrogen bonds in ice are broken
  when ice melts, thereby allowing ice to shrink
  into smaller volume. This signifies that pressure
  favours the denser liquid. This effect is thought
  to contribute the advance of glaciers.
• The same is true for Gallium and Bismuth.

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• In the phase diagram for other substances, the
  solid-liquid phase boundary doesnt lean over to
  the left.
• As can be predicted from the phase diagram for
  CO2 the freezing point rises as pressure is
  applied.

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• A triple point is a point on a phase diagram
  where three phase boundaries meet. For water, it
  occurs at 4.6 Torr and 0.01C. Under these
  conditions all three phases coexist in dynamic
  equilibrium.
• Unlike the freezing and boiling points of a
  substance, which depend on the applied pressure,
  the triple point of water is a fixed property. In
  fact it is used to define the size of the the
  kelvin by definition, there are exactly 273.16
  kelvins between absolute zero and the triple
  point of water. The freezing point of water is
  0.01K below the triple point.

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Critical Properties

• The liquid-vapour phase terminates at a point
  (the point C).
• Imagine a tube containing water and its vapour at
  25C and 24 Torr ,its vapour pressure at 25C
  (point A). As this system is heated, the vapour
  pressure increases (point B and other higher
  points on the curve), but never boils because the
  system is closed and the vapour cannot escape. At
  200C the pressure reaches to 11700 Torr (15.4
  atm, point B) and the vapour is very dense
  because of its high pressure. At point C,
  temperature is 374C and pressure is 218 atm.
  Here, the density of the vapour is so high equal
  to the remainder liquid.

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• At the point C there is no dividing surface
  between the vapour and liquid phase. A single
  uniform substance fills the container. It shows
  the properties of gases. This is a temperature
  (374C) above which the liquid state does not
  exist anymore. This temperature is called the
  critical temperature, Tc, the temperature above
  which water cannot be condensed to a liquid. The
  corresponding pressure is called the critical
  pressure, Pc, of the substance, 218 atm for
  water.
• A substance can be liquefied by applying pressure
  only if it is below its critical temperature. It
  is useless to try to liquefy a gas by applying
  pressure if it is above its critical temperature.

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• Substance Tc (C) Pc (atm)
• He -268 2.3
• Ne -229 27
• Ar -123 48
• Kr -64 54
• Xe 17 58
• H2 -240
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• O2 -118 50
• H2O 374 218
• N2 -147 34
• CO2 31 73
• CH4 -83 49

• The critical temperature of oxygen is -118C,
  so it cannot be compressed to a liquid at r.t.
  and oxygen tanks contain only gaseous oxygen.
• A substance just above its critical pressure is
  called a supercritical fluid. Although it is a
  gas, it is so dens that acts like a liquid and
  can be used as a solvent for liquids and solids.
• Supercritical CO2 can dissolve organic compounds
  and can remove caffeine from coffee beans and
  extract perfumes from flowers without
  contaminating the extracts with potentially
  harmful solvents.