Phases
A phase is defined as a physically distinct but homogenous part of a system
separated from other parts by boundary surface.
Phase diagrams : The effect of changes in temperature or in pressure
or in the composition on the changes of the phase in a system can be understood
easily when it is presented graphically. The conditions under which the
different phases can co-exist are conveniently described by graphs called
phase diagrams.
Figure 17
The line XB is the locus of points (P, T) at which a solid and a liquid can
co-exist. Similarly co-existence of other phases namely liquid-gas and solid-gas
can be represented by lines XC and XA respectively. The point of intersection of
any two curves will indicate the co-existence of all the three phases
solid-liquid-vapor in equilibrium. This point is called a triple point.
Phase diagrams are classified based on the number of components as
- one component system
- two component system
One component system :
The Water System : The simplest and typical example of a one component
system is that of water. Three possible phases depending on pressure and
temperature are ice, water and water vapor.
The line XC is the vapor pressure curve for water which increases by
increasing temperature.
Each of the three lines XA, XB and XC indicates the two phase equilibria
and represents univariant states. The areas enclosed between these lines
represent regions of a single phase, i.e. bivariant states. Thus the
region AXB is ice, BXC is water and CXA is vapor.
X is the triple point where all the three phases (ice, water and vapor)
co-exist. The pressure at this point is found to be 4.58 mm. The freezing point
is 00C at 760 mm and changes by 10 when pressure changes
by 140 atm. Hence at 4.58 mm the freezing point is 0.0070C
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