When we were discussing fundamentals of
thermal engineering, we have discussed various basic concepts of thermodynamics
such as “Concept of pure substance”
and we have studied there that a substance will be termed as pure substance
which will have homogeneous and invariable chemical composition throughout its
mass even if there is any change in phase too.
In simple words, we can say that a pure
substance will have fixed chemical composition and homogeneous throughout the
mass
Today we will see here the fundamentals
of phase diagram of water, in the subject of thermal engineering, with the help
of this post. We will see here each basic characteristic of phase diagram of
water and we will also see here how to read the phase diagram of water.
So let us see, triple phase diagram of water
Triple phase diagram of water is
basically the graphical interpretation of two variables i.e. pressure and
temperature and water phases will be changed due to the variation of these two
variables.
Following figure displayed here
indicates the triple phase diagram of water. Pressure will be displayed here at
vertical i.e. Y-axis and temperature will be displayed here at horizontal axis
i.e. X-axis.
Before going in detail explanation, we
must understand here that why we need to read a phase diagram of water.
As we know that water will have total
three phases i.e. solid, liquid and vapour. In order to secure the information
that when these three phases of water will be in equilibrium condition with
each other or when any two phases will be in equilibrium condition or when
water phase will be changed completely from one phase to other phase or at
which pressure and temperature water will undergo through a phase change, we
need to read the phase diagram of water for answering the above questions.
Triple point
We can see triple point in above phase
diagram of water. Phase diagram of water will locate one point in curve at
which all phases of water will coexist together. At triple point, pressure will
be 4.58 mm of Hg and respective temperature will be 0.00750C.
Let us see one live example to
understand the concept of triple point or concept of coexisting of all three
phases of water at pressure 4.58 mm of Hg and respective temperature 0.00750C.
Let we have one container and container is
having with water, ice and vapour. I mean to say that suppose ice is floating
inside the container and water is also available in container and
simultaneously vapour is also present above the water level surface inside the
container.
In general what will be happened? Ice will be melted and vapour will be
condensed and we will have water in liquid phase only.
Suppose we have
considered the concept of triple point and hence we have maintained the
pressure 4.58 mm of Hg and temperature 0.00750C. Now at this
situation where pressure and temperature are according to triple point, ice
will not be melted in to liquid and vapour will not be condensed in to liquid
and water will have three phases there i.e. liquid, solid and vapour.
This is the basic concept of triple
point in phase diagram of water and we must have to note here the respective
pressure value (4.58 mm of Hg) and temperature value (0.00750C) for
securing the triple point.
Triple point is also termed as non
variant point because degree of freedom at this point will be zero. As degree
of freedom at triple point will be zero and therefore none of the variables
will be changed when all the phases exist.
In simple words, we can say that three phase
or triple point will only be available at a particular pressure and
temperature.
Vapour pressure curve
Vapour pressure curve, as shown in
figure by AB, will represent the equilibrium between liquid phase and vapour
phase. Vapour pressure curve will provide the information about the
combinations of pressure and temperature at which vapour pressure become equivalent
to atmospheric pressure or we can say that we will have information about the combinations
of pressure and temperature at which liquid will be started to boil.
Let us see one example here. We have one
point at vapour pressure curve which will have pressure value equivalent to atmospheric
pressure and temperature value 1000C. We can say that a point will
be termed as boiling point of water at which water will be started to boil. We
have also displayed the boiling point in above figure and this point indicates
that at a pressure equivalent to atmospheric pressure, water will be started to
boil at a temperature of 1000C.
Now as we can observe here from vapour
pressure curve that with increase in pressure value, boiling point of water
will also be increased.
Let us see the point B on vapour
pressure curve i.e. critical point. Critical point is a point beyond which liquid
phase will not exist and there will be only vapour phase beyond the critical
point. For water, critical point will have pressure value 217.75 atm and
temperature value 373.990C.
If water is heated above 373.990C,
then in that situation water will be in vapour phase and could not be converted
in to liquid phase as we have studied above that beyond critical point liquid
phase will not be exist and there will be only vapour phase.
Sublimation curve
Sublimation curve, as shown in figure by
yellow line, will represent the equilibrium between solid phase of water and
vapour phase of water.
If ice (solid phase of water) is converted directly in to vapour phase without
going to liquid phase then such phenomena will be termed as sublimation and it
will be done at quite low pressure. We have seen above that triple point for
water will be at a pressure of 4.58 mm Hg and therefore sublimation phenomena
will be possible below 4.58 mm Hg of pressure.
Fusion curve
Fusion curve, as shown in figure by brown
line, will represent the equilibrium between solid phase of water and liquid
phase of water. Let us see the fusion curve in above figure which is displayed
by brown line.
We can observe here that fusion curve is
slightly aligned towards the vertical axis and hence we can conclude here that
with increase in pressure value, melting point of ice will be reduced.
Let us see the normal freezing point
which is displayed in above figure and we can see here that for pressure of 1
atm, water will be started to freeze at a temperature of 00C. Normal
freezing point of water will also be termed as melting temperature of ice.
We will discuss another topic i.e. "PV diagram for pure substance in thermodynamics"in our
next post in the category of thermal engineering.
Do you have suggestions? Please write in
comment box
Reference:
Engineering thermodynamics by P.K. Nag
Image courtesy: Google
No comments:
Post a Comment