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.

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### Reference:

Engineering thermodynamics by P.K. Nag