 Srikanth V. Kuttkur, Working as Assistant Professor, Department of Mechanical Engineering, Volchian Institute of Technology, Solapur. Today we are going to discuss the properties of pure substance. At the end of this session, students will be able to understand the phase change process of pure substance. Students will be able to draw a property diagram for phase change process. Moving topics will be discussed, first we will discuss what do you mean by pure substance. Then we will move towards the phase change of pure substance, how to draw the phase change process and what do you mean by pure substance. A pure substance is a system which is homogeneous in composition, homogeneous in chemical aggregation and invariable in composition. Homogeneous in composition means whatever the elements are there, these elements are chemically combined. The example is water. If you consist the mixture of water and steam in water, hydrogen and oxygen are combined in the form of H2O and whereas the steam as well as consist of H2O means that is homogeneous in composition. Homogeneous in chemical aggregation, we are going to consider the same example as well as the mixture of water and the mixture of steam means in both way they are chemically combined as H2O and invariable in composition means what the I am going to give the same example that is mixture of water and steam this is this can be considered as a pure substance. But if after some particular period of time that steam may be get converted into mixture of hydrogen and oxygen that is not going to considered as a pure substance. If these three conditions are satisfied then that system is called as a pure substance. Now we will discuss the phase change process of pure substance. Now here we are going to consider the example of generation of a steam at a constant pressure that is of one bar that is a normal atmospheric pressure. Now consider the first diagram in which we are going to consider the water which is at a less than 100 degree Celsius and the pressure exerted by this piston that is one atmospheric pressure means this process is taking place at normal atmospheric pressure. Now as the temperature of this water is less than the saturation pressure means generally obviously it is in liquid state. Now if you are going to supply the heat to this then the temperature of the water goes on increases that will be shown here means if the pressure is one bar the temperature of this water going to increase up to saturation temperature. Now for one atmospheric pressure the saturation temperature of water is 100 degree Celsius. The temperature of the water going to increase now this type of heat is called as sensible heat. Sensible heat means there is change in temperature but there is no change in the phase up to 100 degree Celsius the temperature of the water going to increase. Now once the temperature is reached 100 degree Celsius then there is no change in the temperature. Now whatever the heat supplied is there now that heat will be used to convert the saturated liquid which is at 100 degree Celsius to vapor. Now here if the temperature is 100 degree Celsius means some part of the liquid will get start converted into the vapor means this diagram will consist of water and water vapor which is generated by saturated liquid and vapor mixture means here if suppose for example if 100 molecules of the water are there in this means partly some particles are get converted into vapor and some particles are still remains in liquid state. If you proceed if you supply the heat further then remaining all particles will also get converted into vapor. Now this diagram will consist all the particles are in vapor state still there is a temperature is 100 degree Celsius means what here at this in this diagram all the particles are in liquid state and here all the particles are in vapor state. Whatever this amount is there that is called as the latent heat of vaporization. If you further supply the heat to this then the temperature of this vapor now start increasing. Now that vapor is called as the superheated vapor. Now pause this video for a while and try to draw this entire process or this all the process on TV diagram. Now this will be the TV diagram. Now here we are going to consider temperature versus volume. Now at initially whatever the first diagram is there means at start the liquid is in subcooled state means the temperature of that water is less than 100. Once we are supplying the heat the temperature of the water goes on increasing simultaneously the volume will going to increase means the fister will move upwards. Whatever this 1 to 2 amount of heat is there that is called as sensible heat which can be calculated by mcp delta T where the delta T is nothing but the initial water temperature and the saturated temperature for a particular pressure. Now in this case the temperature is 100 degree Celsius as the pressure is 1 bar. Whatever the initial temperature let us say suppose for example the initial temperature is 40 means the delta T value will be 100 minus 40 that is 60 and CP means specific heat of water means it is the amount of heat required to increase the temperature of the unit mass by unit degree Celsius and whatever the mass is there that you are going to multiply the mcp delta T value that is nothing but the sensible heat of the water. Now once the temperature reach to 100 degree Celsius the temperature will stop and whatever the amount of heat supplied is there that will used to convert the water into vapor. Now at this point all the liquid molecules are in liquid state and the temperature will remain constant. At point 3 there is 2 to 3 means what it will contains a water vapor and the water molecules means there is a mixture. This point 2 is called as a saturated liquid point. If you further provide the heat to the that mixture then all the liquid molecules are going to convert into vapor molecules means at point 4 all the water molecules are in vapor state. Now from 2 to 4 whatever the amount of heat supplied is there that is called as latent heat of vaporization. Now here we have to introduce one term that is called as dryness fraction. Dryness fraction it is nothing but the ratio of steam molecules present in the mixture to the total molecules available in the mixture. Now let us consider the example at point 2 at point 2 all the molecules are in vapor there is no steam molecules means the steam molecules are not available that is why the value of dryness fraction which is denoted by x at point 2 is 0. Now consider at 3 at 3 some parts are some molecules are in steam or vapor remaining particles are in water that is why let us say suppose for example at 3 out of 100 molecules there are 40 molecules are in vapor and remaining 60 molecules are in liquid state means 40 divided by 40 plus 60 you will get 0.4 as dryness fraction at 0.3. At 0.4 all the liquid molecules are in vapor states that is why 100 divided by 100 you will get x as 1. This 2 to 4 whatever the amount of heat supplied is there that is called as the latent heat of vaporization at 1 bar the value is 2 to 5 7 means what suppose for example if you are having the water which is at a saturated level 100 degree Celsius and the mass is 1 kg if you supply 2 to 5 7 kilo joule amount of heat what did that water is there saturated water will get converted into steam. Now if you further supply the heat to that saturated vapor the temperature of this goes on increasing because all the liquid molecules are in vapor state and whatever the amount of heat supplied is there that will tend to increase the temperature of that vapor here that is 5. T5 is the temperature of the superheated steam and the difference between the saturation temperature sorry superheated temperature and saturation temperature that difference is called as the degree of superheat and this amount is calculated by mcp delta T where Cp is now specific heat of water if the specific heat of water is not given you can take that value as in between 2 to 2.2 kilo joule amount of heat means by this way you can represent that entire process on TV that is 1 to 5 at 1 the water is in liquid state and at 5 there is a steam which is a superheated having the value of x that is dynastaction as 1. These are the references thank you.