 Let me complete that question which was asked by Jawaharlal Institute of Technology. So I am going over there. See the question was about supersaturation. The question asked was supersaturation in the nozzle as steam or vapor expands isentropically. But supersaturation is the phenomena when a change of phase if it does not occur or does not commence exactly at the saturation line. So supersaturation is generally general process which may occur when the liquid tries to become a solid, solid tries to become a liquid, liquid tries to evaporate or a vapor tries to condense. Let us look at the case of a nozzle. When steam expands in a nozzle, I am showing a converging diverging nozzle. You have a high pressure P0 and it goes to a low pressure P1. And the process is almost isentropic or in the ideal case it is isentropic. In a real case, it is adiabatic, may not be reversed, it is almost exactly adiabatic. Now if you take say the HS diagram, this is the dry saturated vapor line. Let us say that we have our, this is our P0, P0 T0, so this would be assumed superheated. The phenomena of supersaturation would occur when superheated steam expands and let us assume isentropic expansion and let us say this is P1, so it will expand here. Now as the flow moves, this is the point where it intersects the x equal to one line. And if you look up the pressure here, may be this is the pressure at which it continues. But condensation does not exactly start here, condensation gets delayed and the vapor behaves as if it is superheated for some time beyond this. So although this is, these are the various states showing lower and lower dryness fraction, the steam remains dry and the state is not given by our steam tables of the HS diagram. The state is given as if the superheated states are extended or extrapolated into the two phase zone, that is supersaturation. In principle there is no basic theoretical limit for supersaturation but the flow becomes unstable and any small trigger, small roughness on the wall which causes secondary flow presents of a small impurity, a small speck of dust or for some reason if there is a small, very small level droplet that is sufficient to trigger the process of condensation. And when that happens, there is supersaturation, so you get into a supersaturated state and then there is some sort of a trigger which suddenly, sudden change to the expected state. This expected state if you see the detail usually leads to a loss in efficiency and the isentropic efficiency of the nozzle reduces if it goes through supersaturation. If you want more technical details on this, I refer you to a book by Moore and C. A. Verding. I think the title is two phase flow in turbines and separator. So this is recommended reading particularly for the supersaturation which occurs in steam nozzles. Although they talk of two phase flow, it is essentially the two phase flow of steam water that they are talking about. It is difficult to demonstrate in a laboratory the supersaturation but something which I recommend if you want to show the supersaturated state, I recommend you do the following simple experiment and I am sure you will succeed if not in the first attempt, in the second or definitely in the third attempt. What one should do is take a glass or a beaker, clean it absolutely neatly to the extent possible. Then take a chilled cold drink. A cola I find is better for some reason, something like your favorite cola whatever you decide and pour it in this almost up to the rim. Then cover it up with a dish and put it in the freezer. For some reason I find the older type of freezers evaporated directly on the freezer liner is better rather than the newer type of freezers or the frost free type of freezers and leave it undisturbed. See to it that the refrigerator is otherwise clean, the freezer door is not opened and closed unnecessarily and the refrigerator the freezer does not vibrate. You will find that after 3-4 hours it remains still chilled when you expect it to be frozen and if you find it still chilled in a good freezer in all probability what you have is a super saturated liquid state of that cold drink and all that you have to do is take a spoon, scrape off from the wall of the freezer I tell you should not be a frost free freezer. A few ice particles icicles and just drop them in this and you will notice that in front of your eyes from the point where that icicle is dropped the freezing begins and you can actually see the freezing front progressing and occupying the whole glass within a matter of some 3 or 4 seconds you will find in front of your eyes the whole thing freezing. It does not form a single crystal it is a collection of crystals and that happens because when it freezes the whatever is the gas used for aeration typically carbon dioxide tries to come out and cracks the whole thing but you will notice that and that is a demonstration because if it were to freeze on its own it will already be in solid and if it is not in solid it is still in liquid then what is the reason for a small piece of solid phase triggering that freezing. Do this experiment this is a very recommended experiment and the very simplest and safe experiment and of course you wait a few more minutes keep the glass outside under a fan it will melt and then you can enjoy your cold drink over to you. Sir does there exist any qualitative difference between heat and work because your discussion you did not ask about anything between heat and work qualitative difference. Qualitative difference well if you compare heat and work work I can say it is something like a mechanical phenomena can always be reduced to the rays of a wave that is the definition. So whether you have work and heat this can be shown equivalent to rays of a weight whereas this is not so both are energy in transit between systems system A and system B and our first law helps us define or using first law we have defined heat as energy in transit between system which is not of the work kind is heat. So not of the W type that is it in fact a question was asked whether this energy in transit has to be only work type or of the heat type or is there a third kind and the answer to that is we have not discovered or seen a hint of any third kind of interaction. So both are energies in transit between systems so we cannot say the work contained in a system or the heat content of a system or heat contained in a system. What is contained in a system is only its energy in various forms we call thermal internal energy we should not really call it thermal we should just call it internal energy it is the historical name which still continues and perhaps this is the only difference which I can talk about work and heat. Of course heat has a special status because in the second law the heat interaction multiplied by 1 over T it shown to be equal to delta S in the limiting case where the process is reversing. So work does not appear in the second law at all directly whereas heat appears directly okay I suppose that explains it. Work is termed as high grade energy and heat is termed as low grade energy. Yes but that is because perhaps we are mechanical engineers and we started worrying about levers and rising weights and using water to create power for us. So anything which is almost directly and almost completely convertible to power we thought it to be high grade and since we know that heat is associated with a temperature the so called grade of heat increases as the temperature of the source increases. In the morning we noticed that the if there is a heat interaction Q1 at a temperature T1 the maximum you can work you can extract out of it is Q into 1 minus T0 by T1 that is the reason why heat is called low grade energy and work is called high grade energy but these are our definitions they themselves have not defined themselves as high grade and low grade. There are two different types of interactions and neither work nor heat are energies by themselves they are energies in transit. So we should be careful when you say work is high grade energy and heat is low grade energy all that we can say is work perhaps is one way of transfer of energy from one system to another where a complete transfer at the receiving end into work is possible because when you transfer work you are directly giving work we are transferring heat you are giving it at some temperature work is not associated with any temperature. So if the recipient is not at the receiving temperature and the recipient has to convert it into work then the recipient will say that look I have only the environment temperature to work with you are giving me something at a temperature higher than environment so I can only convert part of that into work. So what is not converted you can say is a thermodynamic stacks of conversion of heat into work and again you will notice that if you provide that heat at the room temperature itself then you can say that it is an energy of no value because it just cannot be converted into work. One of my professors you say used to tell us that anything which is available and cannot be converted to anything useful is work less so he said heat at ambient temperature is a work less form of energy and that is his way of looking at things that is not what is defined in thermodynamics all these terms are something which we define over to you. Sir, last question is this sir this is about entropy yes entropy comes only when there exist inequality in clauses statement and number two it only shows thus the process between any two states is reversible or irreversible. In a way yes entropy is process by which you can determine how good a process is comparing that by dq by t actually we write although we write the second law as in differential form as say ds must be equal to or greater than or equal to dq by t actually this is a defined redefine form using entropy of this form dq by t for a reversible process element must be greater than dq by t for a general or irreversible process element. What the clauses in equality in its equality form allows us to do is we can show that this particular form is an exact differential so we define it as a change in property. So ds is nothing but a short form of dq by t for a reversible process element just the way we can say is dE is nothing but a short form for minus dw adiabatic for a process element where that process element has to be adiabatic that is it.