 Welcome back. Having understood now what is a fluid, now let us try to understand one more fact that what is a pure fluid and what is a pure fluid because if I want to study fluids I would like to study pure fluid and let us try to understand the difference or the definition of what is a pure fluid and what is an impure fluid. So when I define a pure fluid I define it as a substance with a fixed chemical composition throughout is called as pure fluid. For example water, water in liquid form I will call as a pure fluid. The chemical composition is H2O or air, I say gaseous air here, air which is a mixture of several gases but still considered to be a pure fluid in a single phase, for example gaseous air or liquid air. Now air we know is a mixture of various gases, we have got oxygen, nitrogen, helium, carbon dioxide, water vapor, hydrogen may be in few percentage. Even though it is a mixture of several gases we still identify it as a pure fluid in a single phase. For example I have got gaseous air, I will call it a pure fluid, if it is a liquid air I will call it a pure fluid still. So water and air in single phase I will call them as pure fluid by the definition of a substance with a fixed chemical composition throughout I call it as a pure fluid. So with this definition I will call water in liquid form or in single phase form and air pure fluid. The question is what about two phase mixture? If I have got a two phase mixture of water that means two different phases are together maybe liquid and gases, liquid and vapor or again for air I have got gaseous air and liquid air together which makes it a two phase mixture. Is it qualified to be called as pure fluid in a two phase mixture form also? Let us see that thing. So mixture of water and water vapor is a pure substance but mixture of liquid and gaseous air is not. I am making a very important statement over here that if I have got a water, liquid and vapor which is a two phase mixture together I will still call it a pure fluid while if I got a mixture of liquid air and gaseous air I will not call this a pure fluid. I will call it a pure fluid what is the reason? What is the important difference that you have against a mixture of liquid and vapor of water against the mixture of liquid air and gaseous air together or a two phase mixture of air why one is impure and one why one is pure? Basic definition of what we had defined earlier as pure mixture actually comes to our help. So what is the difference? The chemical composition. The chemical composition of water remains the same in both the phases. So whether it is a liquid vapor or even solid for that matter all these three phases are actually H2O because the chemical composition remains always the same. Will it be same for the two phase mixture of air? If I got a gaseous air or liquid air the chemical composition of gaseous air will be different than the chemical composition of liquid air and that is why we call it a impure mixture. So a mixture of gaseous air and liquid air do not have the same chemical composition. For example if I am starting from the gaseous phase 100% gaseous phase of air and go on reducing the temperature of this air the higher boiling component will first get condensed and the liquid now here will have more concentration of higher boiling components like carbon dioxide and the lower boiling components like oxygen and nitrogen would remain in the gaseous phase. It means that the gas will have more of oxygen and nitrogen while the liquid will have more of high boiling component like CO2 that means the chemical composition of the liquid is going to be different than the chemical composition of the gas. That is why I will say that this mixture of liquid air and gaseous air is not going to have the same chemical composition. The chemical composition of gas will be different the chemical composition of liquid air will be different making this mixture as impure fluid. Because of this definition the gaseous air and liquid air will not be called as pure fluid that means if I want to study pure fluid such a two phase mixture will not be qualified to be called as pure fluid while for water that should not be a problem. If I want to study fluid I would like to study pure fluid and when I say pure fluid I would like to refer to water and again in two phases where vapor and liquid are together that will also constitute to be a pure fluid in that case. So the chemical composition plays a very important role in order to decide if the two phase mixture of a fluid is basically have the same composition or not. So when I now say air I have just come to the conclusion that the single phase of air is a pure fluid while single phase, two phase, three phases of water are pure fluid. But I have said that two phase and three phases or different phases together for air will not constitute a pure fluid it will be an impure fluid a very important difference just shown pictorially what we have talked about earlier that water whether it is single phase, two phase or three phase it will always be termed as pure fluid and it is a very important advantage of learning water or taking water as a pure fluid whether it is single phase, two phase or three phase I am not bothered I am studying all the time a pure fluid. Thank you.