 In the last few lectures, what we're doing is we're looking at gas mixtures and properties of gas mixtures. What we're now going to do is we're going to move into a different form of gas mixture and that is where one of the components of our gas mixture is a vapor. And consequently if it's a vapor that means that it can start going through a phase change and so it changes the dynamics of the process a little bit. But what we're looking at now will be gas vapor mixtures and a common application on this is air conditioning. This is an area that many mechanical engineers work in and contribute and it helps give us a more comfortable existence in our buildings given that we have controlled environment. So what we've looked at thus far, we've looked at mixtures of gases and when we did that we looked at mass fractions, mole fractions, PVT behavior as well as properties such as internal energy, enthalpy and entropy. However when we were doing this all of the components of our gas mixture remained in the gas phase. So we're now going to look at the situation where one of our components can go through a phase change and I mentioned that an important application of that is the mixture of dry air and water vapor and that pertains to heating ventilation and air conditioning systems. So air conditioning is one of the applications. Another one that we won't particularly or specifically look at in this course however it is important is atmospheric sciences and so processes such as cloud formation, short and long wave radiation or radiative flux coming from the sun and then energy being radiated back into space that all kind of couples together with climate change. So when we look at global climate change one of the big uncertainties is the contribution of water vapor in terms of the modeling efforts and the place would be the contribution that water vapor would make and consequently this is quite an active area and it's an important area for people doing atmospheric science studies. We won't be looking at that in this course what we'll be sticking with mainly is dealing with processes involving air conditioning which is typically where mechanical engineers would contribute in this area. But as we're doing this what we will be doing is we'll be considering air to consist of two parts. One part is dry air and so this is typically air as we usually think of that. So typically we think of air as being mainly nitrogen and oxygen and that's small amounts of other gases. However when we talk about dry air we are implying that there is no water in the air. Atmospheric air on the other hand which is what we deal with when we're doing heating ventilation and air conditioning it is a mixture of dry air and water vapor. So what we're going to be doing is we're going to be taking a look at how to handle problems and particularly we'll be looking at how to apply the first law and perhaps entropy generation but mainly the first law in heating ventilation and air conditioning applications. And in order to get there we need to come up with the equations that enable us to determine things such as enthalpy and other properties of the air that consists of dry air and water vapor. So that's where we're going to be going in this lecture actually in the next couple of lectures.