 Welcome back. Before we attend following some problems using the first law of thermodynamics, let us see something about properties of a system because we will be using these quite often. Since most of our systems would be simple compressible systems, let us look at the possible type of property variations we have equations of state and the thermal energy relation for simple compressible systems. And we know that simple compressible systems are going to be either gaseous or liquid or solid. And hence let us first look at typical properties of a gas. The special case of gas is an ideal gas. We have already studied properties of this. Next, liquid and solid. When liquid flows, solid does not flow. But liquids and solids are significantly different from gases. So let us look at how the properties will vary. And in particular, just the way ideal gas is an approximation, idealization of the behavior of gases. Let us consider the extreme case of a liquid or solid, incompressible liquid or incompressible solid. Thank you.