In this lecture Prof H. Bijl uses thermodynamic to derive equations which can be used for compressible flows. She starts by deriving equations for a perfect gas, and then applies this to find the relations for an isentropic flow. Next the energy equation is derived to complete a set of relations that can be used to find solutions for an airflow. Lastly as part of an excercise the expression for the speed of sound in a gas is derived.