Two experiments are described in which students explore phase transitions and critical phenomena by obtaining compression isotherms of phospholipid monolayers using a Langmuir trough. Through relatively simple analysis of their data students gain a better understanding of compression isotherms, the application of the Clapeyron equation, the balance between enthalpic and entropic contributions to the chemical potential, critical phenomena, and phase diagrams in general. Students use their data to determine the latent heat of transition, the entropy of transition, and the critical temperature for the liquid condensed to liquid expanded phase transition of a monolayer. Students also gain a general understanding of how molecular level changes in the structure of phospholipids, such as changes in chain length affect the structure and function of biological membranes. (Contains 3 figures and 1 table.)