A phase diagram for a substance that exhibits the liquid crystalline state is shown. (b) If the substance is at a temperature and pressure repre- sented by point 1 and the temperature is increased until the phase changes, what is the phase transition? Describe how the ordering of molecules changes, and propose a reason for the change.

Key Concepts

Phase Diagram

A phase diagram is a graphical representation that shows the phases of a substance as a function of temperature and pressure. It delineates the boundaries between different phases, such as solid, liquid, and gas, and indicates the conditions under which these phases exist. In the context of liquid crystals, the diagram also includes regions for smectic, nematic, and isotropic phases, which are crucial for understanding phase transitions.

Liquid Crystalline Phases

Liquid crystals are materials that exhibit properties between those of conventional liquids and solid crystals. The smectic and nematic phases are two types of liquid crystalline states. In the smectic phase, molecules are organized in layers, while in the nematic phase, they are aligned but not layered. Understanding these phases is essential for predicting how molecular ordering changes during phase transitions.

Phase Transition

A phase transition refers to the transformation of a substance from one phase to another, such as from solid to liquid or from liquid to gas. In the context of the question, as the temperature increases at a constant pressure, the substance may transition from a smectic or nematic phase to an isotropic phase. This transition involves a change in molecular ordering, where the structured arrangement of molecules in the smectic or nematic phase becomes more random in the isotropic phase.