The single-component material models are for modeling pure substances such as air, water or aluminum.
There are three single-component material models in STAR-CCM+: Gas, Liquid and Solid models. Each manages one single-component material representing the pure substance being modeled. The Gas model manages a gas material (i.e. a pure substance in the gaseous phase), the Liquid model a liquid material (i.e. a pure substance in the liquid phase), and the Solid model a solid material (i.e. a pure substance in the solid phase).