Part of a thermodynamic Definable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with having uniform properties (e.g., Mass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much, Mutual arrangement of atoms, molecules or ions that are packed together in a crystal lattice to form a crystal., index of refraction). The most familiar examples of phases are solids (minerals), liquids, and gases. Less familiar phases include plasmas, quark-gluon plasmas, Bose-Einstein condensates and fermionic condensates, strange matter, liquid crystals, superfluids and supersolids.
Some or all content above used with permission from J. H. Wittke.