気体、液体及び固体の分子比較

All matter is composed of a very large number of molecules that move in a constant, random motion. Even in ice and metals, atoms still move. There are three different states of matter: solids, liquids, and gases. In all these states molecules, atoms, or ions are in motion with a specific amount of kinetic energy, which determines the state of matter. In the gaseous state, molecules have a high kinetic energy. The high energy molecules move quickly past each other and are widely separated, making it harder for them to intersect frequently. As a result the attractive forces in gases are weak. With this negligible force of attraction, gases have no definite volume or shape and expand freely to fill in the entire volume of its container. With a lot of space around the molecules, gases have low densities making them easy to compress. When gases are compressed or cooled, their kinetic energy is reduced and consequently the molecules slow down. The molecules move past each other more often and come closer together. This increases the amount of intermolecular forces and transforms the gas to a liquid state. Molecules within liquids are bound by stronger attractive forces, leaving little space around them. Thus, liquids are denser than gases making them difficult to compress. A liquid also has a definite volume and assumes the shape of the container. Despite stronger attractive forces, molecules in liquids can still move freely about one another, and so the liquids can flow or be poured. When liquids are further cooled, the kinetic energy reduces to a point where the particles almost stop moving due to the strong intermolecular forces and can only vibrate at their fixed position. At this point, molecules transform into a three-dimensional conformation state called solids. In any solid, molecules are densely packed, leaving almost no empty space around them. Thus, solids are incompressible, having a definite volume and shape.