Changes in the chemical or physical state of matter subjected to high pressure. The earliest high-pressure chemical process of commercial importance was the Haber synthesis of ammonia from hydrogen and nitrogen developed in Germany prior to World War I. The synthesis of diamonds from graphite developed in the early 1950s is a high-pressure physical process. Raising the pressure on a system may result in several kinds of change. It causes a gas or vapor to become a liquid, a liquid to become a solid, a solid to change from one molecular arrangement to another, and a gas to dissolve to a greater extent in a liquid or solid. These are physical changes. A chemical reaction under pressure may proceed in such a fashion that at equilibrium more of the product forms than at atmospheric pressure; it may also take place more rapidly under pressure; and it may proceed selectively, forming more of the desired product among multiple possible products.