Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Catalysis, alteration of the speed of a chemical reaction, through the presence of an additional substance, known as a catalyst, that remains chemically unchanged by the reaction. Enzymes, which are among the most powerful catalysts, play an essential role in living organisms, where they accelerate reactions that otherwise would require temperatures that would destroy most of the organic matter.

A catalyst in a solution with—or in the same phase as—the reactants is called a homogeneous catalyst. The catalyst combines with one of the reactants to form an intermediate compound that reacts more readily with the other reactant. The catalyst, however, does not influence the equilibrium of the reaction, because the decomposition of the products into the reactants is speeded up to a similar degree. An example of homogeneous catalysis is the formation of sulfur trioxide by the reaction of sulfur dioxide with oxygen, in which nitrogen dioxide serves as a catalyst. Under extreme heat, sulfur dioxide and nitrogen dioxide react to form sulfur trioxide and the intermediate compound nitric oxide, which then reacts with oxygen to re-form nitrogen dioxide. The same amount of nitrogen dioxide exists at both the beginning and end of the reaction.