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.


Carcinogen, any chemical, biological, or physical agent that can potentially be a cause of cancer. The term is most commonly applied to chemicals introduced into the environment by human activity. Researchers label a substance a carcinogen if it causes a statistically significant increase in some form of neoplasm, or anomalous cell growth, when applied to a population of previously unexposed organisms. The modes of cancer initiation are still little understood, however, and efforts to establish the carcinogenic hazards of substances have aroused great controversy. The question of the usefulness of laboratory tests on animals in assessing human risks is particularly complex. The more recent development of short-term tests using cell cultures of microorganisms, however, is considered a major advance in carcinogen research.

Substances indicted as carcinogenic over the past few decades include the pesticides DDT, Kepone, and EDB; the synthetic hormone DES; the artificial sweetener cyclamate; asbestos; and a wide range of other industrial and environmental substances.