Science, Tech, Math › Science Charles's Law Definition in Chemistry Charles Law Definition and Equation Share Flipboard Email Print Charles Law describes the relationship between temperature and volume when mass and pressure are constant. NASA's Glenn Research Center Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated May 06, 2019 Charles's law is a gas law that states gases expand when heated. The law is also known as the law of volumes. The law takes its name from French scientist and inventor Jacques Charles, who formulated it in the 1780s. Charles's Law Definition Charles's Law is an ideal gas law where at constant pressure, the volume of an ideal gas is directly proportional to its absolute temperature. The simplest statement of the law is: V/T = k where V is volume, T is absolute temperature, and k is a constantVi/Ti = Vf/TfwhereVi = initial pressureTi = initial temperatureVf = final pressureTf = final temperature Charles's Law and Absolute Zero If the law is taken to its natural conclusion, it appears the volume of a gas approaches zero and its temperature nears absolute zero. Gay-Lussac explained this could only be true if the gas continued to behave as an ideal gas, which it was not. Like other ideal gas laws, Charles's law works best when applied to gases under normal conditions.