Science, Tech, Math › Science Scientific Definition of a Closed System in Thermodynamics Share Flipboard Email Print Viesturs Jankovskis / EyeEm / Getty Images 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 April 09, 2019 A closed system is a concept used in thermodynamics (physics and engineering) and in chemistry. It differs from an isolated system. Closed System Definition A closed system is a type of thermodynamic system where mass is conserved within the boundaries of the system, but energy is allowed to freely enter or exit the system. In chemistry, a closed system is one in which neither reactants nor products can enter or escape, yet which allows energy transfer (heat and light). A closed system may be used for experiments where temperature isn't a factor.