Science, Tech, Math › Science Anhydrous Definition in Chemistry Share Flipboard Email Print ThoughtCo / Bailey Mariner Science Chemistry Basics Chemical Laws 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 July 09, 2019 Anhydrous literally means "no water." In chemistry, substances without water are labeled anhydrous. The term is most often applied to crystalline substances after the water of crystallization is removed. Anhydrous can also refer to the gaseous form of some concentrated solutions or pure compounds. For example, gaseous ammonia is called anhydrous ammonia to distinguish it from its aqueous form. Gaseous hydrogen chloride is called anhydrous hydrogen chloride to distinguish it from hydrochloric acid. Anhydrous solvents are used to perform certain chemical reactions that, in the presence of water, either cannot proceed or yield unwanted products. Examples of reactions with anhydrous solvents include the Wurtz reaction and the Grignard reaction. Examples Anhydrous substances exist in solid, liquid, and gas forms. Table salt is anhydrous sodium chloride (NaCl).Gaseous HCl is anhydrous, which differentiates it from hydrochloric acid, a solution of 37 percent HCl in water (w/w).Heating copper(II) sulfate pentahydrate (CuSO4·5H2O) yields anhydrous copper(II) sulfate (CuSO4). How Anhydrous Chemicals Are Prepared The method of preparation depends on the chemical. In some cases, simply applying heat can drive off water. Storage in a desiccator can slow rehydration. Solvents may be boiled in the presence of a hygroscopic material to prevent water from returning to the solution.