Science, Tech, Math › Science Is HF (Hydrofluoric Acid) a Strong Acid or a Weak Acid? Share Flipboard Email Print LAGUNA DESIGN/Getty Images 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 December 02, 2019 Hydrofluoric acid or HF is an extremely corrosive acid. However, it is a weak acid and not a strong acid because it does not completely dissociate in water (which is the definition of a strong acid) or at least because the ions it forms upon dissociation are too strongly bound to each other for it to act as a strong acid. Why Hydrofluoric Acid Is a Weak Acid Hydrofluoric acid is the only hydrohalic acid (such as HCl, HI) that is not a strong acid. HF ionizes in an aqueous solution like other acids: HF + H2O ⇆ H3O+ + F- Hydrogen fluoride does actually dissolve fairly freely in water, but the H3O+ and F- ions are strongly attracted to each other and form the strongly bound pair, H3O+ · F-. Because the hydroxonium ion is attached to the fluoride ion, it isn't free to function as an acid, thus limiting the strength of HF in water. Hydrofluoric acid is a much stronger acid when it is concentrated than when it is diluted. As the concentration of hydrofluoric acid approaches 100 percent, it's acidity increases because of homoassociation, where a base and conjugate acid form a bond: 3 HF ⇆ H2F+ + HF2- The FHF- bifluoride anion is stabilized by a strong hydrogen bond between hydrogen and fluorine. The stated ionization constant of hydrofluoric acid, 10-3.15, does not reflect the true acidity of concentrated HF solutions. Hydrogen bonding also accounts for the higher boiling point of HF compared to other hydrogen halides. Is HF Polar? Another common question about the chemistry of hydrofluoric acid is whether the HF molecule is polar. The chemical bond between hydrogen and fluorine is a polar covalent bond in which the covalent electrons are closer to the more electronegative fluorine.