Science, Tech, Math › Science Acid Definition and Examples Chemistry Glossary Definition of Acid Share Flipboard Email Print Three-dimensional model of sulfuric acid, a common strong acid. LAGUNA DESIGN / 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. 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. Updated January 30, 2019 Acid Definition in Chemistry An acid is a chemical species that donates protons or hydrogen ions and/or accepts electrons. Most acids contain a hydrogen atom bonded that can release (dissociate) to yield a cation and anion in water. The higher the concentration of hydrogen ions produced by an acid, the higher its acidity and the lower the pH of the solution. The word acid comes from the Latin words acidus or acere, which mean "sour," since one of the characteristics of acids in water is a sour taste (e.g., vinegar or lemon juice). Summary of Acid and Base Properties This table offers an overview of key properties of acids compared with bases: Property Acid Base pH less than 7 greater than 7 litmus paper blue to red doesn't change litmus, but can return acid (red) paper back to blue taste sour (e.g., vinegar) bitter or soapy (e.g., baking soda) odor burning sensation often no odor (exception is ammonia) texture sticky slippery reactivity reacts with metals to produce hydrogen gas reacts with several fats and oils Arrhenius, Brønsted-Lowry, and Lewis Acids There are different ways of defining acids. When a person refers to "an acid," this usually refers to an Arrhenius or Brønsted-Lowry acid. A Lewis acid is typically called a "Lewis acid." The reason for the differing definitions is because these different acids don't include the same set of molecules. Arrhenius Acid: By this definition, an acid is a substance which increases the concentration of hydronium ions (H3O+) when added to water. You may also consider increasing the concentration of hydrogen ion (H+), as an alternative. Brønsted-Lowry Acid: By this definition, an acid is a material capable of acting as a proton donor. This is a less restrictive definition because solvents besides water are not excluded. Essentially, any compound that can be deprotonated is a Brønsted-Lowry acid, including typical acids, plus amines, and alcohol. This is the most widely used definition of an acid. Lewis Acid: A Lewis acid is a compound which can accept an electron pair to form a covalent bond. By this definition, some compounds which don't contain hydrogen qualify as acids, including aluminum trichloride and boron trifluoride. Acid Examples These are examples of types of acids and specific acids: Arrhenius acidLewis acidHydrochloric acidSulfuric acidHydrofluoric acidAcetic acidStomach acid (which contains hydrochloric acid)Vinegar (which contains acetic acid)Citric acid (found in citrus fruits) Strong and Weak Acids Acids may be identified as either strong or weak acids based on how completely they dissociate into their ions in water. A strong acid, such as hydrochloric acid, completely dissociates into its ions in water. A weak acid only partly dissociates into its ions, so the solution contains water, ions, and the acid (e.g., acetic acid). Learn More Names of 10 AcidsDo You Add Acid to Water or Water to Acid?Intro to Acids, Bases, and pH Continue Reading What Is a Weak Acid? Understand the Bronsted Lowry Theory of Acids and Bases What Is an Arrhenius Acid? Review Your Chemistry Concepts Know the Conjugate Acid Definition in Chemistry Know What a Strong Acid Is in Chemistry What Is a Monoprotic Acid in Chemistry? Key Acids and Bases Definitions What Is a Base in Chemistry? Chemistry Vocabulary Terms You Should Know What Is an Acid-Base Indicator? Review Your Chemistry Concepts What Is a Mineral Acid? Definition and List What Is Neutralization? Review Your Chemistry Concepts What Determines the Strength of an Acid or Base? What Is a pH Indicator? Learn What a Bronsted-Lowry Acid Is in Chemistry What Is a Hydronium Ion?