Acid Definition and Examples

Chemistry Glossary Definition of Acid

Three-dimensional model of sulfuric acid
Three-dimensional model of sulfuric acid, a common strong acid. LAGUNA DESIGN / Getty Images

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 acid
  • Monoprotic acid
  • Lewis acid
  • Hydrochloric acid
  • Sulfuric acid
  • Hydrofluoric acid
  • Acetic acid
  • Stomach 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).

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