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 an 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).
This table offers an overview of the key properties of acids compared with bases.
|Summary of Acid and Base Properties|
|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)|
|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. A person referring to “an acid” is usually referring to an Arrhenius or Brønsted-Lowry acid. A Lewis acid is typically called a “Lewis acid.” The reason for the differing definitions is that these different acids don’t include the same set of molecules:
- Arrhenius Acid: By this definition, an acid is a substance that increases the concentration of hydronium ions (H3O+) when added to water. You might 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 that can accept an electron pair to form a covalent bond. By this definition, some compounds that don’t contain hydrogen qualify as acids, including aluminum trichloride and boron trifluoride.
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 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).