Strength of Acids and Bases

Strong and Weak Acids & Bases

Lithium hydroxide is an example of a strong base.
Lithium hydroxide is an example of a strong base. CCoil/Wikimedia Commons/CC By 3.0
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 April 12, 2018

Strong electrolytes are completely dissociated into ions in water. The acid or base molecule does not exist in aqueous solution, only ions. Weak electrolytes are incompletely dissociated.

Strong Acids

Strong acids completely dissociate in water, forming H+ and an anion. There are six strong acids. The others are considered to be weak acids. You should commit the strong acids to memory:

  • HCl - hydrochloric acid
  • HNO3 - nitric acid
  • H2SO4 - sulfuric acid
  • HBr - hydrobromic acid
  • HI - hydroiodic acid
  • HClO4 - perchloric acid

If the acid is 100% dissociated in solutions of 1.0 M or less, it is called strong. Sulfuric acid is considered strong only in its first dissociation step; 100% dissociation isn't true as solutions become more concentrated. 

H2SO4 → H+ + HSO4-

Weak Acids

A weak acid only partially dissociates in water to give H+ and the anion. Examples of weak acids include hydrofluoric acid, HF, and acetic acid, CH3COOH. Weak acids include:

  • Molecules that contain an ionizable proton. A molecule with a formula starting with H usually is an acid.
  • Organic acids containing one or more carboxyl group, -COOH. The H is ionizable.
  • Anions with an ionizable proton. (e.g., HSO4- → H+ + SO42-)
  • Cations
    • Transition metal cations
    • Heavy metal cations with high charge
    • NH4+ dissociates into NH3 + H+

Strong Bases

Strong bases dissociate 100% into the cation and OH- (hydroxide ion). The hydroxides of the Group I and Group II metals usually are considered to be strong bases.

  • LiOH - lithium hydroxide
  • NaOH - sodium hydroxide
  • KOH - potassium hydroxide
  • RbOH - rubidium hydroxide
  • CsOH - cesium hydroxide
  • *Ca(OH)2 - calcium hydroxide
  • *Sr(OH)2 - strontium hydroxide
  • *Ba(OH)2 - barium hydroxide

* These bases completely dissociate in solutions of 0.01 M or less. The other bases make solutions of 1.0 M and are 100% dissociated at that concentration. There are other strong bases than those listed, but they are not often encountered.

Weak Bases

Examples of weak bases include ammonia, NH3, and diethylamine, (CH3CH2)2NH. Like weak acids, weak bases do not completely dissociate in aqueous solution.

  • Most weak bases are anions of weak acids.
  • Weak bases do not furnish OH- ions by dissociation. Instead, they react with water to generate OH- ions.