A base is strong if it completely produces hydroxide ions in water, and weak if it only partly does so and mostly stays as intact molecules in solution.

Key idea: ionization in water

  • Strong bases are essentially 100% ionized in water, so nearly every formula unit you dissolve becomes ions (for example, NaOH → Na⁺ + OH⁻).
  • Weak bases ionize only partially, so at equilibrium you have a mix of undissociated base and a relatively small amount of ions.

Quick recognition shortcuts

  • Common strong bases (in intro chemistry) are metal hydroxides of group 1 and heavy group 2 metals: LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂.
  • If a base is not one of these typical hydroxides (for example, NH₃, amines like CH₃NH₂, or anions like CO₃²⁻, HCO₃⁻), it is almost always a weak base in water.

Using KbK_bKb​ and pH

  • Strong bases do not have a meaningful finite KbK_bKb​ value in simple models because they are treated as fully dissociated; their solutions have very high pH at a given concentration.
  • Weak bases have a specific KbK_bKb​; a small KbK_bKb​ means the base is weak, because only a small fraction of molecules accept protons from water and form OH⁻.

Structural clues

  • Bases whose basic site is a negatively charged atom that is not strongly stabilized (for example, simple alkoxide RO⁻) tend to be very strong bases.
  • If the negative charge can be spread out or resonance‑stabilized (for example, carboxylate like CH₃COO⁻), the species is much less eager to grab a proton and acts as a weaker base.

Practical “lab feel”

  • A solution of a strong base at modest concentration (like 0.1 M NaOH) has very high pH and conducts electricity strongly because it is full of ions.
  • An equally concentrated weak base solution (like 0.1 M NH₃) has a noticeably lower pH and is a weaker electrolyte, because fewer molecules ionize to form OH⁻.

Information gathered from public forums or data available on the internet and portrayed here.