In a strong base, all base molecules accept donated protons. In a weak base, only a fraction of molecules accepts protons.

Two chemical reaction equations. The first equation shows a strong base dissociating completely into positively charged ions and negatively charged hydroxide ions. The second equation shows a weak base reacting with water on the left side. On the right side there is a positively charged ion (base plus a proton) a negatively charged hydroxide ion. A double sided arrow indicates that this reaction occurs in both directions.

The double arrow shows that the reaction goes in both directions until equilibrium is reached. At that point, the forward reaction occurs at the same rate as the reverse reaction.

For strong acids and bases, that reverse reaction is almost non-existent. This is why only one-directional arrows are used.

What does strong or weak mean?

The terms "strong" or "weak" refer to how readily acid or base molecules dissociate into ions. This measure is represented by the base dissociation constant, Kb.

Whether a base is strong or weak has nothing to do with the concentration or dilution of the solution.

Safety and strong bases

It is important to notice that acids and bases, especially strong ones, are corrosive, and can corrode skin and metal.

Strong and weak acids