Final answer:
If an acid or a base only partially ionises in water, it is called a weak acid or a weak base.
Step-by-step explanation:
If an acid or a base only partially ionises in water, it is called a weak acid or a weak base. In contrast, a strong acid or base will fully ionize in water.
For example, hydrochloric acid (HCl) is a strong acid that completely ionizes in water, while acetic acid (CH3COOH) is a weak acid that only partially ionizes.
This process of partial ionization accounts for the difference in pH and strength between strong and weak acids or bases.