What is a strong base?

• A. Completely ionises to give OH- ions in solution
• B. Partially ionises in solution
• C. Does not ionise in solution
• D. Remains bound to its conjugate acid in solution

Answer: (A)

Strong bases are substances that release hydroxide ions very completely when dissolved in water. The idea of strength here is about how fully the base dissociates into ions in solution. When a base dissociates completely, it splits into its ions—typically a metal cation and OH−—with essentially no undissociated molecules left. That means the solution ends up with a high concentration of OH−, making it strongly alkaline. For example, sodium hydroxide breaks apart into Na+ and OH−, giving a lot of hydroxide ions.

If a base only partly ionizes, it’s weaker because the base and its conjugate acid are in equilibrium, producing far fewer OH− ions and yielding a less alkaline solution. If a substance doesn’t ionize in water, it wouldn’t supply OH− ions in the solution, so it wouldn’t behave as a strong base in this Arrhenius sense. Likewise, if it remains bound to its conjugate acid in solution, that means little or no OH− is produced, which again doesn’t fit a strong base.

So the description that a strong base completely ionises to give OH− ions in solution captures its defining behavior.