What happens when an acid or a base is mixed with water?

The correct answer is option 4. Decrease in the concentration of ions \((H_3O^+/OH^–)\) per unit volume.

Here's why:

When an acid or base is mixed with water, it undergoes a process called dilution. This means that the concentration of the acid or base is decreased because it is spread out in a larger volume of water.

Both acids and bases dissociate in water, meaning they break apart into their constituent ions. Acids typically release hydronium ions \((H_3O^+)\), while bases release hydroxide ions \((OH^-)\).

By adding water, the total number of ions \((H_3O^+ \text{and }OH^-)\) remains the same, but the volume of the solution increases. Therefore, the concentration of ions per unit volume decreases.

Therefore, option 4 correctly describes what happens when an acid or a base is mixed with water.