How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?
When excess base is dissolved in a solution of sodium hydroxide, the concentration of hydroxide ions (OH⁻) increases. This occurs because bases generate hydroxide ions in water.
Bases which are soluble in water are called alkalis. When dissolved, they dissociate to produce OH⁻ ions. For example, NaOH dissociates in water as: NaOH(s) → Na⁺(aq) + OH⁻(aq).
Therefore, adding excess base to the sodium hydroxide solution introduces additional hydroxide ions, resulting in an increase in the overall concentration of OH⁻ ions in the solution.
Explanation
The textbook context clearly states that 'Bases generate hydroxide (OH⁻) ions in water.' When excess base is dissolved in an existing sodium hydroxide solution, the additional base dissociates and produces more OH⁻ ions. This is different from dilution with water (which decreases ion concentration). The question tests whether students understand that adding more base increases hydroxide ion concentration, while adding water decreases it through dilution.