how is the concentration of hydroxide ions 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−)(\text{OH}^-)(OH−) increases , as long as the added base is soluble in water.

Why the concentration increases

• Sodium hydroxide (NaOH) is a strong Arrhenius base, so it dissociates completely in water into Na⁺ and OH⁻ ions.

• Adding “excess base” means you are putting more base (like more NaOH) into the same solution, so more OH⁻ ions are produced by dissociation.

• As a result, the solution becomes more strongly basic: OH⁻ concentration goes up and pH increases (solution becomes more alkaline).

Important condition

• This increase in OH⁻ happens only if the base you add actually dissolves in water.

• If the added base is not soluble and just sits as solid undissolved, then it does not release extra OH⁻ ions, so the OH⁻ concentration in the solution effectively remains the same.

In standard NCERT/CBSE-style questions, the intended answer is: the concentration of hydroxide ions increases when excess base is dissolved in sodium hydroxide solution (assuming the base is soluble).

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Learn how the concentration of hydroxide ions is affected when excess base is dissolved in a sodium hydroxide solution, with a clear Class 10/11 style explanation and key exam point.