1. Initial State:
* Phenolphthalein solution: Phenolphthalein is a colorless indicator in acidic or neutral solutions.
* Sodium hydroxide (NaOH): A strong base.
2. Adding NaOH dropwise:
* As you add NaOH drops, the solution becomes more basic (alkaline).
* Key Point: Phenolphthalein changes color in basic solutions.
3. Color Change:
* At a certain pH: When the solution reaches a slightly basic pH (around 8.2 - 10), the phenolphthalein molecules undergo a structural change. This change causes them to absorb light differently, resulting in a pink color.
4. The reaction:
* NaOH dissociates in water to form hydroxide ions (OH-):
NaOH(aq) → Na+(aq) + OH-(aq)
* The hydroxide ions react with the phenolphthalein, causing the color change.
In summary:
Adding sodium hydroxide dropwise to a phenolphthalein solution causes the solution to become basic. Once the pH reaches a slightly basic range, the phenolphthalein changes color from colorless to pink, indicating the presence of a basic solution.
