Why does sodium in phenolphthalein change color?
Na is a very reactive metal which reacts with H2O to form NaOH, a base. Phenolphthalein is an indicator which turns pink in basic solutions.
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Phenolphthalein is a commonly used pH indicator that undergoes a color change in the presence of acids or bases. It is colorless in acidic solutions (pH less than 7), but turns pink or reddish in basic solutions (pH greater than 7).
The reason phenolphthalein changes color when sodium is present is because sodium hydroxide (NaOH) is a strong base, and it causes the solution to become basic. Sodium hydroxide readily dissociates in water to form Na+ cations and OH− anions.
When sodium hydroxide is added to a solution containing phenolphthalein, the hydroxide ions (OH-) from sodium hydroxide combine with hydrogen ions (H+) from the phenolphthalein molecules. This reaction results in the formation of water.
Phenolphthalein itself is a weak acid that is only slightly dissociated in water. In an acidic solution, the equilibrium between the ionized and non-ionized forms of phenolphthalein favors the non-ionized form, which is colorless. When the solution becomes basic, the equilibrium shifts towards the ionized form, which has a pink/red color. This change in equilibrium is responsible for the color change of phenolphthalein.
So, in summary, sodium hydroxide (NaOH) reacts with phenolphthalein, causing an increase in the concentration of hydroxide ions (OH-) in the solution. This shift in the equilibrium of phenolphthalein causes it to change from colorless to pink or reddish, indicating the presence of a base.