What happens when you mix sodium bicarbonate solution and phenolphthalein?

Mixing sodium bicarbonate solution (NaHCO3) with phenolphthalein results in a color change from colorless to pink. This occurs due to the following reaction:

NaHCO3 + H2O → Na+ + HCO3- + H2O

H2O + CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

The sodium bicarbonate solution contains bicarbonate ions (HCO3-), which react with water to form carbonic acid (H2CO3). Carbonic acid is a weak acid that partially dissociates into hydrogen ions (H+) and bicarbonate ions.

Phenolphthalein is a weak acid-base indicator that changes color in response to changes in pH. In acidic solutions, phenolphthalein remains colorless, while in basic solutions, it turns pink.

The presence of hydrogen ions (H+) produced by the dissociation of carbonic acid causes the solution to become slightly acidic, resulting in a colorless phenolphthalein solution. However, upon further addition of sodium bicarbonate solution, the concentration of bicarbonate ions increases, which shifts the equilibrium of the carbonic acid reaction towards the formation of more bicarbonate ions and water.

As a result, the solution becomes less acidic and the pH increases. When the pH reaches approximately 8.2-10, phenolphthalein undergoes a color change, turning the solution from colorless to pink, indicating the presence of a basic environment.