One colorless liquid is added gradually to another colorless liquid, and at one moment the solution becomes pink
name: Titration of NaOH with HCl using phenolphthalein
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The neutralization reaction between base and acid occurs in the solution. Concentration of hydrogen ions is changed - pH increases.
Phenolphthalein presence in the acidic solution is in the colorless molecular form H2Ind. When the base is added, phenolphthalein turns to the ion form Ind2-. When all the acid is neutralized, and pH becomes about 8.2, the solution becomes pink.