Question

Phenolphthalein in acid shows:
a.) blue
b.) red
c.) pink/purple
d.) colorless
e.) orange

Answer 1

Hint: Phenolphthalein is compound with a chemical formula \[{{C}_{20}}{{H}_{14}}{{O}_{4}}\], which is also used as an indicator for acid-base titrations. Indicators turn their color when subjected to pH of different substances respectively.

Complete step by step solution:

Phenolphthalein is slightly soluble in water and it usually dissolves in alcohols for experimental use. It is a weak base, which can easily form \[{{H}^{+}}\] ions in solution.
The phenolphthalein molecule is generally colorless, and its ion is pink. When a base is added to it, the equilibrium shifts, leading to more ionization as \[{{H}^{+}}\] ions are removed.
When phenolphthalein is added in an acid, it turns colourless. In aqueous solution, phenolphthalein dissociates into ions. The pink colour of the solution is due to the concentration of \[{{\ln }^{-}}\] ions in the solution. Under acidic conditions, the concentration of the \[{{\ln }^{-}}\] solution is very low and concentration of \[{{H}^{+}}\] ion is high; hence it is colourless.
It is also colourless in neutral solution, with an increase in pH it turns pink and turns colorless in acidic solution.
Phenolphthalein remains colorless throughout the range of acidic pH levels but begins to turn pink at a pH level of 8.2 and continues to a bright purple in stronger alkalines.

Therefore, the correct answer is (d).

Note: Along with phenolphthalein, Methyl orange is also a pH indicator frequently used in titration because of its clear and distinct color variance at different pH values. It shows red color in acidic medium and yellow color in basic medium.