Phenolphthalein is a weak acid and is colorless in solution. It is generally used to find out the endpoint of titration. As the An unknown solution is colorless when tested with phenolphthalein but causes the indicator phenol red to turn red it indicates the pH of the solution is 8.0 or above.
Phenol red is an indicator whose color can exhibit a gradual transition from color yellow to red.
Phenol red is actually a water-soluble dye as is used as a pH indicator, changing from yellow to red over pH 6.6 to 8.0.
Thus, it can be concluded that the pH of the given unknown solution is in range 8.0 or above.
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