Question

What is the $pH$ range in which phenolphthalein is colorless?
A) $0 - 8$
B) $8 - 10$
C) $10 - 12$
D) $12 - 14$

Answer 1

We know that Phenolphthalein is an acid-base indicator. An acid-base indicator is also called a $pH$ indicator. Indicators are usually a substance which changes its color with change in $pH$ of the solution.

Complete step by step answer:
We can draw the structure of phenolphthalein as,

We know that the molecular formula of Phenolphthalein is ${C_{20}}{H_{14}}{O_4}$. It is a weak acid. It is a yellow crystalline solid and easily dissolves in alcohols and slightly soluble in water.
Acid-base indicators are generally weak acids or bases which dissociate when dissolved in water and form ions.
Now, consider a weak acid indicator with the formula$H{I_n}$, it dissociates in water as follows.
$H{I_n}\left( {aq} \right) + {H_2}O \rightleftharpoons {H_3}{O^ + }\left( {aq} \right) + {I_n}^ - \left( {aq} \right)$
$Acid\left( {ColorA} \right)$ $Conjugate\,base(colorB)$
The dissociation of Phenolphthalein in water is,

Phenolphthalein indicator used in a solution which changes its color at higher $pH$. If a weak acid is titrated against a strong base the solution changes its color when the $pH$ solution is greater than 7. Phenolphthalein remains colorless in the acidic $pH$ levels and it turns to pink at $pH = 8.2$ and continues to a bright magenta at $pH = 10.$ The color change is due to the ionization of the phenolphthalein which in turn changes the shape of the molecules.

Therefore, the correct option is A. .

Note: We know that $pH$ is the concentration of hydrogen ion in the solution.
The mathematical expression of $pH$ is,
$pH = \log \left[ {{H^ + }} \right]$
A solution which has $pH$ less than seven then is considered as acidic and a solution which has $pH$ greater than seven then it is considered as basic.