p-Nitrophenol (p-NP) can be used as a pH indicator because its color changes depending on the pH of the solution. In acidic or neutral conditions (below pH 5.4), p-NP is colorless or lightly colored.

 p-Nitrophenol (p-NP) can be used as a pH indicator because its color changes depending on the pH of the solution. In acidic or neutral conditions (below pH 5.4), p-NP is colorless or lightly colored. However, in alkaline solutions (above pH 7.5), it turns yellow. This color change makes it a useful indicator for determining the pH of a solution. 

Here's a more detailed explanation:

pH Range:

p-NP is colorless below a pH of 5.4 and yellow above a pH of 7.5. 

Color Change Mechanism:

The color change is due to the ionization of p-NP. In acidic solutions, it exists as a neutral molecule. However, in alkaline solutions, it forms a negatively charged anion (4-nitrophenolate or 4-nitrophenoxide), which absorbs light at different wavelengths and appears yellow. 

Applications:

p-NP is used as an indicator in various applications, including titrations, analytical chemistry, and laboratory work. 

Other Uses:

Besides being an indicator, p-NP is also used in the synthesis of pharmaceuticals, dyes, and agrochemicals. It can also be used to darken leather.