Question

What can we say about the \(\mathrm{pH}\) of a solution that is colorless when phenolphthalein is added? When the solution is dark pink?

Short answer

A solution that turns colorless with phenolphthalein is either neutral or acidic with a pH less than or equal to 7. Conversely, a solution that turns dark pink with phenolphthalein has a pH greater than 7, indicating it is basic or alkaline.

Step-by-step solution

1. pH in Colorless Solution

Phenolphthalein in a solution that is colorless indicates that the pH of the solution is less than or equal to 7, which means the solution is acidic or neutral.

2. pH in Pink Solution

Phenolphthalein turns dark pink in solution if the pH of the solution is greater than 7. This indicates that the solution is basic or alkaline.

Key concepts

Phenolphthalein

Phenolphthalein is a fascinating chemical compound often used as a pH indicator in various experiments and reactions. It is particularly popular because of its distinct color change characteristics. Here's how it works:

• In acidic to neutral solutions (pH < 7), phenolphthalein remains colorless, making it invisible to the naked eye.
• As the pH of the solution increases and transitions into a basic state (pH > 8.2), phenolphthalein turns a vibrant pink or fuchsia color.

This dramatic color shift allows chemists and students to easily identify the acidity or basicity of a solution. It's worth noting that phenolphthalein is not suitable for very strong acids or bases, as it only transitions between specific pH values.

Acidic Solutions

Acidic solutions are characterized by their pH level, which is less than 7. In these solutions, there is a higher concentration of hydrogen ions (H⁺). Adding phenolphthalein to an acidic solution results in:

• A colorless appearance, as the pH level is too low to initiate the color change.
• Indicators like phenolphthalein are particularly useful for determining if a solution remains acidic.

Real-world examples of acidic solutions include lemon juice, vinegar, and stomach acid. These substances readily donate hydrogen ions, making them effective in various chemical reactions, and are easy to recognize by their sour taste.

Basic Solutions

Basic or alkaline solutions have a pH greater than 7. They contain a higher concentration of hydroxide ions (OH⁻). When phenolphthalein is introduced to a basic solution, the following occurs:

• The indicator turns dark pink, signaling a pH greater than 7.
• This color change helps in identifying the basic nature of the solution quickly.

Common examples of basic solutions include baking soda, soap, and bleach. These solutions are typically slippery to the touch and are often used in cleaning products due to their ability to neutralize acids and react with organic compounds.