Question

An unknown solution is colorless when tested with phenolphthalein but causes the indicator phenol red to turn red. Use this information to find the approximate pH of this solution.

Short answer

The pH of the solution is between 6.8 and 8.2.

Step-by-step solution

Recall phenolphthalein behavior

Phenolphthalein is colorless in acidic and neutral solutions and turns pink in basic solutions with a pH typically above 8.2.

Recall phenol red behavior

Phenol red is yellow in acidic solutions (pH below 6.8), red in neutral to slightly basic solutions (pH 6.8 to 8.4), and turns pink at pH above 8.4.

Analyze the given information

Since the solution is colorless with phenolphthalein, its pH is less than 8.2. It turns red with phenol red, indicating a pH between 6.8 and 8.4.

Determine the approximate pH range

Combining the information from both indicators, the solution must have a pH between 6.8 and 8.2.

Key concepts

Phenolphthalein Behavior

Phenolphthalein is an indicator frequently used in chemistry to determine the pH of a solution. It is valuable because it exhibits a distinct change in color depending on the pH level.
When phenolphthalein is added to a solution:
- In acidic solutions (pH < 7), it remains colorless.
- In neutral solutions (pH around 7), it also stays colorless.
- In basic solutions (pH > 8.2), phenolphthalein turns pink.
This means that if a solution remains colorless when phenolphthalein is added, its pH must be below 8.2. Remember, a color change with phenolphthalein is a clear indication of its pH behavior, making this indicator very useful for pH testing.

Phenol Red Indicator

Phenol red is another important indicator used to measure pH. Unlike phenolphthalein, phenol red shows different colors within a certain pH range:
- In acidic environments (pH < 6.8), phenol red appears yellow.
- In neutral to slightly basic solutions (pH 6.8 - 8.4), it turns red.
- At pH levels above 8.4, phenol red becomes pink.
This color change makes phenol red a useful tool for determining the approximate pH of a solution. For instance, if phenol red turns red in a solution, it suggests that the pH lies between 6.8 and 8.4. This clear color variation allows for an easy and quick assessment of the pH level, making it highly practical in various scientific applications.

Solution Acidity

Understanding the acidity or basicity of a solution is crucial in many scientific and everyday applications. Acidity is determined by the concentration of hydrogen ions (H+) present in a solution. The pH scale, ranging from 0 to 14, measures how acidic or basic a solution is:
- Acids: Solutions with a pH below 7. They contain higher concentrations of H+ ions.
- Neutral: A pH of exactly 7, indicating equal concentrations of H+ and OH- ions.
- Bases: Solutions with a pH above 7. They contain fewer H+ ions and more hydroxide ions (OH-).
Indicators like phenolphthalein and phenol red are helpful for determining a solution's pH. For example, if a solution is colorless with phenolphthalein but turns red with phenol red, we can infer its pH to be between 6.8 and 8.2. This information can be vital in fields like chemistry, biology, environmental science, and even medicine, where the pH of a solution can influence processes and reactions. Always remember, knowing a solution's acidity can provide critical insights into its properties and behavior.