Problem 68 - A Complete Resource Book in Chemistry for JEE Main

Question

Phenolphthalein acts as an indicator within a \(\mathrm{pH}\) range of (a) \(8.2-10\) (b) \(6.2-7.2\) (c) \(4.2-6.2\) (d) \(2.2-4.2\)

Step-by-Step Solution

Answer

The correct answer is (a) 8.2-10.

Step 1: Understanding Indicator Ranges

Phenolphthalein is a chemical compound that is commonly used as an acid-base indicator. It changes color at specific pH ranges, transitioning from colorless in acidic solutions to pink in basic solutions.

Step 2: Identifying the Correct pH Range

Phenolphthalein changes color specifically in basic environments. Its effective pH range for this color change is commonly known as between 8.2 and 10.0. Other pH values, such as the ones provided in options b, c, and d, do not support the color change of phenolphthalein.

Step 3: Confirming the Correct Answer

Given the options, only (a) has a pH range between 8.2 and 10, which is when phenolphthalein acts as an indicator by changing color to pink. Therefore, option (a) is the correct choice.

Key Concepts

Phenolphthalein

Phenolphthalein is a fascinating chemical compound widely used as an acid-base indicator in various scientific experiments. It is highly valued for its distinct and visually dramatic color change when transitioning from an acidic to a basic environment. This characteristic makes it particularly useful in laboratory settings for determining the acidity or basicity of a solution.

• In acidic solutions (pH less than 8.2), phenolphthalein remains colorless.
• In basic solutions (pH greater than 8.2), it changes to a vibrant pink color.

This color transformation is due to a structural change in the phenolphthalein molecule, which affects its electronic configuration and how it absorbs and reflects light. This indicator is selected often because it provides a clear visual signal, aiding researchers and students in observing the completion of a chemical reaction.

pH Scale

The pH scale is a numerical representation used to specify the acidity or basicity (alkalinity) of an aqueous solution. It ranges from 0 to 14, with 7 being neutral, values below 7 indicating acidity, and those above 7 indicating basicity. The scale itself is logarithmic, which means each whole number change on the pH scale represents a tenfold increase or decrease in hydrogen ion concentration.

• A decrease in pH corresponds to an increase in acidity.
• An increase in pH indicates heightened basicity.

Understanding the pH scale is crucial in chemistry, biology, and environmental science, as it influences the behavior of substances and organisms in solution. For example, everyday items such as lemon juice (pH around 2), neutral water (pH 7), and household ammonia (pH about 11) showcase the spectrum and usability of pH measurements. Recognizing where substances fall on the pH scale helps in predicting their chemical behavior and potential reactions.

Color Change

Color change is a critical concept when working with indicators like phenolphthalein. This transformation provides a visual cue that a chemical reaction has occurred or is at a particular stage. In the context of acid-base reactions, such as those involving phenolphthalein, the color change is often used to signal the point at which the solution has reached a specific pH level or has been neutralized.

• Phenolphthalein starts colorless and turns pink as the pH moves into the basic range.
• This color change usually occurs sharply around the pH 8.2 range, providing a clear indicator during titration experiments.

Color changes like this one serve as powerful tools in chemistry, making abstract concepts tangible and experiments easier to interpret. They're essential for visually displaying data and allowing scientists to obtain quantitative information from qualitative observations. Understanding and utilizing color change effectively can lead to more accurate and reliable experimental results.