Problem 28 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The colour of light absorbed by an aqueous solution of \(\mathrm{CuSO}_{4}\) is: (a) orange-red (b) blue-green (c) yellow (d) violet

Step-by-Step Solution

Answer

The aqueous solution of CuSO4 absorbs orange-red light.

Step 1: Understanding the Complementary Colors

To determine the color of light absorbed by a substance, we need to know about complementary colors. Colors absorbed and colors observed (seen) are complementary. This means they are on opposite sides of the color wheel. For example, if a solution absorbs blue light, it will appear orange, and vice versa.

Step 2: Identify the Observed Color of the Solution

An aqueous solution of copper(II) sulfate, \(\mathrm{CuSO}_{4}\), appears blue. This is an important observation because it tells us which color is seen.

Step 3: Use Color Wheel to Determine Absorbed Color

Since the \(\mathrm{CuSO}_{4}\) solution is blue, we use the color wheel to locate blue's complementary color. The complementary color of blue is orange-red. This means the solution absorbs orange-red light.

Step 4: Select the Correct Answer

Given the options, the correct answer must reflect the absorbed color. Since we've determined copper(II) sulfate absorbs orange-red light, the correct choice is (a) orange-red.

Key Concepts

Complementary Colors

In the world of colors, complementary colors play a fascinating role. Imagine colors sitting opposite each other on a color wheel. These are known as complementary colors. When a substance absorbs a certain color, the color we see is the complementary one on the opposite side of the wheel.
For example:

• If a solution absorbs blue light, it appears orange to our eyes.
• Similarly, if a red color is absorbed, the solution will look green.

This happens because our eyes perceive what is left over when certain wavelengths of light are absorbed. This understanding of complementary colors is crucial when distinguishing between absorbed and observed colors in solutions.

Color Wheel

The color wheel is a visual representation of colors arranged in a circular format. It shows relationships between different colors. Artists have used it for centuries to mix and match colors harmoniously.
In a standard color wheel:

• Primary colors like red, blue, and yellow are the building blocks.
• Secondary colors like green, orange, and violet are made by mixing two primaries.
• Tertiary colors are formed by mixing a primary and a secondary color.

But in terms of chemistry and physics, the color wheel helps determine which colors are absorbed and which are visible in a solution. When a copper(II) sulfate solution looks blue, the color wheel helps us identify that it absorbs orange-red, which is blue’s complementary color.

Copper(II) Sulfate Solution

Copper(II) sulfate, often seen in schools as bright blue crystals, is more than just a pretty color. Its blue color when dissolved in water gives us valuable clues about the behavior of light with materials.
In aqueous copper(II) sulfate:

• The solution appears blue to the human eye.
• This appearance indicates that the solute absorbs light primarily in the orange-red portion of the spectrum.

This color change is essential for understanding absorption and reflection of light. Copper's sulfate ions, when interacting with water molecules, set this specific absorption pattern that results in the blue color we associate with the solution. By understanding how specific solutions absorb certain light, chemists can derive properties and behaviors of various chemical substances.