Problem 11 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a basic dye? (a) alizarin (b) indigo (c) aniline yellow (d) congo red

Step-by-Step Solution

Answer

The basic dye is (c) aniline yellow.

Step 1: Identify Characteristics of Basic Dyes

Basic dyes are water-soluble and can easily attach to negatively charged surfaces such as wool, silk, and acrylic fibers. These dyes generally carry a positive charge and are known for their bright, vibrant coloration. Identify the characteristic that defines basic dyes to proceed to the next step.

Step 2: Analyze Each Option

Now we consider each dye from the list: (a) alizarin, which is an anthraquinone dye, does not fit basic dye characteristics; (b) indigo is a vat dye mainly used in cotton dyeing; (c) aniline yellow is an azo dye, which is basic; (d) Congo red is an anionic dye used as an indicator and in histology staining. Only one color in the options is basic.

Step 3: Select the Correct Basic Dye

Given the analysis, aniline yellow fits the description of a basic dye because it is a vibrant, water-soluble dye that positively charges and interacts with negatively charged materials. It is widely used to dye fabrics like silk and wool.

Key Concepts

Dye Characteristics

Dyes are substances that impart color to a material. Understanding their characteristics is essential for determining their application and adaptability to different materials. Basic dyes are a subset of dyes that exhibit specific features making them unique from others. Basic dyes are known for:

• Intensity: These dyes typically offer very bright and vibrant colors, which makes them popular for materials that need to stand out.


• Positivity: They carry a positive charge, which plays a critical role in their interaction with materials.


• Affinity towards Charged Surfaces: They have a high affinity for materials such as wool, silk, and acrylic fibers, which typically have a negative charge.

This positive-negative charge relationship is crucial as it enhances the bonding of the dye to the fabric, ensuring strong, vivid coloration.

Water-Soluble Dyes

Water-solubility is a hallmark of basic dyes. This property greatly influences how the dye is used and processed. A dye that dissolves easily in water is advantageous for several reasons. Firstly, water-solubility allows:

• Efficient Dye Application: Since the dye can be uniformly distributed in water, it provides even coverage over the fabric, resulting in a consistent appearance.


• Simple Preparation: Being water-soluble simplifies the preparation process and reduces the need for additional chemicals, making it more economical.


• Versatility: These dyes can be used at a wide range of pH levels, making them suitable for various dyeing applications.

Basic dyes exhibit water-solubility not only due to the chemical structure of the dye but also because of the ionic interactions facilitated by their positive charges.

Positively Charged Dyes

The positive charge characteristic of basic dyes is integral to their functionality. This feature allows them to bond strongly to materials that have negative charges, such as certain textiles. The interaction is driven by electrostatic attractions, much like magnets with opposite poles attracting. Here's how this plays out in practice:

• Strong Bonding: The positive charge ensures a robust bond with negative sites on the material, enhancing colorfastness.


• High Affinity for Natural Fibers: Natural fibers like wool and silk naturally possess negative charges due to their structure, making basic dyes particularly effective.


• Enhanced Absorption: This strong attraction allows the dye to be absorbed deeply into the fibers, ensuring vibrant and lasting colors.

Thus, the positively charged nature of basic dyes not only influences their binding ability but also their ultimate usage and effectiveness in dyeing processes.