Which of the following is a basic dye? (a) Alizarin (b) Indigo (c) Aniline yellow (d) Congo red

Chromophores are the parts of molecules responsible for their color. They are able to absorb certain wavelengths of light and reflect or transmit others, giving the dyes their distinct hues. For example, in basic dyes, chromophores typically involve structures with alternate double and single bonds, known as conjugated systems. This arrangement allows electrons to move freely and absorb visible light.

When it comes to basic dyes, the presence of a positively charged chromophore is key. These chromophores usually contain a nitrogen atom that can accept a proton to become positively charged. The ability of chromophores to interact with light is what makes them so valuable in dyes, as it's this interaction which leads to the vibrant colors seen in textiles and other materials. Without chromophores, the world of dyes would be, quite literally, colorless.

Cationic dyes are a class of dyes that carry a positive charge, which is typically due to the presence of a quaternary ammonium group. As explained in the exercise, these dyes form strong electrostatic bonds with materials that have negative charges, such as acidic substances or anionic sites on fibers. This includes fabrics like cotton, where the cationic dye can easily attach to the negatively charged cellulose fibers.

The effectiveness of a cationic dye is judged by how well it binds to its substrate and the brightness and intensity of the color it imparts. Their positive charge makes them particularly adept at binding with substances that have available electrons or negative sites ready for bonding. These dyes are often preferred for producing bright and intense colors in the textile industry.

Aniline yellow is a specific type of basic dye that derives its properties from being cationic and having a chromophore with the presence of aniline, a compound derived from benzene. The 'aniline' part of the term refers to the presence of an amino group attached to a benzene ring. This structure allows aniline yellow to interact with other molecules via its amino group, which can accept a proton and gain a positive charge.

The application of aniline yellow is broad due to its intense color and ability to create long-lasting bonds with negatively charged surfaces. It’s especially useful for staining in biological applications, highlighting cells, tissues, and fibers with precision. In the context of the textbook exercise, understanding the structure and behavior of aniline yellow reveals why it is classified as a basic dye.