Step 1: Understanding the Question:
The question asks to identify the specific class of textile dye that colors fabrics by chemically bonding with the fiber molecules via covalent bonds.
Step 2: Detailed Explanation:
Different dyes use different chemical mechanisms to adhere to textile fibers:
Direct Dyes: These adhere to the fabric (usually cellulose) through weaker intermolecular forces like hydrogen bonding and Van der Waals forces. They tend to have lower wash fastness.
Reactive Dyes: These contain a specific reactive group (like a halotriazine or vinyl sulfone) that undergoes a chemical reaction with the hydroxyl ($-OH$) groups of cellulose fibers (like cotton) or the amino groups in protein fibers (like silk/wool). This reaction forms a strong, permanent covalent bond, making the dye highly resistant to washing out.
Vat Dyes: These are insoluble dyes that must be chemically reduced to a soluble form before applying. Once in the fiber, they are oxidized back to their insoluble state. They do not form covalent bonds.
Disperse Dyes: Used primarily for synthetic hydrophobic fibers like polyester, these dyes dissolve directly into the fiber structure forming a solid solution, without chemical bonding.
Step 3: Final Answer:
Reactive Dyes are the ones that form a covalent bond with the fiber.