Bleaching hair is unequivocally a chemical change, not merely a physical alteration to the hair shaft. This process involves the application of strong chemical agents that fundamentally change the molecular composition of the hair’s internal components. Hair is primarily composed of a fibrous structural protein called keratin, and the bleaching procedure chemically modifies this protein and destroys the molecules responsible for color. The resulting change is permanent and irreversible, which is the defining characteristic of a chemical reaction.
Chemical vs. Physical Changes
A physical change alters the form or appearance of a substance without changing its core chemical identity. For instance, cutting a hair strand short or melting an ice cube are physical changes, as the substance remains chemically the same, just in a different shape or state. These changes are often reversible, meaning the substance can return to its original state through simple means.
A chemical change, by contrast, occurs when a substance is fundamentally transformed into a new substance through a chemical reaction. The original atoms and molecules rearrange to form new compounds with different properties. For example, burning wood converts cellulose and lignin into ash, carbon dioxide, and water vapor. Since new materials are created and the process cannot be easily reversed, bleaching falls squarely into this category.
How Bleach Alters Hair Pigment
Hair color is determined by melanin, a pigment naturally embedded within the hair’s inner core, the cortex. The two main types are eumelanin (brown and black shades) and pheomelanin (red and yellow tones). Bleaching agents, typically containing hydrogen peroxide, initiate an oxidation reaction to remove this pigment.
The bleaching solution also contains an alkaline agent, such as ammonia, which serves a dual purpose. The alkaline substance first raises the outer layer of the hair, the cuticle, allowing the other chemicals to penetrate the hair shaft. Once inside the cortex, the hydrogen peroxide acts as an oxidizer, causing a loss of electrons in the melanin molecules. This oxidation permanently breaks down the large, colored melanin molecules into smaller, colorless compounds that can no longer absorb visible light.
The Impact on Hair Structure
The chemical reaction that destroys the melanin pigment also damages the hair’s protein structure. The alkaline agent in the bleach forces the overlapping cells of the cuticle layer to lift and separate. This compromise in the cuticle’s integrity makes the hair shaft more vulnerable and porous, which is why bleached hair often feels rougher.
Inside the cortex, the oxidizing agents attack the keratin protein itself by breaking the disulfide bonds. These bonds, formed between sulfur atoms in the amino acid cystine, are responsible for the hair’s strength, shape, and elasticity. When the bleach converts cystine into cysteic acid, it disrupts the internal protein matrix, leading to a loss of structural integrity. This degradation makes the hair weaker, more brittle, and highly susceptible to breakage.