Ace the Louisiana Cosmetology Exam 2026 – Glam Up Your Career!

A flat iron changes the hair's structure primarily by altering the hydrogen bonds within the hair. Hair is composed of keratin, a protein that includes various types of bonds that maintain its shape and structure. Among these, hydrogen bonds are responsible for most of the hair's elasticity and temporary shape. When heat is applied through a flat iron, it opens up the cuticle of the hair and allows these hydrogen bonds to break. This process enables the hair to be reshaped into a smoother and straighter form. Once the hair cools, the hydrogen bonds can reform in the new shape, thus altering the hair’s overall appearance.

Hydrating the hair, locking in moisture, and adding shine are not the primary functions of a flat iron. While using heat may create the illusion of shinier hair and contribute to a smoother look, these effects are secondary outcomes rather than the primary way that a flat iron functions in changing hair structure. The fundamental mechanism of a flat iron involves the manipulation of the hair’s internal bonds, particularly the hydrogen bonds, which is why this answer is the most accurate.