Blow drying hair is a common routine, often raising questions about its effects on hair’s structure, specifically if it constitutes a chemical change. Understanding the differences between physical and chemical transformations is key.
Physical Versus Chemical Transformations
Changes in matter are categorized as physical or chemical, based on whether the substance’s molecular composition is altered. A physical change involves altering a substance’s form or appearance, but not its chemical identity. For instance, melting ice into water or shredding paper are physical changes; the substance remains chemically the same, just in a different state. These changes are often reversible.
In contrast, a chemical change results in the formation of a new substance with a different chemical composition. This involves breaking old chemical bonds and forming new ones. Burning wood into ash and smoke, or cooking an egg, are examples of chemical changes. Chemical changes are not easily reversible and typically involve the production or absorption of energy.
The Mechanics of Blow Drying
Blow drying primarily removes water through evaporation. Heat accelerates the transition of liquid water into vapor. This affects the hair’s water content, not its keratin protein structure; the hair remains chemically the same.
Hair’s keratin contains numerous weak hydrogen bonds. When hair is wet, these hydrogen bonds break, making hair flexible. As the hair dries with a blow dryer, these hydrogen bonds reform, locking the hair into its new shape. This temporary breaking and reforming of hydrogen bonds is a physical process, as the keratin’s chemical composition is not permanently altered. While excessive heat can damage the hair’s surface, its fundamental chemical structure does not change during typical blow drying.
When Hair Truly Undergoes Chemical Change
Hair undergoes true chemical changes through processes that intentionally alter its molecular structure. Hair dyeing, for example, involves chemical reactions modifying natural pigment molecules. Permanent dyes use agents like hydrogen peroxide to oxidize and remove color, while other chemicals form new, larger color molecules within the hair shaft.
Similarly, perming or relaxing treatments involve breaking and reforming the hair’s stronger disulfide bonds, which are chemical cross-links in the keratin structure. Chemicals like thioglycolates break these bonds, allowing reshaping, then a neutralizer reforms them in the new configuration. These processes result in permanent changes to the hair’s chemical makeup, unlike the temporary changes seen with blow drying.