Scientific Concepts and Hair Care
Domain 1: Scientific Concepts and Hair Care
This domain tests how hair is built, grows, and responds to water, heat, and chemicals-and how that science guides safe product and service choices. Expect questions on hair structure, bonds, pH, growth cycles, hair analysis, and scalp/hair disorders.
Structure and chemistry
- Hair has three layers: cuticle (protective outer scales), cortex (bulk; melanin and bonds), and medulla (central core, often absent in fine hair).
- The cortex contains peptide bonds and side bonds. Disulfide bonds are strong and are broken/reformed by chemical texture and waving services; hydrogen bonds are broken by water and reset by heat/drying; salt bonds are pH-sensitive.
- pH is logarithmic: pH 5 is ten times more acidic than pH 6. The hair/scalp acid mantle is about pH 4.5–5.5. High-pH products swell the cuticle and increase porosity.
- Growth cycles: anagen (active growth), catagen (transition), telogen (resting/shedding). Hair grows about 0.5 inch (1.25 cm) per month on average, though this varies.
Hair analysis
Assess texture (diameter), density (number per square inch), porosity (ability to absorb moisture), elasticity (stretch and return), and scalp condition. Porosity is tested by sliding fingers along a strand; elasticity by wet-stretch. Chemical services require a strand test and patch test where applicable.
Worked example
A client with lightened, fragile hair asks for a permanent wave. Wet-stretch shows the strand stretches and does not return to shape; the cuticle feels rough and absorbs water quickly. Reasoning: poor elasticity indicates compromised cortex; high porosity means uneven absorption. A perm would further break disulfide bonds and risk breakage. The correct response is to decline the chemical service, recommend conditioning and a trim, and re-evaluate elasticity before any waving service.
Traps
Don't confuse density with texture: density is how many hairs, texture is strand diameter. Don't call the cuticle living-the visible hair is dead keratin. Don't assume high porosity means strong hair; often it means damage. Remember that water breaks hydrogen bonds, not disulfide bonds.