Hairdressing Theory and Professional Practice
What this domain tests
This section examines the science underneath the scissors - hair and scalp structure, the growth cycle, bonds and pH - together with the professional standards a Level 5 stylist must hold without being reminded: presentation, confidentiality, hygiene discipline and scope of practice.
Core science you must be able to state
- Three layers: cuticle (overlapping scales controlling porosity and shine), cortex (the bulk, holding melanin and all the bonds), medulla (a loose core, often absent in fine hair).
- The shaft is dead keratinised protein; the follicle in the scalp is the living structure that builds it.
- Bonds: disulphide (broken by perming/relaxing, re-formed by neutraliser), hydrogen (broken by water, reset by drying and styling), salt (broken by pH change), peptide (broken by bleach - not re-formed).
- Growth cycle: anagen 2–7 years (about 85% of scalp hair), catagen 2–3 weeks, telogen roughly 3 months. Shedding of 50–100 hairs a day is normal; growth averages 1–1.25 cm a month.
- Hair and skin sit near pH 4.5–5.5 (the acid mantle); alkaline products swell the cuticle and raise porosity.
Professional practice
Clean uniform, hair tied back, short unvarnished nails, closed shoes, minimal jewellery. Client records are confidential under GDPR and the Data Protection Act 2018. Work only within your training: refer scalp disease, unexplained hair loss or a suspected reaction to a GP or pharmacist rather than treating it in the salon.
Worked example
A client has 12 cm of hair, wants 30 cm, and trims 1 cm every eight weeks. Annual growth is 1.25 × 12 = 15 cm; trims remove about 6.5 cm, so net gain is 8.5 cm a year. The shortfall is 18 cm, so 18 ÷ 8.5 ≈ 2.1 years - around 25 months, not the 14 months a growth-only sum would suggest.
Traps that cost marks
Swapping hydrogen and disulphide bonds; calling the visible hair shaft living; describing bleach damage as reversible; confusing porosity (how fast hair takes up moisture) with elasticity (stretch and return).