Chemistry, Ingredients, and Product Selection
What this domain tests
Chemistry questions test pH, functional ingredients, formulation types, and safe product selection.
Core rules
- pH measures hydrogen-ion activity on a 0–14 scale. Skin sits near pH 4.7–5.5; low-pH products can sting, while high-pH cleansers can disrupt the barrier.
- Surfactants clean; humectants attract water, such as glycerin; emollients soften; occlusives reduce water loss, such as petrolatum. Emulsions combine oil and water with an emulsifier.
- Acids: AHAs such as glycolic and lactic exfoliate superficially; BHA, salicylic acid, is oil-soluble. Efficacy depends on concentration, pH, and free-acid level, not percentage alone.
- Antioxidants such as vitamin C and tocopherol can oxidize; packaging and storage matter. Preservatives prevent microbial growth; natural does not mean safe or preservative-free.
- Read INCI labels; patch-test; avoid mixing incompatible actives such as strong acids and retinoids without guidance.
Worked example
A 10% glycolic product at pH 4.5 may exfoliate less than a 5% product at pH 3.2. Reasoning: more acid is in the free, active form at lower pH, so concentration alone does not predict effect.
Traps
Believing higher percentage always means stronger; ignoring pH; assuming chemical-free; using expired vitamin C; mixing exfoliating acids with retinoids on sensitive skin.