Chemistry, Products, and Ingredient Knowledge
Chemistry questions test product behavior: pH, surfactants, emulsions, actives, preservatives, and incompatibilities. You do not need to formulate, but you must predict irritation and efficacy.
Core concepts
- pH is logarithmic: pH 3 is ten times more acidic than pH 4. Skin surface is about pH 4.5-5.5; barrier repair favors mildly acidic products.
- Surfactants have water-loving and oil-loving ends; they lift oil and dirt. Harsh surfactants can strip lipids.
- Emulsions mix oil and water with emulsifiers; lotions are oil-in-water or water-in-oil. Stability and preservation matter.
- Humectants attract water, such as glycerin and hyaluronic acid; emollients soften, such as oils and esters; occlusives block water loss, such as petrolatum and dimethicone.
- AHAs such as glycolic and lactic acid exfoliate; free-acid availability depends on pH. BHAs such as salicylic acid are oil-soluble. Retinoids increase turnover and photosensitivity. Antioxidants such as vitamin C can oxidize.
- Preservatives prevent microbial growth; natural does not mean safe. Check expiry and storage.
Worked example
A 10% glycolic product at pH 3.5 is more exfoliating than the same percentage at pH 5.5 because more acid is free. That also means higher irritation risk. Patch test and start slowly.
Traps
Thinking pH 2 is twice as acidic as pH 4; mixing acids or retinoids aggressively; assuming essential oils are hypoallergenic; using expired vitamin C; ignoring preservative needs in water-based products; judging strength by percentage alone.