Chemistry, Product Knowledge, and Ingredient Function
Chemistry that shows up on the exam
- pH runs 0–14, and skin sits near 4.5–5.5. Lower means more acidic. For acids, both pH and concentration matter - the free-acid value, not the percentage alone, sets AHA strength.
- Solutions are dissolved; suspensions are dispersed solids; emulsions combine oil and water using an emulsifier. Oil-in-water feels lighter; water-in-oil is heavier and more occlusive.
- Surfactants clean by lowering surface tension; they are the core of cleansers and strip the barrier when too harsh.
- Humectant pulls water in (glycerin, hyaluronic acid, urea). Emollient softens and fills cracks (esters, oils). Occlusive blocks water loss (petrolatum, dimethicone). These are routinely confused.
- Exfoliating acids: AHAs (glycolic, lactic, mandelic) are water-soluble and act at the surface; BHA (salicylic acid) is oil-soluble, reaches the pore, and suits oily or acne-prone skin.
- Antioxidants (vitamins C and E, niacinamide, green tea) fight free radicals; vitamin A derivatives increase turnover; preservatives prevent microbial growth in water-containing products.
Traps: assuming "natural" or "organic" means non-irritating; assuming a higher percentage is always stronger; expecting oils and water to mix without an emulsifier; and forgetting that water-based products need preservatives.
Rule of thumb: read the first five ingredients - that is most of the formula - and match the vehicle (lotion, cream, gel, serum) to the skin type before matching the active.