Chemistry and Product Knowledge
What this domain tests
Chemistry items test pH, acids and bases, functional ingredients, formulation compatibility, preservatives, and sunscreen. You need to predict how products behave when layered and why some combinations irritate.
Core rules
- pH measures hydrogen ion activity; skin surface is about pH 4.5–5.5. AHAs work at low pH; neutralized glycolic acid is much less exfoliating.
- pKa determines ionization. When pH is below pKa, more acid is un-ionized and can penetrate.
- Surfactants clean by emulsifying oils; humectants bind water, such as glycerin and hyaluronic acid; emollients soften, such as esters and oils; occlusives block water loss, such as petrolatum and dimethicone.
- Antioxidants including vitamin C, vitamin E, and ferulic acid stabilize free radicals, but vitamin C is unstable at high pH and in light.
- Avoid layering strong acids with retinoids in the same session; benzoyl peroxide can oxidize some retinoids; many actives need patch testing.
- Preservatives are necessary in water-containing products; preservative-free usually means single-use or anhydrous.
Worked example
A client wants to use a 10% glycolic serum at pH 3.5, a 0.5% retinol, and a vitamin C serum at pH 3.0 every night. The combination is likely irritating. Recommend alternating nights, buffering with moisturizer, and using sunscreen. The reasoning: three low-pH or cell-turnover actives increase barrier disruption, and vitamin C at low pH can sting already compromised skin.
Traps
Do not judge an acid by percentage alone; pH matters. Do not mix niacinamide and vitamin C as an absolute rule, because modern formulations are often stable. Do not claim chemical-free products; all matter is chemical.