Chemistry, Products, and Ingredient Knowledge
Chemistry, products, and ingredient knowledge tests whether you understand pH, formulation, and interactions well enough to choose and combine products safely.
pH and acids
Skin's acid mantle is about pH 4.5–5.5. AHAs such as glycolic and lactic acid are water-soluble and need a low pH to exfoliate; BHA (salicylic acid) is oil-soluble and works in the pore. Percentage matters, but pH and free-acid level can matter as much. A product can be strong on paper and weak in the bottle if the pH is wrong.
Ingredient families
- Humectants attract water: glycerin, hyaluronic acid, sorbitol.
- Emollients soften and smooth: oils, esters, fatty alcohols.
- Occlusives block water loss: petrolatum, dimethicone, beeswax.
- Antioxidants: vitamin C, vitamin E, ferulic acid.
- Cell-communicating or smoothing: retinoids, peptides, niacinamide.
- Surfactants clean; preservatives prevent microbial growth; emulsifiers hold oil and water together.
Interactions and labeling
Retinoids plus strong AHAs or BHAs increase irritation. Do not layer multiple strong exfoliants in one service. Vitamin C is unstable at high pH and in light; packaging matters. 'Natural' and 'chemical-free' are marketing terms, not safety guarantees. Essential oils can sensitize. Patch test where indicated, follow shelf life, and preserve water-containing products.
Worked example
A 15% L-ascorbic acid serum at pH 3.5 is likely effective; the same percentage at pH 6 is largely neutralized and less stable. The reasoning: vitamin C needs a low pH to remain in its active form and penetrate. The exam may ask you to choose the better formulation, not just the higher percentage.
Traps
- Assuming a higher percentage is always stronger.
- Ignoring pH when comparing acids.
- Believing 'natural' means hypoallergenic.
- Mixing retinoid, AHA, and scrub in one service.
- Forgetting that water-based products need preservatives.
- Using essential oils neat on skin.