6.1 Matter, pH & Product Chemistry

Why This Matters for the Exam

The Safety and Infection Control domain of the PSI cosmetology theory exam assumes you can explain what products are made of and how they interact with hair at the chemical level. A meaningful share of the written test draws on chemistry basics, and pH questions alone appear in several forms: identifying a product's pH, predicting what an alkaline solution does to the cuticle, or choosing the correct finishing product after a chemical service. Master this section and you can reason through any product-chemistry question, even one built around a product you have never used.

Matter and Its States

Matter is any substance that occupies space and has physical and chemical properties (mass). All matter exists in one of three physical forms:

• Solid — rigid, with a fixed shape and volume, like ice or a bar of soap.
• Liquid — has a definite volume but takes the shape of its container, like water or shampoo.
• Gas — has no fixed volume or shape and expands to fill any space, like steam or the vapor from an aerosol can.

Elements, Atoms, Molecules, and Compounds

An element is the simplest form of chemical matter — a substance that cannot be broken down into a simpler substance without losing its identity. There are about 90 naturally occurring elements, each with its own symbol (O for oxygen, H for hydrogen, C for carbon). The atom is the smallest particle of an element that still retains the properties of that element. When atoms join together chemically, they form a molecule: two or more atoms joined by a chemical bond. A molecule of water (H2O) is two hydrogen atoms bonded to one oxygen atom. A compound is a substance made of two or more different elements chemically united in fixed proportions — water, hydrogen peroxide, and sodium hydroxide are all compounds.

Physical vs. Chemical Changes

A physical change alters the form or physical properties of a substance without a chemical reaction and without creating a new substance. Ice melting, water evaporating, hair being cut, and salt dissolving in water are all physical changes — usually reversible. A chemical change alters the chemical composition of a substance and produces one or more new substances through a chemical reaction that cannot easily be reversed. Salon examples you should be able to classify on the exam:

Pure Substances and Mixtures

A pure substance is matter with a fixed chemical composition — a single element or a single compound, like pure hydrogen peroxide. Most salon products are mixtures: physical combinations of two or more substances in any proportion, held together by physical rather than chemical bonds. The exam expects you to tell three kinds apart:

• Solution — a stable, uniform mixture of two or more miscible substances. The solute (the substance being dissolved) is evenly distributed in the solvent (the substance doing the dissolving). Solutions stay mixed and will not separate on standing. Example: perm neutralizer diluted in water.
• Suspension — an unstable mixture of undissolved particles dispersed in a vehicle. The particles are larger than those in a solution and settle out over time, so suspensions usually carry a "shake well" direction. Example: calamine lotion.
• Emulsion — a mixture of two or more immiscible substances (normally oil and water) united with the help of a binder or emulsifier such as gum, wax, or soap. Emulsions are milky and can separate over time. In an oil-in-water (O/W) emulsion, tiny droplets of oil are dispersed in water; these lighter emulsions form most moisturizing lotions and hair conditioners. In a water-in-oil (W/O) emulsion, droplets of water are dispersed in oil, producing heavier, richer products like cold creams.

The pH Scale

pH stands for potential hydrogen and measures the acidity or alkalinity of a substance by quantifying its hydrogen ions. The scale runs from 0 to 14:

• 7 is neutral — pure water sits here.
• Below 7 is acidic — the lower the number, the more acid.
• Above 7 is alkaline — the higher the number, the more alkaline.

The scale is logarithmic: each whole-number step represents a tenfold change. A solution at pH 5 is 10 times more acidic than one at pH 6 and 100 times more acidic than one at pH 7. That is why small pH differences between products matter enormously.

Healthy hair and skin are naturally acidic. The acid mantle — the protective barrier made of sebum, sweat, and dead cells — keeps skin and hair at about pH 4.5–5.5. On the exam, anything described as pH-balanced means formulated in this 4.5–5.5 range to match hair and skin.

What pH Does to Hair

Alkaline solutions soften and swell the hair fiber, lifting and opening the cuticle. This is deliberate in services that must reach the cortex: permanent waving lotions (about pH 9.0–9.6), chemical relaxers (up to pH 13+ for lye formulas), and permanent or high-lift haircolor all rely on alkalinity to open the cuticle and reach the bonds inside. But an overly alkaline product left on too long can strip the hair's natural protective oils and dissolve the fiber itself. Acidic solutions contract and harden the hair, closing and smoothing the cuticle. That is why acid-balanced shampoos and conditioners (pH 4.5–5.5) seal the cuticle after chemical work, and why many perm neutralizers are acidic; their oxidizing agent rebuilds disulfide bonds while the formulation returns hair toward its normal pH.

On the exam, if a question says a client's hair feels rough and gummy after a service, suspect alkaline damage. If it asks why a colorist finishes with an acidic conditioner, the answer is to close the cuticle and lock in color.