Only products that contain _____ can have a pH.

The correct answer is related to the nature of pH and what it measures within substances. pH is a scale used to specify the acidity or basicity of an aqueous solution. To have a measurable pH, a product must be in a form that allows for water to engage in dissociation to form hydroxide ions (OH⁻) or hydrogen ions (H⁺).

Water, being a polar solvent, allows other compounds to dissolve and interact, making it essential for pH measurement. Products formulated without water do not provide that environment necessary for such dissociation, thus they lack a pH value. This is why water is fundamental to the concept of pH; it is the medium through which acids and bases exist and can be measured.

Substances like alcohol, oil, and various solvents might not inherently contribute to a measurable pH as they do not facilitate the same kind of ionization as water does. For example, while alcohol and solvents can dissolve other compounds, they do not provide the same level of ionic dissociation that water does. Consequently, it is only those products containing water that will have a defined pH, which is crucial for many cosmetic formulations and their efficacy.