What term would describe two substances that will not mix at all?

The term that describes two substances that will not mix at all is "immiscible." When two liquids are immiscible, they do not dissolve in each other, which results in the formation of distinct layers when they are combined. A familiar example of this is oil and water; they do not mix and instead create separate layers due to their differing polarities.

The other terms don't fit this description. "Partially miscible" refers to substances that can mix to some extent but not entirely, resulting in a situation where they can exist in both a mixed and unmixed state. "Miscible" denotes substances that fully dissolve in each other, creating a homogenous solution without any separation. "Saturated," on the other hand, describes a solution that has reached its maximum concentration of solute at a given temperature and pressure, and does not relate to the mixing characteristics of different substances. Thus, "immiscible" is the precise term for substances that completely refuse to mix.