What type of solution is casein found in within fresh milk?

Casein is a type of protein found in milk that exists primarily in a colloidal suspension. In fresh milk, casein molecules aggregate to form casein micelles, which are large, stable structures that remain dispersed throughout the liquid. This colloidal solution is characterized by its ability to scatter light, giving milk its opaque appearance.

The reason "colloidal solution" is the correct answer lies in the nature of how casein interacts with water. In a colloidal solution, the dispersed particles (in this case, casein micelles) are larger than typical molecules found in true solutions and do not settle out like particles in a suspension. They are also not uniformly soluble like crystals in a crystalline solution or completely clear as seen in an aqueous solution.

When considering the possible alternatives, an aqueous solution refers to a homogenous mixture where substances are entirely dissolved, which does not apply to casein. A crystalline solution involves solid crystals dissolved in a solvent, which is not the case with casein as it does not form crystal structures in milk. An opaque solution describes the visual characteristic of the solution but does not pertain specifically to the nature of casein or how it behaves in milk.

Thus, the classification of casein in milk as a