Milk is a solution of many different things. It is also a colloid - that strange combination of suspension and solution. Colloids can usually be distinguished from solutions by the fact that they tend to be cloudy rather than clear.
How is milk a solution?
Terry Prothero, Studier of Labels and Purveyor of Truth
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Re: What exactly is milk regarding solutions and mixtures?
Milk is both a "mechanical mixture" (in this case, called an "emulsion") and a solution.
The solution is a mixture of proteins (such as casein and whey proteins) and sugars (such as lactose) in water.
The emulsion, or mixture, is the milk fat suspended in the solution with the proteins as the "emulsifier" or stabilizer.