What properties of milk qualify it as heterogeneous?
The fact that the fat (the dispersed phase) and the water (the continuous phase) are not miscible.
Milk is essentially a colloidal dispersion of fat in water. Various proteins and other naturally occurring surfactants stabilise the dispersion, allowing the fat droplets to remain suspended in the water without settling* or coalescing. However, the fact remains that the fat and water components cannot be mixed together to form a solution. There are, therefore, two distinct immiscible liquid phases present, which is why it is a heterogeneous mixture.
Compare this with sugar in water, where the solid sugar dissolves to form a single homogenous phase - that would be a homogenous mixture.
- Some settling of heavier fat particles can occur, which is why you observe "top of the milk" where the creamier fraction partially separates. However, by and large milk remains as a stable dispersion.