A mixture which is consists of only a single phase is known as a homogeneous mixture.
In this type of mixture, the components must be evenly distributed throughout which represents only one state of matter that can be present in the mixture.
Example: A mixture of sugar in water, where the solid sugar dissolves to form a single homogenous phase that would be a homogenous mixture.
Heterogeneous mixture:
A mixture which is consists of two or more phases that are immiscible to each other is known as a heterogeneous mixture.
The components of heterogeneous mixtures can usually be separated by simple means.
Example: Oil and water when present together in a solution, they do not mix evenly, but instead create two separate layers forming a heterogeneous mixture.
Milk is a homogeneous or heterogeneous mixture:
Milk is essentially known as a colloidal dispersion of fat in water where various proteins and other naturally occurring surfactants, allow the fat droplets to remain suspended in the water without setting or coalescing.
However, as we know 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 milk is a heterogeneous mixture.
Hence, milk is considered as a heterogeneous mixture.