Milk is a mixture. It is not an element or a compound. Milk is composed of various substances such as water, fats, proteins, lactose (a type of sugar), and minerals. These components are not chemically bonded to each other, which means they can be separated by physical methods like filtration or centrifugation.
For example, when milk is left to stand, the cream (which contains fat) rises to the top. This separation occurs because the different components of milk have different densities and properties, allowing them to be separated without any chemical reactions taking place.
In summary, milk is a heterogeneous mixture because it consists of different substances that are not uniformly distributed and can be separated physically.