Question

Why is milk a colloid?

Answer 1

A colloidal solution or a colloidal dispersion in which the particles aren’t small enough to form a perfect solution yet aren’t large enough to get suspended or settle down at the bottom of the container.

Complete answer:
A colloid solution is a type of homogenous solution in which the particles get dispersed through the bulk but are still not completely miscible.
Milk is an important dairy product that is known for its high nutritional values. Macronutrients like proteins are present in milk in adequate amounts. But along with proteins, milk also contains saturated and unsaturated fats that do not have a very high solubility in fluids like water and any other polar liquids.
The colloidal solution formed by milk is a result of the fine globular structures formed from the fatty acid chains present in milk that get dispersed throughout the bulk or the volume of the solvent. This dispersion is such that the fat globules do not settle on standing yet do not completely mix with water and remain dispersed and distributed in the solution.
$\Rightarrow$ Thus, the dispersion of fat globules in a liquid solvent (mainly water) makes milk a colloid.

Note:
Milk can be transformed from a colloid into a suspension by causing the precipitation of the fats and proteins present in milk. Adding sour substances to warm milk leads to the formation of cottage cheese and the globules aggregate and precipitate out.