Final answer:
Motor oil, sand, and sawdust are heterogeneous mixtures, while milk is a homogeneous mixture.
Step-by-step explanation:
a. Motor oil is a heterogeneous mixture because it is made up of two or more substances that are visibly distinguishable from each other. It consists of a liquid base along with various additives and dissolved contaminants.
b. Sand is a heterogeneous mixture as well, as it is composed of tiny particles of different minerals and rocks. These particles do not dissolve or mix uniformly in each other.
c. Sawdust is also a heterogeneous mixture since it is composed of small pieces of wood that are not chemically combined with each other. Each piece of sawdust retains its individual properties.
d. Milk is a homogeneous mixture, specifically a colloidal suspension where the milk proteins and fats are evenly distributed in the liquid portion. When milk is left undisturbed, no visible separation occurs.