Final answer:
Saltwater is a solution, air is a solution, gold is an element, and milk is a colloid.
Step-by-step explanation:
a) Saltwater is a homogeneous mixture, also known as a solution, because it consists of two substances, salt and water, that are evenly distributed on the molecular level. The salt particles dissolve in the water to form a transparent solution.
b) Air is a homogeneous mixture or a solution in which gases like nitrogen, oxygen, and carbon dioxide are evenly distributed. The different gases in air cannot be visibly separated and mix uniformly.
c) Gold is a pure substance and an element. It is composed solely of gold atoms and cannot be separated into different substances by physical means.
d) Milk is a colloidal mixture or a colloid. It consists of water, proteins, fats, and other substances that are dispersed throughout the liquid but do not settle. Colloids have larger particles compared to solutions.