Final answer:
Air is a homogeneous mixture, gold is an element, saltwater is a homogeneous mixture, and sugar is a compound. Each substance is classified based on its composition and the uniformity of its components.
Step-by-step explanation:
To answer the student's question regarding the classification of substances, we will determine whether each one is a pure substance or a mixture, and if it is a pure substance, whether it is an element or a compound. If it's a mixture, we will specify whether it is a homogeneous mixture or a heterogeneous mixture.
Substance Classifications:
Air: A homogeneous mixture because it is a blend of different gases such as nitrogen, oxygen, and small amounts of other gases, which are evenly distributed.
Gold: An element because it consists of only one type of atom and cannot be broken down into simpler substances by chemical means.
Saltwater: A homogeneous mixture, also known as a solution, because it is a mix of salt (sodium chloride) and water that is uniform throughout.
Sugar: A compound because it is made up of more than one element (carbon, hydrogen, and oxygen) chemically combined in a fixed ratio. This is known as sucrose, and regardless of the source, its composition and properties are consistent.