* Pure substance: A pure substance consists of only one type of molecule. Examples include water (H₂O), oxygen (O₂), and gold (Au).

* Mixture: A mixture is a combination of two or more substances that are not chemically bonded. Each substance retains its own properties.

When you combine liquid and air, you are creating a mixture.

* Liquid: A liquid is a state of matter that has a definite volume but takes the shape of its container.

* Air: Air is a mixture of gases, primarily nitrogen and oxygen.

Therefore, liquid plus air is a heterogeneous mixture because the components are not evenly distributed throughout the mixture. You can usually see the different components (like bubbles of air in a liquid).