Students know properties of solid, liquid, and gaseous substances, such as sugar (C₆H₁₂O₆), water (H₂O), helium (He), oxygen (O₂), nitrogen (N₂), and carbon dioxide (CO₂).

The different physical properties of solids, liquids, and gases connects back to the molecular basis of matter. Some of the texts make this connection, and others do not.

In gases, the atoms and molecules have very loose attractions to one another. Therefore, a gas does not have any definite shape, and it will expand or contract to fill whatever volume is available to it. In a liquid, the particles have stronger attractions and connections. As a result, a liquid has a definite volume. However, the particles of a liquid can still move around enough so that it will still adapt itself to fit the shape of its container. In a solid, the particles connect with each other very strongly. As a result, a solid has both a definite volume and a definite shape. It does not readily change its shape to fit a container.