Matter most commonly exists as a solid, liquid, or gas; these are the three common . Solids have a definite shape and volume. Liquids maintain a definite volume but take the shape of their container. Gases have neither definite shape nor volume and expand to fill their container. (Figure 75.1)

Liquids and gases are classified as because they yield to , unlike solids which resist such deformation. The extent to which fluids flow in response to these forces depends on a property called , which we’ll explore further in Viscosity and Laminar Flow; Poiseuille’s Law.

In solids, atoms are tightly packed in a regular structure. They vibrate but cannot easily move past one another. The forces between atoms act like stiff springs, resisting compression and shearing forces. This rigidity explains why solids maintain their shape and volume.

By contrast, in liquids, atoms are still in close contact but are free to slide past one another. Liquids resist compression like solids, but they do not resist shearing forces and thus can flow. Liquids stay in an open container due to interatomic attraction, unless the container leaks.

In gases, atoms are spaced far apart and interact weakly. They move freely and rapidly, colliding with one another and the container walls. Because of the large spacing, gases are easily compressed and will escape from an open container. Both liquids and gases are considered fluids, but gases are distinct in being highly compressible.