14.1 Fluids

Quite simply, a fluid is a substance that flows. Because they flow, fluids take the shape of their container rather than retaining a shape of their own. You may think that gases and liquids are quite different, but both are fluids, and their similarities are often more important than their differences.

The detailed behavior of gases and, especially, liquids can be complex. Fortunately, the essential characteristics of gases and liquids are captured in simple molecular models that are related to the ball-and-spring model of solids that we presented in 《 SECTION 5.2.

MODEL 14.1 Molecular model of gases and liquids

Gases and liquids are fluids—they flow and exert pressure.

  • Gases

    • Molecules move freely through space.

    • Molecules do not interact except for occasional collisions with each other or the walls.

    • Molecules are far apart, so a gas is compressible.

    The figure illustrates gas particles depicted as spheres distributed sparsely in a square box.

  • Liquids

    • Molecules are weakly bound and stay close together.

    • A liquid is incompressible because the molecules can’t get any closer.

    • Weak bonds allow the molecules to move around.

    The figure illustrates a few liquid particles depicted as spheres concentrated in the bottom half of the square box and in close contact with each other.