Fluids and their Properties

The distinction between fluids and solids arises from differences at the microscopic and macroscopic levels.

Microscopic Structure

In a solid, molecules are held in fixed positions by strong bonds between them. They vibrate around equilibrium positions but cannot move past neighbors.

Fluids include both liquids and gases. Their molecules can move past one another. In a liquid, molecules stay close but slide freely. In a gas, molecules are widely spaced and move rapidly with minimal interaction.

Macroscopic Behavior

The defining test is how a material responds when pushed along its surface.

A solid deforms by a fixed amount and holds its shape. It resists this sideways push.

A fluid cannot resist a sustained sideways push while at rest. It flows continuously under any such force. This is why fluids conform to their container while solids maintain a fixed shape.