Fluid
Definition and meaning of Fluid in chemistry.
Fluid is any substance that continuously deforms, or flows, under an applied shear stress, no matter how small that stress is. This category includes both liquids and gases (and plasmas), distinguishing them from solids, which resist shear deformation and hold a fixed shape.
In more detail
Unlike a solid, a fluid has no fixed shape and takes the shape of its container because its constituent particles can slide past one another with relatively little resistance. Liquids are fluids with a fixed volume but variable shape, while gases are fluids that expand to fill any available volume. The resistance a fluid offers to flow is called viscosity, and the study of fluid behavior, including pressure, flow, and viscosity, is central to fluid mechanics and has wide application in chemistry, from solution behavior to gas laws.
Key facts
| Field | Physical Chemistry |
|---|---|
| States included | Liquid, gas, plasma |
| Key property | Viscosity (resistance to flow) |
| Governing behavior | Deforms continuously under shear stress |
Water and air are both fluids: water flows to conform to the shape of a glass while keeping a constant volume, and air expands to fill an entire room, illustrating the liquid and gas subtypes of fluid behavior.
Frequently asked questions
Is a fluid the same as a liquid?
No. Liquid is one type of fluid; fluid is the broader category that also includes gases and plasmas, since all of these flow under shear stress.
Why don't solids count as fluids?
Solids resist shear stress and maintain a fixed shape unless a large enough force causes permanent deformation or fracture, whereas fluids yield to any shear stress, however small, and flow continuously.