Fluid
Definition and meaning of Fluid in chemistry.
A fluid is any substance that continuously deforms or flows when a sideways physical force is applied. In chemistry and physics, this broad category includes both liquids and gases. A fluid will always change its shape to match its container.
In more detail
Many students mistakenly think the word fluid only means a liquid. In science, gases and plasmas are also considered fluids. A solid resists changes to its shape.
Its atoms or molecules are locked tightly into rigid structures. A fluid behaves differently because its constituent particles can slide past one another with very little resistance. This freedom of movement allows a fluid to yield to applied forces.
A sideways pulling force is known as shear stress. Even a tiny shear stress will cause a fluid to flow continuously. Liquids are fluids that always maintain a fixed total volume.
A liquid will simply pool at the bottom of a container. Gases are fluids that completely lack a fixed volume. A gas will expand freely to fill any available empty space.
The internal friction that a fluid offers against flowing is called viscosity. Honey has a high viscosity because it flows very slowly. Water has a low viscosity because it pours very easily.
The study of how fluids move and exert pressure is called fluid mechanics. Chemists rely on these physical principles to understand how solutions mix and how gases react in closed systems.
Key facts
| Field | Physical Chemistry |
|---|---|
| States Included | Liquid, gas, plasma |
| Key Property | Viscosity measures the internal resistance to flow |
| Governing Behavior | Deforms continuously under shear stress |
| Volume of Liquids | Fixed volume but variable shape |
| Volume of Gases | Variable volume and variable shape |
Water and air are two very common fluids with different physical behaviors. If you pour water into a glass, it flows to match the bottom shape. The liquid also keeps a constant total volume. If you pump air into a tire, the gas flows to fill the entire inner space. Both substances demonstrate the classic flowing nature of a fluid.
Frequently asked questions
Is a fluid the exact same thing as a liquid?
No. A liquid is just one type of fluid. The fluid category also includes gases and plasmas because they all flow under pressure.
Why do scientists not count solids as fluids?
Solids resist shear stress and maintain a fixed rigid shape. A fluid will yield to a pushing force and flow continuously.
Does air count as a fluid?
Yes, air is a gas. All gases are considered fluids because their particles freely flow and slide past one another.