Defining Fluids and Liquids
The main difference is that 'fluid' is a broad category, while 'liquid' is a specific type of fluid. A fluid is any substance that can flow and takes the shape of its container. A liquid is one specific state of matter that fits this description. Therefore, all liquids are fluids, but not all fluids are liquids.
Section 2: Characteristics of a Fluid
A fluid is any substance that continuously deforms or flows when subjected to a shear stress. This defining characteristic means fluids lack a fixed shape. The category of fluids includes liquids, gases, and even plasmas. Their molecules are not locked in place and can move relative to each other.
Section 3: A Practical Example with Liquids
A liquid is a specific state of matter that has a definite volume but no fixed shape. Unlike a gas (another type of fluid), a liquid will not expand to fill the entire volume of its container; it maintains a distinct surface. For example, if you pour 100 mL of water into a 1-liter bottle, it remains 100 mL of water and takes the shape of the bottom of the bottle, but it doesn't expand to fill the whole liter.
Section 4: Why the Distinction Matters
This distinction is critical in physics and engineering. Principles of fluid dynamics, like pressure, buoyancy, and viscosity, apply to all fluids, including both air (a gas) and water (a liquid). Recognizing that both are fluids allows scientists to apply the same fundamental laws to understand the behavior of very different substances.