What is the difference between fluid and liquid?
I'm thinking of this in the context of drink plenty of fluids.
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A fluid can either be a liquid or a gas; as such, fluid is a hypernym of liquid in this regard. In your example
Drink plenty of fluids
it's only idiomatic to use fluids instead of liquids. (I wonder what else could be drunk besides a liquid/fluid, anyway!) You also hear things like:
Your body needs lots of fluids
In day-to-day usage, one will find liquid only used when the state of matter of a substance is in question or important to note. For example:
"How's the brownie mix looking?" "It's still basically liquid!"
When ice-cream is involved, melted is more likely to be used instead of liquid. One may also find liquid used in a financial context:
They think I'm liquid but, man, I'm flat broke!
Fluid just sounds smoother and it rolls off the tongue much easier. That probably explains why it's more commonly used in place of liquid. People would probably use gas if they wanted to be specific. Here are some common idiomatic phrases:
From the above examples, you may notice that fluid tends to be used for a liquid substance that has a specified/particular purpose. Thus, transmission liquid would be unidiomatic, for instance.
In general usage, "liquid" and "fluid" are synonyms.
In a scientific context they are not.
"Liquid" is a state of matter: solid, liquid, gas, plasma, Bose-Einstein condensate
"Fluid" describes certain properties generally attributed to substances in a liquid state
Fluids display such properties as:
* not resisting deformation, or resisting it only lightly (viscosity), and
* the ability to flow (also described as the ability to take on the shape of the container).
The degree of fluidity depends on the substance and the prevailing conditions (e.g. temperature and pressure).
Consider water, motor oil and maple syrup. At room temperature, they are all liquids. As the temperature drops, all three liquids get denser and flow more slowly.
At 32° F / 0° C water becomes a solid (ice) and no longer flows. Motor oil and maple syrup continue to flow, but the syrup flows more slowly.
In general usage it has become "less fluid".
As a noun, Merriam-Webster's explicitly defines liquid as "a fluid."
a fluid (as water) that has no independent shape but has a definite volume and does not expand indefinitely and that is only slightly compressible
Unless you are writing in a scientific or explicitly academic context, I'd say the two are interchangeable. Note that as adjectives, they do have slightly different meanings.
A fluid is anything that flows. so naturally it includes both liquids and gases. thus it is clear that all liquids are fluids but not all fluids are liquids, as fluid also contain substances in gaseous state. but also fluid is a subset of matter So, the difference between liquids and fluids boils down to their viscosity as fluids have a higher viscosity such as oils than liquids. Fluids can be gases also whereas liquids arte altogether different than gases.