What's the best adjective to describe the low viscosity of liquids such as water and alcohol?

One that came to mind is 'runny', but then some honeys are runny, despite actually being inherently viscous. Also 'thin' seems somewhat unsatisfactory when describing a liquid.

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    ..........yummy? – Rusty Tuba Dec 30 '14 at 14:29
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    Alcohol is not. – Drew Dec 30 '14 at 16:27
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    Volatile, perhaps? – MikeTheLiar Dec 30 '14 at 16:27
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    @mikeTheLiar: "volatile" means "easily evaporated", which is likely correlated to unviscousness, but not the same thing. – Eric Lippert Dec 30 '14 at 18:10
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    "Alcohol flows easily." – Nick Matteo Dec 30 '14 at 18:51

Which is the best answer will depend partly on the degree of formality (i.e. what register) you are aiming for. Centaurus's suggestion of inviscid is very suitable in an academic or scientific context, but for general purposes you might consider either runny or free-flowing.

You are right, of course, to say that some honeys are runny; but the implication of using that term is not that those honeys have a particularly low viscosity, but only that they flow much more readily than stiff honeys.

  • I think you're right about 'runny'. – EmmaV Apr 26 '15 at 0:32
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    Who has ever heard of a runny bear or wine being runny? On the other hand, running would fit better e.g. running water – Mari-Lou A Jul 30 at 11:45

I think you are looking for "inviscid" (having very low viscosity) or "mobile"

  • inviscid (adj) - "having negligible, or zero, viscosity" - TFD

In common parlance, a fluid that has zero viscosity is known as an inviscid fluid. A liquid is said to be viscous if its viscosity is substantially greater than water's, and may be described as mobile if the viscosity is noticeably less than water's.

  • mobile (adj) Characterized by an extreme degree of fluidity; moving or flowing with great freedom. - Wikipedia
  • He covers governing equations, ideal-fluid flow, viscous flows of incompressible fluids, the compressible flow of inviscid fluids, and methods of mathematical analysis.

  • Mercury is a mobile liquid.

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    So is 'mobile' the word then? Because 'inviscid' seems to be the opposite of 'solid'. – Mitch Dec 30 '14 at 16:29
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    I'm not sure about this. If 'inviscid' means "having zero viscosity", then water and alcohol can't be inviscid. The only things that have zero viscosity as far as I know are superfluids such as liquid helium. – EmmaV Dec 30 '14 at 17:21
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    Better: negligible viscosity. – Francis Davey Dec 30 '14 at 18:47
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    How about "relatively inviscid"? – Hot Licks Dec 30 '14 at 18:49
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    I don't think that "inviscid" is a good answer to this question. No non-super fluid is "inviscid"; at best, a real fluid may be "of low viscosity" (or "relatively inviscid", as Hot Licks suggests). I certainly have never heard a fluid being called "inviscid" in "common parlance" - are all the people you talk to physicists? – senshin Dec 30 '14 at 23:21

I suggest fluid: (from TFD)

  • (General Physics) capable of flowing and easily changing shape
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    This was my natural reaction to the question title if you want colloquial/natural voice – Daenyth Dec 30 '14 at 17:22
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    A viscous fluid is still a fluid though. – Jon Hanna Dec 30 '14 at 21:00
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    A viscous fluid is still a fluid, but perhaps it is not a fluid fluid. – Shai Dec 31 '14 at 0:06
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    @DavidHammen: You are thinking of the noun form of fluid, which as you say is not a good fit, but Josh61 is thinking of the adjective, which does mean "capable of flowing". – Nate Eldredge Dec 31 '14 at 0:49
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    @smci That's actually a myth, glass isn't a fluid/liquid unless molten: io9.com/… – BrianH Dec 31 '14 at 9:29

All fluids are viscous. Some fluids have low viscosity, while other fluids have high viscosity. I'm not aware of a common word, even a common technical word, that will fit in your blank. "Inviscid," from another answer, is not in common use and conjures a technical meaning that I don't think you're after.

A similar example from the sciences: all objects contain heat. The analogy is that "viscosity" is to "viscous" as "heat" is to "hot"; you're searching for "cold," but I don't know that you'll find it.

I would abandon the parallelism and describe what the fluids do: treacle is viscous, but alcohol flows freely.

  • I was about to say this same thing. From a scientific perspective, all liquids are viscous/have viscosity. All matter that has a temperature above absolute zero are hot/have heat. "Cold" is a subjective term. Liquids simply have more or less viscosity than other liquids. Alcohol/water/liquor have low viscosities/are not highly viscous. – miltonaut Jan 1 '15 at 5:38

If you want an adjective, I'd suggest "liquid" or "fluid", though "a thin liquid" is entirely correct idiomatically

  • Both "liquid" and "fluid" apply to treacle. – nnnnnn Dec 31 '14 at 5:16
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    @nnnnnn: As nouns, yes. As adjectives, not in my usage. – keshlam Dec 31 '14 at 23:01

Use "watery" (Ref: Dictionary.com):

8. resembling water in fluidity and absence of viscosity:
"a watery fluid."

This is technically correct, easily understood, and quite often applies in more than one sense ("watery soup", for example).

"Dilute" also applies in some contexts (Solutions, colloids, the aforementioned soup or honey).


Thin is sometimes used in this context as an antonym to thick.

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