Treacle is viscous; alcohol is ____?

Solution 1:

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.

Solution 2:

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.

Solution 3:

I suggest fluid: (from TFD)

• (General Physics) capable of flowing and easily changing shape

Solution 4:

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.