Pouring Solids, Acting as Fluids
Date: Summer 2012
Since you can pour sand into a cup, why is it not considered a liquid?
"Pouring" does not make something a liquid. For example, gases can be
poured as well. The definition of a solid is based on the fixed
position of its molecules/atoms relative to each other. In a liquid,
the molecules/atoms are free to move relative to each other (mix) yet
keep the same volume (in a gas, the atoms/molecules can expand in
volume based on pressure). Although you can rearrange different grains
of sand, the atoms in each grain of sand are fixed. Thus, it is a
Hope this helps,
Granular solids such as sand can share some of the properties we associate with liquids (i.e., flow), but sand would still be considered a solid at the particle level. A sand particle is solid and does not deform or flow, and its molecules are close packed together in a rigid structure. In a liquid the molecules are also close together, but they are free to move relative to one another. This gives the liquid the ability to flow. Sand also flows, but only because it exists as a mass of particulates in which the solid particles can move relative to each other. So, in summary, sand does exhibit one of the properties we associate with liquids (flow), but not all of them.
John C. Strong
You can call sand a liquid if you wish, but you will be alone.
Wikipedia defines a liquid as:
Liquid is a form of matter with a definite volume but no fixed shape. A liquid is made up of tiny vibrating particles of matter, such as atoms and molecules, held together by forces called chemical bonds.
The part that disqualifies sand as a liquid is that it does not consist of “tiny vibrating particles of matter, such as atoms and molecules, held together by forces called chemical bonds.”
I think you have read, or been told that “a liquid” is a material that has a definite volume but takes the shape of the container it occupies. While all this is true, it is incomplete. Sand, for example, can be poured as you point out; however, the volume of the sand does not occupy the entire volume available to it.
The definition of “a liquid” is one of those terms that everyone agrees on, in general, but the definition fails when one tries to give a definition that applies to all cases. The ability to “pour” is just one property of a liquid. Glasses (honey or window panes) are liquids, but they do not “pour” – at least not on a short time scale. A “liquid” in the absence of gravity may not conform to the shape of its container. This is because when gravity is negligible, surface tension becomes a dominant property. Liquids do not have a definite volume if the pressure is high enough. In those cases, the liquid does not have a definite constant volume. It is compressible.
You have raised a common general problem in science. There are few, if any, truly general definitions. In fact, what seems worrisome is an important property of scientific definitions. They are “falsifiable”. That is, there are always conditions where a definition does NOT apply. Definitions are not truly “universal” – even in mathematics. One example: (-1)^1/2, the square root of minus one is undefined in the real number system, but in the algebra of complex numbers, it is well defined, with well-defined properties.
Your question is “easy to ask” but “difficult to answer”.
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Update: November 2011