Ask A Scientist Chemistry Archive

  
name         Krista
status       other
grade        other
location     PA

Question -   How do you find the volume and density of a powder?
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There are two ways to define the volume and/or density of a powder. One is
the packing volume, or density. This refers to the volume/density of the
powder including the voids. Obviously this is going to depend upon how hard
you pack the powder, but sometimes this definition is important if, for
example, you are interested in the packing of soil. The more common
definition if volume/density of a powder is the volume/density of the solid
material comprising the powder. This determination is as follows:
Use a liquid that will not dissolve or react with the solid powder. It will
also be necessary to make sure that no air remains adsorbed on the powder
when the powder and liquid are mixed. Often this is not a problem, but for
very fluffy powders like baking flour some steps would have to be taken to
assure that the air is removed. For the present, assume that is not a
problem. Weigh out a known amount of powder, call it Wp. Weigh out a known
volume of liquid, Vl, call that weight Wl. The ratio: Wl / Vl = Dl, the
density of the liquid. Now mix the weighed powder, Wp and a sufficient
amount of the liquid so that the volume of the powder, Vp, (which you do not
know yet) + the volume of the liquid = the original volume of the liquid,
Vl. This tell you how much liquid was replaced by the powder particles. So
we have three weights: Wl the weight of the known volume of liquid, Wp the
weight of the dry powder, and Wmixture = Wp + (Wl - Wd) where Wd is the
weight of liquid that has been replaced by the powder.
Since you know the density of the liquid, Dl = Wl / Vl, from above, you can
calculate the volume of liquid that has been replaced by the powder, so this
is equal to the volume of powder, Vp, because the powder and liquid have not
reacted, dissolved or otherwise interacted. And since you know the weight of
the powder Wp you know its density, Dp = Wp / Vp.

Vince Calder
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