# Water filters, urine, and beer

Kragen Sitaker kragen@pobox.com
Sun, 14 Apr 2002 03:11:49 -0400 (EDT)

```Dan Kohn writes:
> Essentially, this was the point of my experiment.  Eugene, can you
> provide any info on the particulate sizes of the chemicals that make
> beer and urine beer and urine?  I'm trying to remember my high school
> chemistry on the difference between a colloid, suspension, and solution.
> (http://www.factmonster.com/ce6/sci/A0847314.html).
>
> The Waterworks can filter 0.2 micron particles.  What is the size of
> dissolved salt?  Why does reverse osmosis
> <http://www.howstuffworks.com/question29.htm> work when a ceramic
> filter doesn't?  Inquiring minds want to know.

Well, IIRC, a carbon atom is about an angstrom (0.0001 micron) across.
I think the actual dissolved-salt ions are about that size, but a
little smaller than a water molecule; but I think they have to go
through pores while attached to water molecules.  I'm offline at the
moment (in Socorro for my mom's fiftieth birthday), so I can't look
that up.  But I'm guessing you need pores of one nanometer (ten
angstroms) or smaller to do reverse osmosis.

I think urea molecules are considerably larger than salt ions, on the
order of five or ten angstroms across.

--
/* By Kragen Sitaker, http://pobox.com/~kragen/puzzle5.html */
char b[2000],m[]={1,-80,-1,80};main(){int i,x=1000,s=2000,d=0;while(1+(i=
getchar()))switch(i){case'f':b[x]=1;case'g':x=(x+m[d/2]+s)%s;d--;case'+':d+=2;
case'-':d+=7;d%=8;}for(i=0;i!=s;i++)putchar(" #"[b[i]]);}

```