Ask A Scientist© General Science Archive

name         John
status       student
age          15

Question -   I am doing a science experiment on fog lights, and I was
wondering if dry ice would be a good substitution for fog. If it is, how
does the dry ice simulate fog?
------------------------------------------------
It's not a substitute; it's the real thing.  Fog is nothing but a cloud
on the ground, and a cloud is nothing but a collection of water
droplets small enough to be so easily blown around by the wind that the
gravitational force on them is negligible by comparison.  (This is
possible, by the way, because the gravitational force on a water
droplet is proportional to its mass, which is proportional to the
droplet's radius cubed.  The wind force on a droplet is proportional to
its surface area, which is proportional to the radius squared.  So, for
a small enough droplet the wind force blows away the gravitational
force.  But I digress.)

Dry ice makes the air around it (including the water dissolved in that
air) cold, and cold water molecules aren't whizzing around fast enough
to hit each other and bounce off.  They stick together when they
collide, and make a droplet.  Of course, next thing you know, the
droplet is getting creamed by an air molecule which could knock off a
water molecule, so the droplet's size depends on the ratio of numbers
of air molecules to water molecules, and on the speed at which they are
moving (i.e., on the temperature).

Anyway, to make fog, all you need is cold air with some water dissolved
in it.  The fact that dry ice (frozen carbon dioxide) also sublimes and
this puts lots of carbon dioxide in the fog is ignorable here.

Tim Mooney 
==================================
Well, dry ice itself isn't much like fog, but you can make fog using dry
ice.  Fog is an aerosol of water droplets suspended in air.  It forms when
humid air is cooled, so that the concentration of moisture in the air is
above the condensation point at the new temperature.  The humidity then
condenses into small droplets, which are fog.

You can make your own fog by exposing humid air to dry ice.  The dry ice
chills that air enough to cause this condensation process to occur.  A
convenient way to make dry ice fog is to put a chunk of dry ice into warm
water.  The solid dry ice becomes carbon dioxide gas, carrying with i
tdroplets of water.  In short, a fog.

Richard Barrans Jr., Ph.D.
Chemical Separations Group
=======================================