Ask A Scientist Chemistry Archive

name         Erica D.
status       student
age          12

Question -   Hello,
My name is Erica. I am in year 7 and I have a question to ask. My
teacher taught us about sublimation. I asked her," I understand the
process of how a solid can turn into a gas without melting. How can a gas
turn into a solid without becoming a liquid?" Then she said," Try and
research it on the Internet." I try to search for some sites, but all
they tell us is the process of a solid into a gas. Can you please help me?
----------------------------------------
A gas can turn into a solid without becoming liquid much in the same way
that a solid dissolved in a liquid can form solid crystals.  The gaseous
molecules diffuse about until they strike the surface of a crystal made of
like molecules, to which they can attach, making the crystal grow.  It
does not need to form a liquid first.

Frost and snow form this way.

Richard E. Barrans Jr., Ph.D.
Director of Academic Programs
PG Research Foundation, Darien, Illinois
==============================================================
The reverse sublimation happens just like sublimation in reverse gear.
Sublimation occurs from the surface of a solid when the temperature is high
enough for a molecule(s) on the surface to have enough kinetic energy to get
"bumped" into the gas phase. Reverse, or inverse, sublimation occurs when
the vapor in the presence of a solid phase has some molecules that are "lazy
enough" to stick to the surface. These adsorbed molecules "skate" across and
around the surface until they find a location in the crystal lattice to find
a "home". They have enough energy to move around on the surface. Some could
get "bumped" again and return to the vapor phase, but more have enough
energy to wander around until they happen to find a vacant spot in the
solid's crystal lattice. This is also promoted by the vibrations of other
molecules of the solid on the surface. You can picture this if you (thought
experiment, you do not have to do it) think of placing a tennis ball on a
sheet or blanket and then "whip" the sheet or blanket. The tennis ball is
propelled in the direction of the wave that the "whipping" produces. The
molecule will drop into a place in the solid crystal lattice if it finds a
vacant site. Its kinetic energy is transferred to the rest of the solid as
thermal vibrations. This is one of the processes that produces frost on cold
winter days.
     Now if the temperature is low enough, something else can also happen.
The vapor molecule with sufficiently small kinetic energy strikes the
surface, but because the temperature is much lower than the equilibrium
temperature (where sublimation, and inverse sublimation occur in equal
amounts) neither the molecule has sufficient kinetic energy to "skate"
across the surface, nor do the molecules on the surface of the solid have
sufficient kinetic energy to propel the molecule along the surface. The
molecule then just "sticks" where it hits. This is more or less a random
process and the solid has little or no order like a crystal. This is called
a GLASS. The "glassy" state is called meta-stable, because it has a higher
energy than the crystal solid at the same temperature and would "like" to
form a crystal, but there is not enough thermal energy available for the
molecules on the surface to "look around" and find a site in the crystal
lattice. If the glass is then heated to a sufficiently high temperature
there will be a transition from the glassy to the crystalline form of the
substance. Depending upon a number of factors, this may or may not occur.
For example, window glass (assume it is just silicon dioxide) has a higher
energy than a quartz crystal (also silicon dioxide), but even when it is
heated to high temperature and allowed to cool slowly, it almost always
reforms the glass rather than converting into the lower energy crystalline
form. In this case the viscosity of the molten glass is too great to allow
enough motion of the molecules to move around and form a crystal lattice.
     I know this is a long answer, but your perceptive question deserved a
detailed response. I hope it gives you some feel for what processes are
going on.

Vince Calder
==============================================================