MadSci Network: Chemistry

Re: What happens to the molecules in a physical change for a solid to a liquid?

Date: Sat Dec 11 02:07:27 1999
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Chemistry
ID: 944443404.Ch

Greetings, Sara:

This is a very complex issue, which has been under study for decades.  The 
answer I offer here will cover considerable territory, because many things 
in science are related -- and it is fun to digress.  Still, I hope you 
will find it satisfactory.

The first thing to keep in mind is that all atoms and molecules are 
constantly vibrating.  Some of this vibration is an inherent part of the 
existence of any particle, and some of it is simply an indication of how 
little freedom a particle has to move about.  Consider a tiny tiny box 
that contains just one particle.  If the box is only slightly larger than 
the particle, then an overall motion of the particle, bouncing off the 
walls, will be practically indistinguishable from a stationary vibration. 
 If the box is considerably larger than the particle, then the particle 
might be imagined able to smoothly move about the interior of the box.  
However, real particles will vibrate anyway, even while moving significant 
distances before hitting the walls of a container.

For many molecules, an altogether different type of vibration is possible. 
 Let me portray a water molecule as  <  (oxygen on the left, with two 
hydrogens extending towards the right).  WITHOUT ROTATING, it is possible 
for the molecule to vibrate to the opposite  >  position.  And it need not 
stop there; it is able to vibrate back and forth:
  from  <  to  >  to  <  to  >  ...
This type of vibration is frequently associated with the absorption or 
emission of radiant energy (radio microwaves, usually, but it depends on 
the molecule).

The next thing to keep in mind is that the overall rate of all types of 
vibration is highly dependent upon the temperature:  the more heat, the 
more vibes, and that includes overall physical motion of particles.  
Inside a typical object, say food in an ordinary oven, if its surface is 
heated, the heavily vibrating particles there will collide with interior 
and less-active neighboring molecules, and transfer heat to them.  Then 
those molecules can transfer heat to THEIR neigbors, and gradually the 
whole chunk of food gets warmer via this 'conduction' process.  A 
microwave oven is designed to generate radiant-energy waves that cause 
water molecules to vibrate as described in the previous paragraph; this 
vibration is indistinguishable from heat to the neighboring molecules in 
food.  Microwave energy penetrates the food completely, is absorbed by 
water molecules throughout, causes them to vibrate against and thereby 
heat up their neighboring molecules -- and the longer this goes on, the 
more the food cooks.  Since it bypasses the time needed for heat to 
conduct from the surface to the interior of food, a microwave is much more 
efficient than a conventional oven.

Now let's consider a solid substance about to be melted.  Its atoms or 
molecules are all very close to each other, and for any one particle in 
the middle of the solid, this can easily be imagined as being the 
equivalent of confinement inside a tiny box.  The REASON all the particles 
are so close together is the third thing to remember:  everything 
attracts.  All atoms and molecules experience an attraction for each 
other, and depending on the type of particles, the amount of attraction 
may be large or small.  (This attraction is electrostatic, not 
gravitational.  Even though an atom may be electrically neutral, it still 
consists of distinct particles with distinct electric charges, which are 
seldom evenly distributed.  'Holes' in the distribution of electrons 
allows the positively-charged nucleus of one atom to attract the 
negatively-charged electrons of neighboring atoms.  These attractions are 
known as 'van der Waals forces', after the fellow who first figured it 
out.)  Atoms and molecules possessing well-distributed electrons don't 
attract each other much, and are generally gases even at fairly low 
temperatures; particles that attract each other a lot generally stay 
clumped into solids even at pretty high temperatures.  SOME of the rest of 
the time, groups of particles may exist together in the liquid state.

The liquid state is relatively uncommon in the Universe, compared to the 
other six known states of matter.  [In order of increasing density of 
particles, there are plasmas, gases, liquids, and solids, followed by 
degenerate matter (found in white-dwarf stars), neutronium (in neutron 
stars, of course), and 'singularitum' (is there a better name for matter 
in the state of infinitely-shrinking-towards-a-single-mathematical-point 
that occurs inside a black hole?).  I might note that plasmas, which are 
mixtures of ionized atoms and loose electrons, often exist in a wide range 
of densities:  Where the Sun's outer atmosphere becomes the Solar Wind is 
a very-low-density plasma; if one journeyed to the center of the Sun, one 
would encounter plasma all the way, gradually increasing in density.  The 
very core of the Sun consists of plasma at the maximum possible density, 
known as degenerate matter -- a cubic centimeter might contain a metric 
ton of mass -- and the overall average density of all the Sun's mass of 
plasma is about 1.4 times that of water on Earth.]

The main reason why liquids are less common in the Universe than solids or 
gases is because they require two things which are not always found 
together in the right amounts:  temperature and pressure.  If one starts 
with a cool solid substance and heats it, individual particles in it will 
start vibrating faster and faster.  Eventually some of them will break 
loose from the attractions of nearby other particles.  What is to keep 
them from flying out of this description altogether?  For some substances, 
such as frozen carbon dioxide (dry ice), it is NORMAL on Earth for the 
solid to directly dissipate (or 'sublime') into the gaseous state, as it 
is heated.  For other substances, such as water, the Earth's atmosphere 
provides one aspect of a barrier.  A water molecule that has broken free 
from an ice cube will most certainly bounce off some air molecules, and 
very likely find itself heading right back to the ice cube!  The second 
aspect of the barrier consists of the attraction of water molecules for 
each other; it is fairly easy for the returning molecule to become trapped 
again (and this does not happen to carbon dioxide, because the attraction 
between those molecules is much less).  The third aspect of the barrier is 
simple Physics:  When a fast-moving particle collides with a slower-moving 
particle, USUALLY the fast-moving particle slows down, and the 
slower-moving particle speeds up.  (It is the fundamental basis of the 
process of conduction, mentioned earlier.)   This phenomenon means that a 
returning water molecule will often lose enough energy that it can no 
longer escape the attractions of the neighboring molecules.  The returned 
molecule may still possess enough energy to move fairly freely on the 
surface of the ice cube; as a result, a growing film of liquid water can 
cover the cube.

One of the more complicated aspects of the preceding description involves 
the fact that when a moving particle encounters a relatively massive and 
non-moving object, the particle can often bounce off with very little loss 
of energy.  For carbon dioxide, a molecule returning to a dry ice cube 
encounters exactly this situation, so the molecule can re-escape, and the 
cube can sublime.  For a regular ice cube, the film of liquid on its 
surface prevents a clean bounce; the returning molecule encounters a 
MOVING water-film molecule, so it can lose energy easily and stay confined 
by the three-aspect barrier.  Yet now one can imagine a chicken-and-egg 
situation:  How does the film of water form when it seems to require 
itself to maintain itself?  The answer lies in the gradual rise in 
temperature, which permits the surface molecules of an ice cube to loosen 
enough to start forming the film, but not enough to completely escape -- 
so that when some DO completely escape, and get bounced back by the 
atmosphere, there are enough loose molecules to provide the third aspect 
of the barrier.

The less the pressure of the atmosphere, the less is its effectiveness as 
the first aspect of the barrier described above.  This is well known to 
anyone who ever lived at sea level, and then moved to a high altitude.  
The boiling point of water is the temperature at which a majority of 
moving surface molecules can overcome the atmosphere's tendency to bounce 
them back into their container.  (When only a minority of molecules can 
penetrate the atmosphere's barrier, we call the process 'evaporation'.)  
The higher the altitude, the lower the boiling point.  If the atmospheric 
pressure is low enough, plain old water ice will sublime from solid to 
gas, just like dry ice.  It works the other way, too, of course.  Pressure 
cookers were invented to compensate for the atmosphere's weakness as a 
barrier to water's boiling, at high altitudes.  (At sea level, the boiling 
point of water inside a pressure cooker will be significantly higher than 
100 degrees Celsius, because it effectively adds to the atmospheric 
barrier.)  And if enough atmospheric pressure surrounded dry ice, then 
heating it will yield liquid carbon dioxide, just as you might expect.

As has been stated in lengthy fashion, when a solid substance begins to 
melt, its constituent particles move faster, and break away from the 
attraction that previously held them in place.  If there is an appropriate 
container such as sufficient air pressure, then intermittent mutual 
attractions, as they bounce off each other, can be sufficient to let them 
form the liquid state.  Since each particle moves about more freely than 
when in the solid state, it is generally true that it takes up more space, 
and on the whole, the liquid occupies more space than the solid.  Even for 
liquid water, which takes about 10% LESS space than when solid, there is 
plenty of space between molecules.  (Ice floats on liquid water since it 
is less dense.  Out in space near the planet Jupiter is the moon Europa, 
which has a surface fully covered by ice.  The temperature is so cold that 
the ice does not sublime into the vacuum of space.  The available evidence 
strongly suggests that the interior of Europa is warm enough to melt ice; 
the liquid state is possible on a moon with little gravity and no 
atmosphere, because the exterior layer of ice forms an effective 

The fact that there is significant space between the particles making up a 
liquid is one of the keys to understanding most solvents.  The other key 
fact concerns the nature of the molecules of the solvent, and the nature 
of the substance which you might wish to dissolve.  While all the 
particles of any one substance attract each other equally well, different 
particles from different substances are not always so cooperative.  Since 
water molecules get along well with sugar molecules, a sugar cube placed 
in a cup of water will gradually disappear, as molecule after molecule 
breaks off and fits itself into the spaces between water molecules.  But 
water molecules do not get along well with most plastic molecules, so a 
pile of plastic dust poured into a cup of water will just sit there.  
Since other substances, such as benzene, do mesh well with various 
plastics, those substances are used when it is desired to dissolve 
plastics.  And like sugar in water, the process is the same:  molecule 
after molecule breaks off from the solid substance, and fits itself into 
the spaces between the particles of the liquid.

It may be true that just about any substance can be dissolved in some 
special-enough liquid.  The chemical element mercury is well-known for its 
abiltiy to dissolve gold, silver, and other metals.  Bauxite, or aluminum 
oxide (the stuff of which sapphires are made) is dissolved in a molten 
salt known as cryolite, before electrolysis separates the aluminum from 
its ore for our use.  And even though most chemistry-lab equipment is made 
of glass because few chemicals affect it, there ARE substances which can 
react with it chemically (hydrofluoric acid, for one), and probably there 
are substances which can dissolve glass like so much sugar in water 
(although I don't know the name of one offhand -- oxalic acid, perhaps?).

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