Re: I would like to know about freezing things, and how and why they change.

I would like to know about freezing things, and how and why they change.  
At home in my freezer I freeze many things, but when de-frosted they all 
return to normal.  Is there anything, that when freezed does not return to 
the original substance?  If things are frozen does the density change?  
Does anything not freeze, for instance- metal, does it really freeze, or 
just get cold. How does freezing happen? 

Wow, what a question!

Essentially you have asked 4 things, of which I will try to answer in this 
order:
1.)	How does freezing happen?
2.)	Does the density of a substance change when frozen?
3.)	Is there any substance that is converted to a new substance by 
freezing and subsequent thawing?
4.)	Is there any substance that does not freeze?

     Your questions cut nearly to the heart of the study of stuff, or 
matter to be precise.  I can tell you have been thinking deeply on these 
topics, and I congratulate you.  You have asked structured questions about 
the world, and appear to have tested the validity of your hypotheses.  No 
good scientist can do more than that!  Nevertheless, there are some pieces 
of information that you know already by heart but you have not 
considered.  You know from experience that matter can be classified into a 
few very general categories.  Sure, matter can be trees and sea water and 
birds and the air that we breathe, but more generally, all pure 
substances, or stuff that is made of only one type of element or compound, 
occurs as either a gas, a liquid, or a solid.  These three ways that 
matter can exist are called states of matter.  Take pure water for 
example.  Water can exist as solid ice, liquid water, and gaseous water 
vapor.  If you think about that for a minute, you will realize that solid 
water tends to be “cold”, liquid water tends to be “warm” and water vapor 
tends to be “hot”.  In fact, you can change from one to another by 
changing the “heat energy” of a substance.  To convert ice to liquid water 
you have to increase the heat energy of the water by increasing the 
temperature to above 0 degrees Celsius (at normal atmospheric pressure).  
To convert solid aspirin to liquid aspirin, you would have to increase the 
temperature of the aspirin to above 143 degrees Celsius.  The opposites 
are also true.  To freeze water you have to remove thermal energy by 
decreasing the temperature to below 0 degrees Celsius.

     Why does this change occur?  Matter is made of discrete pieces.  
Generally these pieces are atoms or combinations of atoms.  I will call 
them for simplicity, molecules.  Molecules are in constant motion, 
jiggling this way and that, vibrating and rotating.  When you measure the 
temperature of a substance, you are measuring the amount of jiggling that 
the molecules of a substance are doing (and not really the vibrations or 
the rotations; that is more complicated.)  Precisely, temperature is a 
measure of how fast, on average, molecules are moving (or at least how 
much momentum they have, which takes into account how heavy they are, but 
don’t worry about that).  As you increase temperature, or add heat energy, 
you increase the speed of the molecules.  As you decrease temperature, you 
are slowing the molecules down.  In a gas, molecules are flying this way 
and that with essentially no heed for one another; however, molecules in 
solids or liquids interact strongly with each other, compared to molecules 
in a gas, and in various ways.  I am talking here about inter-molecular 
interactions between two different molecules and not intra-molecular 
interactions, which could be called chemical bonds.  These intermolecular 
interactions have specific strengths.  If the stuff that you are heating 
is a solid, and you heat the substance enough, you will eventually make 
the molecules juggle around so much that you break the intermolecular 
interactions that hold the solid together, and the solid melts to liquid.  
Let’s look at the system from the other direction.  In a liquid that is 
freezing, you are reducing the speed of the molecules until the 
interactions are stronger that the energy (momentum) inherent to the 
moving molecule.  The molecules will become trapped and the liquid becomes 
a solid.  

(You should, by this point, be able to answer your own question as to 
whether solid metal will freeze.  It’s already frozen.) 

     Density is a measure of how much volume a given mass of substance 
occupies and will increase as the volume of a specific mass decreases.  
When you “remove” heat energy, molecules tend to get closer together.  
Because the distance between molecules is getting smaller, the volume that 
the substance occupies will also get smaller and the density larger.  
Changes in density also tend to accompany changes in state as 
intermolecular forces lose or gain control.  There is no rule that says 
density must increase or decrease, but most of the time it will change, 
and most of the time the solid will be denser than the liquid.  There are 
some exceptions.  When you freeze water (which is really quite an odd 
substance despite being one of the most abundant molecules on earth) the 
molecules of water stop sliding past one another and start to stick 
together in an ordered arrangement.  The ordered arrangement of water 
takes up more space than the sloppy, slippery sliding arrangement water 
had when liquid.  As a result, water expands as it freezes by about 10%.  
You should never freeze stuff, especially water, in closed glass jars 
because the jars will break as the water expands.  I remember reading in 
my old chemistry textbook that some researchers froze water in a steel 
ball with inch-thick sides and the expanding water generated enough 
pressure to burst the steel walls!  That having been said, most stuff will 
get smaller when it freezes but you can be sure there are plenty of 
exceptions.  

     Because freezing is a change of state, melting a frozen hunk of 
matter will generate the same liquid substance with which you started.  
This is the answer to your question, pure and simple.  If we look, though, 
we can find some apparent exceptions to this “rule”.  One apparent 
exception could be freezing salt water.  Let’s say you dissolved salt into 
water and allowed the salt water to begin freezing, but only allowed half 
of the water freeze, dumping the rest of the unfrozen water down the 
sink.  If my rule were true, you would expect the frozen water to melt 
into the same salt water that you started with, right?  Well, if you 
rinsed off the frozen water and then melted it and tested it for salt by 
determining how well it conducted electricity, or more simply just by 
tasting it, you would find that the melted water was more or less salt-
free! (You should never “taste” experiments since that could get you in 
trouble, but if you are using food-grade salt and water, tasting in this 
case is fine).  There is a phrase that I very much like that says “the 
exception proves the rule”, meaning that if you look very carefully at 
exceptions you will often find inherent properties that when included in 
your analysis show you the exception was anything but.  There are facts 
about this system that are a dead give-away.  I told you that a PURE 
substance, when frozen and then melted, will be the same, start to 
finish.  But here we are dealing with a mixture of two things: salt 
dissolved in water.  Now things are trickier and all bets are off, but the 
explanation is the answer to some other question.  

     Finally, to my knowledge, there are no substances that do not 
freeze.  You could make a good argument that glasses do not freeze, but 
this is again the exception that proves the rule.  Glasses, in the 
simplest sense, are fluids that have cooled so fast that they have not had 
the chance to crystallize.  I don’t know if there exists any experimental 
evidence to back me up, but theoretically, if given the time (like 50 
bazillion years), glasses will eventually crystallize and “freeze.”  

     There is at least one substance that ALMOST never freezes.  Helium 
can be cooled as a gas to almost 2.17 degrees above the absolutely lowest 
temperature that you can go (Absolute zero, 0 Kelvin, or 0 K, where all 
thermal motion stops, or at least where every aspect of the substance is 
in the lowest possible energy state).  Everything else like air and water 
would be frozen solid.  Below 2.17 K helium turns to a liquid (there is 
another very strange, fermionic, Helium-3, that does not condense until 
you reach <0.03 K but we will ignore that for the moment).  You would 
think that if you cooled helium further it would become a solid, but that 
is not the case.  Helium will not freeze!  Well…that is not quite true.  
If you increased the pressure on liquid helium while keeping the 
temperature at 1 K until you reached thirty times the pressure you and I 
are feeling right now from our atmosphere, helium would freeze.  I don’t 
know of any other material that reaches this low of a temperature as 
anything but a solid, so I would say there are no substances that do not 
freeze.  This question is truly subtle and I could imagine a professor of 
quantum thermodynamics making a lecture from it.  I can contribute little 
more.  There is one other “substance” that someone might argue may 
not “freeze”.  This substance, called a Bose-Einstein condensate, is 
really more of a state of matter that a substance that can freeze and 
thaw.  


     Don’t stop experimenting and learning.  Take a chemistry class as 
soon as you can.  It is never too early.  Also, do not accept my answer 
without thinking about and understanding the concepts.  There is much that 
I have left out (and you never know, I could be wrong!)

     
Sincerely, 


Chris Reiter