Re: Why can't you reach absolute zero as said by thethird law of thermodynamics

Entropy is, roughly speaking, a measure of the disorder in a system.
The Second Law of Thermodynamics states that in a thermally isolated
system, the entropy tends to increase.  The entropy of one part of the
sytem can decrease as long as the entropy of the rest of the system
increases by a greater amount.  For example, it is possible to decrease
the entropy of water to form ice in a freezer, but to accomplish this
the freezer motor must heat up the room and raise its entropy.  The
entropy of the system (freezer + room) will always increase.

One might wonder if the temperature of one part of a system can be
reduced to absolute zero at the expense of the rest of the system.
    "The experimental evidence is to the contrary, and it is
    reasonable to take tentatively as a further principle of
    physics the following statement: It is impossible by any
    procedure, no matter how idealized, to reduce any system to the
    absolute zero of temperature in a finite number of operations.
    This position was first enunciated by Nernst and has been
    elevated to the position of a fundamental law by Fowler and
    Guggenheim who call it the Third Law of Thermodynamics."
    (from Zemansky "Heat and Thermodynamics," 2nd ed. 1943 p.279)

So the brief answer to your question is that you can not reach absolute
zero experimentally, and that *IS* the Third Law of Thermodynamics.
There are many equivalent ways to state the Third Law, one of them
being "As the temperature of a system approaches absolute zero, the
entropy approaches a constant independent of all parameters of the
system."  This version of the Third Law can be derived from Zemansky's
version, and vice versa.

Perhaps, after reading this response, you have two concerns:
1) How can the Third Law be a "law" when it is just a statement of an
   experimental observation?
2) Why is it impossible to reach absolute zero experimentally?

Let me address them in turn:
1) ALL the "laws" of physics are statements of experimental
   observations.  Even the esteemed Law of Conservation of Energy is 
   merely the summary of observational evidence.  If one of the existing
   laws were found to be violated, it would either be modified to fit 
   the new data or it would be abandonned.

2) Atoms obey the rules of Quantum Mechanics, and one of the rules
   is that an atom can not have a definite position and simultaneously
   a definite momentum.  To achieve absolute zero temperature, the
   atoms must have definite positions (at the lattice sites in your
   example of a crystal) and definite momenta, namely zero.  If they 
   had nonzero momenta, they would move and bump into neighboring
   atoms and the random motion would give a nonzero temperature.
   Therefore, the atoms of the crystal are always moving; this 
   movement which is present even at extremely cold temperatures is 
   called "zero-point motion."

--Randall J. Scalise    http://www.phys.psu.edu/~scalise