Salt Melting Points
If I mix two salts, the melting point of this mixed salts
is lower than the melting point of each salt. Why? (The salts could be
ammonium nitrate and monoammonium phosphate).
Most chemical substances, not just salts, behave this way. The reason is
that crystals consist of molecules arranged in a very specific order in a
repeating lattice. Introducing molecules of substance B into a crystal of
substance A would disrupt the crystal packing. So, when a molten mixture of
two different substances is cooled, the crystals that form when the mixture
solidifies tend to be a mixture of crystals of pure A and crystals of pure
B, not crystals containing both A and B.
This lowers the melting (freezing) point because it requires energy to
separate A and B, which were mixed in solution, into segregated crystals.
Crystals of pure A and pure B won't form as easily from an A/B mixture as
they would from pure liquid A or B. The mixture needs to be cooled down
further to begin to form a solid. In thermodynamic terms, the entropy loss
upon forming segregated crystals from a liquid mixture is greater than the
entropy loss upon simply freezing a pure liquid substance. Consequently,
cooling the mixture down to the freezing point of either pure component
doesn't remove enough entropy from the mixture to get crystals to form.
Now, there are exceptions. Sometimes, two different substances can form a
solid mixture in which molecules of both appear in the repeating lattice.
The "co-crystal" may even be more stable than the crystals of either pure
component, with a HIGHER melting point. However, even in these cases, a
mixture of the two components that is not in exactly the right ratio for the
co-crystal (say, 1:1) will have a lower freezing point.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
Click here to return to the Chemistry Archives
Update: June 2012