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Name: Jose
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 2000

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.
Assistant Director
PG Research Foundation, Darien, Illinois

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