[AdiDex]

Why double salts form....??
Which kind of force act between simple compounds to form double salts....??
 

[Arkcon]

Your question is very general, and unfortunately, very technical.  Lets try to figure this out stepwise:  Can you give us an example of a double salt, and some simpler compounds made of the same elements?

[AdiDex]

Yep i know some....
Mohr Salt - (NH₄)₂Fe(SO₄)₂·6H₂O
Potash Alum -K₂SO₄.Al₂(SO₄)₃.24H₂O
And another one is Carnalite

[Arkcon]

There's a summary, poor though it is, on this page here:  http://en.wikipedia.org/wiki/Double_salt  Briefly, double salts form simply because they can -- the two ions fit in the same crystalline lattice so when present together form the double salt.  Examples have been known for centuries and you may find some insights into what you want to know by looking at specific examples:
http://en.wikipedia.org/wiki/Tutton_salt
http://en.wikipedia.org/wiki/Alum

[AWK]

Double salt in water solution in fact is a physical mixture of ions. As a crystal it is formed because of  good packing which is energetically favorable.

[AdiDex]

That means it is just a rearrangement of ions..
They just rearrange themselves to achieve a more stable lattice..
Is this 100% true.,.
Or anything else should be added to my statement....??

[Borek]

AWK already gave you the best possible answer:

As a crystal it is formed because of  good packing which is energetically favorable.

[AdiDex]

That means it is just a rearrangement of ions..
They just rearrange themselves to achieve a more stable lattice..
Is this 100% true.,.
Or anything else should be added to my statement....??

[Borek]

In the end EVERY chemical reaction can be explained by thermodynamics - either by stability of the products, or by the entropic effects. Creation of the double salts is not different.