Question

When calcium nitrate crystal are exposed to air a solution is formed

Answer 1

Well, the simplest reason is that it is a "hygroscopic " compound. So we know that a hygroscopic will draw the moisture from the air whenever exposed to air .
Calcium nitrate is also a hygroscopic one so whenever exposed to air it absorb moisture from the air  so more and more moisture absorption result in it's molecules dissolve in it and form a solution. it is actually a soluble hygroscopic salt of white color