Question

How common salt is used as freezing mixture?

Answer 1

Residual Volume : The volume of air remaining in the lungs after a maximal expiratory effort.

Role : Residual Volume is important because it prevents the lungs from collapsing. It also prevents the inside surfaces of the lungs touching, thus creating friction. It also prevents the inner surfaces of the lungs sticking together. 
Another reason why we have a residual volume is that gaseous exchange is continuously occurring so after we have expelled as much air as possible (expiratory reserve volume) gaseous exchange is still occurring. 

Answer 2

Answer:

it is called freezing mixture because it lowers the freezing point of water or ice