Hai priyanka,for law of conservation of linear momentum to be valid the following two conditions must be satisfied.        1)net external force acting on the system must be zero.(the system is isolated)2)no particles should leave or enter into the system.(the system is closed)                                                                                                                    THEREFORE FOR ISOLATED AND CLOSED SYSTEMS LAW OF CONSERVATION OF LINEAR MOMENTUM IS VALID.I HOPE U UNDERSTOOD. CHEERS....

Dear

 In entire physics it is yet to discover if in a closed isolated system momentum is changing spontaneously

   

i.e.

If  in your sytem there is no external force momentum of your system must remain same

 

 

No law is physics is as sacred as conservation of angular and linear momentum

 

Newtons third law fails in Electromagnetics but not conservation of momentum