For an isolated system (where the net external force ${\vec{F}}_{ext}=0$), the total linear momentum is always conserved. Internal forces (like those during a collision or explosion) appear in equal and opposite pairs according to Newton's Third Law, so their net impulse on the entire system is zero. While internal forces can change the individual momenta of parts of the system and can change the system's kinetic energy (e.g., converting it to heat), the total $\sum \vec{p}$ remains constant.