What does it mean to say that momentum is conserved?

Conservation of momentum. So long as no external forces are acting on the objects involved, the total momentum stays the same in explosions and collisions. We say that momentum is conserved. You can use this idea to work out the mass, velocity or momentum of an object in an explosion or collision.

10.063g / 10.00 ml = 1.0063 g/ml which rounds off to 4 sig figs as: 1.006 g/ml