Conservation of linear momentum expresses the fact that a body or system of bodies in motion retains its total momentum, the product of mass and vector velocity, unless an external force is applied to it. In an isolated system (such as the universe), there are no external forces, so momentum is always conserved.
Postulates of Kinetic Molecular Theory of Gases:
A gas consists of a collection of small particles traveling in straight-line motion and obeying Newton's Laws.
The molecules in a gas occupy no volume (that is, they are points).
Collisions between molecules are perfectly elastic (that is, no energy is gained or lost during the collision).
There are no attractive or repulsive forces between the molecules.