To help you visualize the principle of Conservation of Momentum, I suggest that you watch the video below.
The collision shown in this video is elastic (Where Kinetic Energy & Momentum are conserved)
Bravo to the video creator.
In real life situation, the particle of gas are moving at high speed and collides with each other. The figure below shows you the real situation for the conservation of momentum
The momentum before and after collision can be calculated using this formula (m= mass, u =initial velocity & v= final velocity)
Figure above shows you an example of elastic collision where both bodies move separately after colliding)