Looking Further into Dalton's Law

The pressure of one gas (in this case, the oxygen) is caused by the collisions of the oxygen particles against the sides of the container.

If anything interferes with those collisions it should affect the pressure. So, if the nitrogen has no effect on the pressure of the oxygen, it would seem that the nitrogen does not interfere with the ability of the oxygen to collide with the walls of the container.

This is true to an extent, since gas particles are so far apart, they miss each other much more often than they collide, so one gas can essentially “ignore” another when they are both at relatively low densities (the particles aren't packed too closely together).

However, reading that last paragraph carefully (seeing words like “to an extent,” essentially,” and “relatively”) should indicate that this isn't the whole truth.

The reality is that, of course, particles of one gas will bump into particles of the other gas, and if one particle was going to hit the wall before the collision and now won't, there should be an affect on the pressure.

So, what's really going on? How can a second gas not change the pressure of the first gas?

The answer lies in the fact that collisions between gas particles are elastic – that is, no energy is lost when they collide. So, if a particle of oxygen was going to hit the wall with a certain amount of energy, that energy will still be carried to the wall, even if it is the nitrogen that carries it rather than the oxygen.

From the wall's perspective, there has been no change, the same energy hits the wall that would have if the collision had never happened.