# Pressure change during free expansion of an ideal gas

In free expansion W=0, even though volume changes and if q=0, then temperature and internal energy do not change. But what about pressure? I will present some arguments, please tell me which one(if any) is right, and also give your own arguments.

Consider ideal gas in an adiabatic container separated from vacuum side by a movable partition. Following are the arguments:

1. Pressure of a gas is its momentum transfer to a wall/boundary, and momentum is mass times velocity. But R. M. S velocity of an ideal gas is a function of its temperature and since temperature does not change in free expansion, therefore velocity should remain same, and therefore pressure also remains same.

2. Since temperature and amount of substance remain constant, therefore according to boyle's law, pressure should decrease.

3. Since volume increases, therefore the distance travelled by a molecule to hit the side wall also increases, decreasing the velocity, thus decreasing the pressure. But here the counter argument could be that the number of collisions between molecules, will also decrease as volume increases, which may neutralise the effect of increased volume on velocity.