What is “Ideal” about an ideal gas? The picture we have of gases is molecules bouncing around like hard balls. We know that when we play billiards it makes a difference if we hit the ball at the center or off center. If we try to write equations for bouncing molecules, being careful if they hit on or off center, it will be very hard. We assume for simplicity that the molecules are so small that it makes no difference.

 

We also ignore the actual shapes of the molecules.

 

Another thing we notice that if we drop a ball to the floor and it bounces, it usually will not reach the height that we dropped it from, i.e., collisions are not completely elastic. The ball can stick a little bit to the floor. Again, for simplicity we ignore this, and assume that the collisions between the molecules are completely elastic, that is, they are very hard and not at all sticky..

 

With these assumptions, it becomes very easy to calculate the physics of gases. Gases that obey these assumptions are called ideal gases. If we have to be concerned about the actual sizes of the molecules or their stickiness, then the gases are called real gases. Real gases are not discussed in high school.