Why is it volume increase as temperature increases at constant pressure?
You’ve hit upon it already, no pun intended. Think of pressure as being proportional to the number of collisions between the gas molecules and the walls of the container. If something can expand, and you hit it harder, then it will expand. As it expands, and there is more volume, then there are the same number of collisions between the gas molecules and the walls of the container as before — hence, a constant pressure. And by the way, if you are thinking of pressure in terms of the force exerted on the walls of the container, then remember that mathematically you are integrating that over then entire area of the container. In other words, you are getting the sum of the forces. So as the force goes up, and the area goes up the force per area stays the same, and pressure is force per area.
