Chemical reaction is most often occurring with the constant parameter of ______?
Likely, we're talking about pressure.
Chemical reactions (the fun ones, anyway) often take place at constant pressure (such as in a coffee-cup calorimeter).
If a process is open to the air, it tends to not induce a pressure change in the entire room (provided the room is sufficiently large). My professor calls this "lab bench" conditions.
On the other hand, processes in a rigid, closed container, have constant volume (such as in a bomb calorimeter). This tends to be experimentally-induced, rather than naturally occurring.
In an instance like that, you cannot easily have constant pressure, because for example, if gases are produced and then expand, they push on the rigid container and increase the pressure of the system.