Open and Closed Systems
When the blue cap is off, the syringe system is open. That means that air particles are free to go in and out of the syringe. However, when the blue cap is on, the syringe system is closed, and the air particles in the syringe can’t get out and air particles on the outside can’t get in. Drag the plungers of the two syringes below to see the difference between an open system and a closed system.
As you can see, in the closed system, the number of air particles in the syringe always stays the same. And in the open system, when you pull the plunger up, more air particles come in; and when you push the plunger down, some of the air particles go out.