Revisiting Scenario H

So let’s revisit Scenario H and try to visualize the air particles inside of the syringe at each step.

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When we take off the blue cap and pull the plunger up to 35, air particles (represented by the 35 green dots) enter the syringe. Without the blue cap back on, the syringe is an open system and air particles can still get in and out. So when we push the plunger down to 20, some of the air particles (represented by 15 of the green dots) leave the syringe and there are only 20 green dots remaining.