Final answer:
The pressure inside a syringe is due to gas molecules colliding with the syringe walls, and this pressure can change with volume adjustments, according to Boyle's Law. Decreasing syringe volume increases pressure; increasing volume decreases it. Changes in temperature also affect the pressure.
Step-by-step explanation:
A gas exerts pressure on the inside of a syringe due to the collisions of gas molecules with the syringe's walls. Each collision exerts a force on the walls, and the sum of these collisions results in gas pressure. According to Boyle's Law, if you decrease the volume of the syringe by pushing the plunger in, the gas molecules have less space to move, which leads to more frequent collisions and thus increased pressure. Conversely, pulling the plunger out increases the volume and the gas pressure decreases, due to less frequent molecular collisions. For example, when a syringe is filled with hot steam and then cooled, the steam condenses and its pressure decreases, allowing the higher atmospheric pressure to push the plunger in. This illustrates how changes in temperature can affect gas pressure inside a container.