It would be very unadvisable to place a can of soup over a campfire without venting the can. As the can heats up, it may explode. The kinetic-molecular theory explains why. The air inside the rigid can of soup is given more kinetic energy by the heat coming from the campfire. The kinetic energy causes the air molecules to move faster and they impact the container walls more frequently and with more force. The increase in pressure inside may eventually exceed the strength of the can and it will explode. An additional factor is that the soup may begin boiling which will then aid even more gas and more pressure to the inside of the can.
Beth grabbed her container of chili from he fridge and tossed in the microwave for 4 minutes. However, she forgot to vent the lid and after several minutes the dish exploded spraying chili all over the inside of the microwave.
Did the liqued increase or decrease the pressure inside the dish??