Question

Draw a cylinder with a movable piston containing six molecules of a liquid. A pressure of 1 atm is exerted on the piston. Next draw the same cylinder after the liquid has been vaporized. A pressure of one atmosphere is still exerted on the piston. Is work done on the system or by the system?

Short answer

Answer: During the vaporization process under a constant pressure of 1 atm, work is being done by the system as the molecules change from the liquid state to the vapor state and push the piston up.

Step-by-step solution

Draw the initial state with liquid molecules

Draw a cylinder with a movable piston at the top. Inside the cylinder, sketch six molecules of liquid. Label this drawing as "Initial State." It's important to have a clear visualization of this scenario where a pressure of 1 atm is exerted on the piston, and the molecules are in a liquid state.

Draw the final state with vaporized molecules

Draw another cylinder, similar to the one in Step 1, with the same movable piston at the top. Inside the cylinder, sketch six molecules, but this time, represent them as vaporized (gaseous) molecules. Label this drawing as "Final State." Notice that the piston is pushed up in comparison to the initial state as the vaporized molecules require more space than liquid molecules, exerting a pressure of 1 atm on the piston.

Observe and compare piston position

Compare the initial and final state drawings and observe the position of the pistons. In the final state, as the pressure inside the cylinder is still 1 atm, the piston would have been pushed up, giving more space for the vaporized molecules.

Determine if work is done on the system, or by the system

Review the change in the piston position as observed in the drawings. Since the piston has moved up against the constant atmospheric pressure, work is done by the system (the liquid molecules vaporizing). The system expands in the presence of constant external pressure, and the piston moves up, which means the system is doing work on pushing the piston up. In conclusion, work is being done by the system during the vaporization process, where the molecules change from the liquid state to the vapor state under a constant pressure of 1 atm.