[sun]

All right, so there's a figure showing the reaction of zinc metal with hydrochloric acid in a beaker open to the atmosphere at pressure P. Underneath the figure explaining the process, the textbook reads the following: "The beginning of the reaction. The constant pressure of the atmosphere has been replaced by a piston and weight to give an equivalent pressure."

The volume of the system has increased, so the piston and weight are lifted upward. Work was done. However, I do not understand what the textbook means when it says specifically, "to given an equivalent pressure." I believe there are gaps in my knowledge, and I'm misreading it.

[Borek]

The constant pressure of the atmosphere has been replaced by a piston and weight to give an equivalent pressure."

The pressure that is now exerted by the piston is exactly the same as the pressure that was exerted by the atmosphere. Nothing more sophisticated.