Question

Vapour is injected at a uniform rate in a closed vessel which was initially evacuated. What will happen to the pressure in the vessel?
It will increase continuously.
It will first decrease continuously.
It first increases then decreases.
It first increases then becomes constant.

Answer 1

Hint:At a given temperature, the pressure at which the vapours are at a thermodynamic equilibrium with its condensed state is called Saturated Vapour Pressure (SVP). Use the concept of Saturated Vapour Pressure to solve the above question.




Complete answer:
On injecting vapours in a closed vessel, the pressure starts to increase.
As it is injected at a uniform rate, the pressure of the closed vessel increases at a uniform rate as well.
It increases and reaches a point where it becomes equivalent to the Saturated Vapour Pressure (SVP). At this point, the vapours start to condense.
On injecting the vapours beyond achieving this Saturated Vapour Pressure, the vapours continue to condense. This means that the pressure is not increasing further, that is, it has achieved a constant value.
Hence, on injecting vapours at a uniform rate in a closed vessel which was initially evacuated, the pressure first starts to increase and then becomes constant.
Thus, the correct option is D.



Note: In the given question, it is important to note that at the point of Saturated Vapour Pressure, the vapours are at thermodynamic equilibrium with the liquid form. On injecting more vapours, the system will go back to its state of equilibrium, and hence, the vapours will condense. In this way, the pressure remains constant.