# Why is pressure and volume of a gas in an open container always constant?

I've read this line in a book

When air is heated in an open vessel, pressure is always atmospheric pressure and thus constant, and volume of the gas is constant.

So how are pressure and volume of the gas constant in this case?

• What book and what page? Jun 20, 2021 at 12:59

Pressure tries to expand the volume. Assuming that atmospheric Pressure is not changing here and the total pressure inside object is doubled by heating gas inside it, will result in expansion of object(if it can be stretched) when it starts increasing its volume the pressure on the walls starts decreasing (because increase in volume decreases number of molecules hitting the particular area of conatiner) this continues unitll pressure becomes same as in atmosphere. This will result in double volume of object. Here pressure remains same while volume changes. But as the conatiner is open in this case, you can't get more volume and it only depends on container

The volume of gas in an open container is the volume of the container. The pressure of the gas is that of the surrounding atmosphere. This need not be constant as it changes with the weather.

that is not the case in real life but i am pretty sure you came across the statement in a book related to competitive exam or in a solution of a question.

While solving questions we generally assume that the conditions in which we are working on remains constant so we assume that the atmospheric pressure and volume of container wont change.

The statement you mentioned is just an assumption we take while solving questions.

Volume is constant because the volume of the vessel will be constant. (There will be no geometrical change in the vessel)

Pressure is constant as in an open vessel, the Atmosphere exerts pressure on the open surface uniformly and constantly.