Why is volume of a gas in an open container assumed to be constant? I've read this in a book

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

How is volume of the gas constant in this case?
 A: When the container is open to atmosphere, gasses may exit or enter. This keeps the pressure inside equal to the pressure outside. this also keeps the volume of the gas inside the container the same unless the container itself expands or contracts. I believe that the statement should have been clearer on these points.
A: The pressure and volume will be constant in an open container .
REASON : when the container is open the pressure inside the container and outside the container will be same . At the same time volume inside and outside the container will be same
Thanku
A: First of all, the volume occupied by a gas is the volume of free space available for it to move freely.
When we open the vessel which contains the gas, some gas molecules escape due to more availability of free space and their random motion.
Not all gas molecules escape due to the small difference in pressure inside the vessel and outside the vessel....Some stay within the container
The molecules that stay within the container occupy the entire container(like when the vessel was closed)
That's why we say volume of the gas is constant
A: The volume of gas in an open container is the volume of the container itself. But volume can change if the container is expanding or contracting(if given in the question else let it be constant).... The pressure of the gas(in an open vessel) is equal to the surrounding atmospheric pressure as gases may exit or enter the container, so). This meanwhile need not be constant as it changes with the weather.
But considering you're studying for competitive exams or school, then assume that pressure and volume will be constant!
