Explanation:
When gas in sealed container is heated, it molecules gain kinetic energy, and their average speed increases. This exert more force on the walls of the cylinder with valve, and they also collide with walls of the cylinder frequently. Therefore, gas is under greater pressure as the heat increase. As soon as the valve opens the gas will escape out of the cylinder. And if the valve is not opened in time, the extreme pressure could even rapture the cylinder.
