Considering the Charles's Law, if you decrease the temperature of the gas by turning down the flame, the volume of the 400 L ballon decreases.
Charles's Law consists of the relationship between the volume and temperature of a certain amount of ideal gas, which is maintained at a constant pressure.
For a certain amount of gas at a constant pressure, as the temperature increases, the volume of the gas increases and as the temperature decreases, the volume of the gas decreases because the temperature is directly related to the energy of movement of the gas molecules.
That is, the volume is directly proportional to the temperature of the gas.
In this case, taking into account the Charles's Law, at constant pressure, if you decrease the temperature of the gas by turning down the flame, the volume of the 400 L ballon decreases.
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