What happens when a given amount of gas at a constant temperature increases in volume?
The gas would decrease in pressure.
According to Boyle's law which states that the volume of a fixed mass of gas is inversely proportional to its pressure at a constant temperature, an increase in volume would cause a decrease in pressure.
If we want to take it even further, a decrease in pressure would cause the gas molecules to spread further apart from each other. As this happens, the intermolecular forces of attraction approach zero and the gas behaves as an ideal gas.
(Kinetic Theory of Gases: Molecules in a gas exert no force on one another)