Final answer:
According to the ideal gas law, the volume of gas must have decreased if the pressure doubled from 2 bar to 4 bar while temperature and the amount of gas remained constant.
Step-by-step explanation:
If a volume of 20 liters of gas is kept in a container with flexible walls at constant temperature and the pressure is observed to increase from 2 bar to 4 bar, we can deduce what happened to the volume of the gas by using the ideal gas law, PV = nRT, where P is pressure, V is volume, n is the amount of gas in moles, T is the temperature, and R is the gas constant. Since n and T are constant, and R is a constant value, the increase in pressure implies that the volume must have decreased. This is because the product of pressure (P) and volume (V) must remain constant when n, R, and T are held constant according to the ideal gas law.