The observation that is inconsistent with the ideal gas law is:
When temperature is held constant and volume increases, the pressure increases.
According to the ideal gas law (PV = nRT), if the temperature (T) is held constant and the volume (V) increases, the pressure (P) must decrease, assuming the amount of gas (n) is constant. This relationship is described by Boyle's Law, which states that pressure is inversely proportional to volume when temperature is constant.