Final answer:
Air's pressure, volume, and temperature are treated as one substance when considering ideal gas behavior, particularly under low pressure and high temperature where the gases behave according to Charles's Law, with volume directly proportional to temperature at constant pressure.
Step-by-step explanation:
Air's pressure, volume, and temperature can indeed be treated as the characteristics of one substance in many scientific contexts, particularly when considering the behavior of ideal gases. The correct answer to the student's question is (a) One. In the ideal gas approximation, the substance's volume is proportional to its temperature and inversely proportional to its pressure. This relationship is articulated by the ideal gas law, which provides a reasonable approximation of the behavior of real gases under conditions of low pressure and high temperature, where the gas molecules are far apart and the interactions between them are minimal. However, at high pressure and low temperature, real gases deviate from the ideal gas behavior because intermolecular forces and the volume of the particles become significant. Volume is directly proportional to temperature when the pressure is constant, which reflects Charles's Law, one of the fundamental gas laws.