If steam occupies 100. L at 400. K and 101.3 kPa, what will its volume be at 100. K and 202.6 kPa? The combined gas law predicts that at one quarter the temperature, and double the pressure, the volume of the vapor will be cut to an eighth of its original or 12.5 L. There is a problem, however. At 100. K (–173°C), steam is a chilly chunk of ice. The gas laws don't apply. Long before the phase change occurs, the volume of the particles, and the attractions between the particles become significant, and the linear relationship between temperature and volume break down. This PowerPoint presentation examines the assumptions of the gas laws and discusses the conditions under which they are most accurate. Free work sheets on this topic are available on my website at www.evanschemistrycorner.com.