Final answer:
Vapor pressure lowering occurs when a nonvolatile substance is dissolved in a volatile liquid, resulting in a decrease in the liquid's vapor pressure. This alteration in vapor pressure affects the phase graph by shifting the vapor pressure-temperature curve downwards.
Step-by-step explanation:
Vapor pressure lowering occurs when a nonvolatile substance is dissolved in a volatile liquid, resulting in a decrease in the liquid's vapor pressure. This can be explained by considering the effect of the added solute molecules on the vaporization and condensation processes of the solvent. The presence of solute molecules decreases the surface area available for solvent vaporization, leading to a slower rate of vaporization and a lower vapor pressure. This alteration in vapor pressure affects the phase graph by shifting the vapor pressure-temperature curve downwards.