In a pressure versus time plot for runaway reactions, what does Curve B represent?

• A. Single-phase gas
• B. Two-phase flow (vapor and liquid)
• C. Liquid pool spreading
• D. Solid-liquid mixture

Answer: (B)

The key idea is recognizing a two-phase region where vapor and liquid coexist. In a runaway scenario, heat drives liquid to vapor, so liquid and vapor are present at the same time. This vapor–liquid mix controls the pressure because energy goes into generating vapor (latent heat) rather than just raising temperature, so the pressure changes in a characteristic way during boiling rather than behaving like a pure gas. Curve B on a pressure–time plot typically shows this boiling behavior: the pressure rises as vapor forms, but the rate is moderated by the phase change, producing a distinct, gradual increase or plateau while boiling continues. Once all liquid has vaporized, the curve would evolve toward the single-phase gas behavior with a different slope.