Which option correctly describes the phase described for acetone as a fuel?

• A. Evaporation
• B. Vapor-liquid equilibrium
• C. Condensation
• D. Sublimation

Answer: (B)

Vapor-liquid equilibrium is the situation where both liquid acetone and its vapor coexist and exchange molecules at a given temperature and pressure. In a fuel context, acetone can evaporate from the liquid into the air above it, while some vapor can condense back into liquid. This balance—evaporation continuing while condensation catches up—defines the equilibrium between the two phases. Evaporation alone is only the process of liquid turning to gas, without implying coexistence at a steady state; condensation is the reverse; sublimation would involve solid to gas, which isn’t relevant for acetone under typical conditions. Since the fuel system often has both liquid acetone and acetone vapor present at the operating conditions, the correct description is vapor-liquid equilibrium.