Along the binodal curve, the two phases are in equilibrium. True or false?

• A. True
• B. False
• C. It represents the three-phase region
• D. It is a vertical line

Answer: (A)

The binodal curve marks the boundary where a mixture can separate into two phases in equilibrium. Along this curve, the two phases are in thermodynamic balance with each other, so the two-phase mixture exists in equilibrium at the boundary. In a binary phase diagram, points inside the curve correspond to a two-phase region with a tie line connecting the compositions of the coexisting phases; the binodal itself is the limit of that two-phase region, where equilibrium between the two phases is just achieved. So the statement is true. The alternative descriptions—such as representing a three-phase region or being a vertical line—do not describe what the binodal boundary represents.