What does the binodal curve indicate in a phase diagram?

• A. The curve marks the boundary between single-phase and two-phase regions, with two-phase below.
• B. The curve indicates the temperature at which solid forms.
• C. The curve marks the critical point only.
• D. The curve is a horizontal line at fixed composition.

Answer: (A)

The binodal curve is the boundary that tells you when a mixture will separate into two distinct phases versus remain as a single homogeneous phase. In a binary phase diagram, outside the curve the system is one phase for that temperature and overall composition; inside the curve, the mixture is unstable and splits into two coexisting phases. The compositions of those two phases at a given temperature are found at the ends of the tie-line drawn across the two-phase region.

That’s why this option is the best: it directly captures that the binodal marks the transition between single-phase and two-phase behavior, with the two-phase region lying inside the curve. The other statements don’t fit because the binodal is not simply the temperature for solid formation, it is not only a single critical point, and it is not a fixed horizontal line at a single composition—the boundary moves with temperature and composition and defines a dome-shaped region.