In a typical phase diagram, in which region does a substance exist in both the solid phase and the liquid phase simultaneously?

• A. Along the solid-liquid equilibrium line (melting curve)
• B. In the area labeled 'liquid'
• C. In the area labeled 'solid'
• D. Along the liquid-gas equilibrium line (vaporization curve)

Answer: (A)

Verified step by step guidance

1. Understand that a phase diagram shows the state of a substance (solid, liquid, gas) at different temperatures and pressures.
2. Recognize that the lines on a phase diagram represent equilibrium conditions where two phases coexist in balance.
3. Identify the solid-liquid equilibrium line (also called the melting curve) as the boundary where solid and liquid phases coexist.
4. Note that along this line, the substance can exist simultaneously as both solid and liquid because the system is at the melting/freezing point for given pressure and temperature.
5. Conclude that the correct region for coexistence of solid and liquid phases is along the solid-liquid equilibrium line, not inside the solid or liquid regions or along the liquid-gas equilibrium line.