In a phase diagram, at what temperature does the solid phase begin to melt?

• A. At the triple point
• B. At the boiling point
• C. At the critical temperature
• D. At the melting point

Answer: (D)

Verified step by step guidance

1. Understand that a phase diagram shows the states of matter (solid, liquid, gas) of a substance at different temperatures and pressures.
2. Identify the melting point on the phase diagram as the temperature at which the solid phase starts to change into the liquid phase under a given pressure.
3. Recognize that the triple point is where solid, liquid, and gas phases coexist in equilibrium, but melting begins specifically at the melting point, not necessarily at the triple point.
4. Note that the boiling point is the temperature where liquid turns into gas, which is different from melting.
5. Understand that the critical temperature is the highest temperature at which a liquid and gas can coexist, unrelated to the melting process.