In a typical phase diagram for a pure substance, what does point 'A' most commonly represent?

• A. The boiling point, where liquid turns into gas at 1 atm pressure.
• B. The melting point, where solid turns into liquid.
• C. The critical point, where the liquid and gas phases become indistinguishable.
• D. The triple point, where solid, liquid, and gas phases coexist in equilibrium.

Answer: (D)

Verified step by step guidance

1. Understand that a phase diagram for a pure substance shows the regions of stability for solid, liquid, and gas phases as a function of temperature and pressure.
2. Recognize that specific points on the phase diagram correspond to important phase transitions or unique conditions, such as melting point, boiling point, critical point, and triple point.
3. Recall that the boiling point is the temperature at which liquid turns into gas at 1 atm pressure, and the melting point is where solid turns into liquid.
4. Know that the critical point is the end of the liquid-gas boundary where the two phases become indistinguishable.
5. Identify that the triple point is the unique condition of temperature and pressure where solid, liquid, and gas phases coexist in equilibrium, which is commonly labeled as point 'A' on a phase diagram.