Under which conditions are propane (C_3H_8) molecules in a liquid state?

• A. At temperatures below the melting point and low pressure
• B. At temperatures above the boiling point and low pressure
• C. At temperatures below the boiling point and pressures above atmospheric pressure
• D. At temperatures above the critical temperature and any pressure

Answer: (C)

Verified step by step guidance

1. Understand the phase behavior of propane (C\_3H\_8) by considering its phase diagram, which shows the states of matter (solid, liquid, gas) at various temperatures and pressures.
2. Recall that the melting point is the temperature at which propane transitions from solid to liquid at 1 atm pressure, and the boiling point is the temperature at which it transitions from liquid to gas at 1 atm pressure.
3. Recognize that for propane to be in the liquid state, the temperature must be between the melting point and boiling point, and the pressure must be sufficient to maintain the liquid phase (usually above atmospheric pressure if temperature is near boiling point).
4. Note that at temperatures below the melting point, propane is solid, and at temperatures above the boiling point and low pressure, it is gaseous.
5. Understand that above the critical temperature, propane cannot be liquefied regardless of pressure, so liquid propane exists only below the critical temperature and at pressures above atmospheric pressure.