Concept:

The boiling point is the specific temperature at which a liquid's internal vapor pressure becomes exactly equal to the external atmospheric pressure pushing down on it.

Formula:

$$ P_{\text{external}} \downarrow \implies \text{Boiling Temperature} \downarrow $$

Solution:

• Under standard 1 atm (760 torr) pressure, the normal boiling point of ethanol is \( 78^\circ\text{C} \).



• If we reduce the external pressure to half (e.g., placing it in a partial vacuum or on a very high mountain), the atmosphere provides much less downward resistance against vapor formation.



• Consequently, the ethanol does not need to be heated as much to match this lower pressure. It will achieve equilibrium at a lower temperature.



• Therefore, the boiling point will be less than \( 78^\circ\text{C} \).

Why other options are incorrect:

The boiling point doesn't stay the same (C) or increase (A) because thermodynamic equilibrium rules dictate otherwise. It does not reduce exactly to half (D) because the relationship between vapor pressure and temperature is logarithmic (Clausius-Clapeyron equation), not strictly linear.