Question

A liquid boils when its vapor pressure ________.

A) equals the external pressure.
B) exceeds the external pressure.
C) equals the equilibrium vapor pressure.
D) is between the equilibrium vapor pressure and the external pressure.
E) None of the other choices is correct.

Answer 1

A liquid boils when its vapor pressure equals the external pressure, which is the correct answer A. The normal boiling point is when this occurs under a pressure of 1 atm.

Step-by-step explanation:

A liquid boils when its vapor pressure equals the external pressure. Therefore, the correct answer to the question is A) equals the external pressure. The boiling point of a liquid is the temperature at which its equilibrium vapor pressure is equal to the pressure exerted on the liquid by its gaseous surroundings, typically the earth's atmosphere. In open containers, this is the atmospheric pressure. The normal boiling point of a liquid is its boiling point under a pressure of 1 atm (101.3 kPa).

As the temperature of a liquid increases, the vapor pressure also increases until it reaches the point where it is equal to the external atmospheric pressure. When this happens, bubbles of vapor form within the liquid and rise to the surface, signifying boiling.