The boiling point of a pure substance is defined as the temperature at which:
a) the kinetic energy of the molecules begins to increase
b) the vapor pressure equals the external pressure
c) the molecules of the substance break apart
d) the liquid phase can be completely evaporated
in advance

B) the vapor pressure equals the external pressure.

Explanation: Boiling point is the temperature at which pressure exerted by the vapors on the liquid is equal to the atmospheric pressure surrounding the liquid.

The kinetic energy begins to increase when the liquid is heated.

The molecules move farther from each other when liquid changes to vapor form.

The liquid phase can be completely evaporated if we continue to boil.

c. is the answer

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