What happens when the vapor pressure of a liquid is equal to the external pressure? The liquid freezes The intermolecular forces become stronger in the liquid The liquid boils Dispersion forces become weaker in the liquid An equilibrium exists between the liquid and the solid state.

The liquid boils.

Explanation:

Vapor pressure is simply defined as the pressure exerted on a substance (solid/liquid) by the vapor of the substance collected just at the top of the surface of the substance. In concise words, it is the pressure of Vapor that is in contact with its solid or liquid state.

For a liquid, it is the pressure of the Vapor gathering at the top of the surface of the liquid.

When this Vapor pressure matches the external pressure, the temperature stays constant and the molecules of the liquid all through the liquid can gain enough energy, rise to the surface of the liquid and break free in gaseous form; thereby, boiling.

The definition of boiling point basically explains that it is the point at which temperature stays constant, and the vapour pressure of the liquid matches the atmospheric/external pressure around the liquid and its liquid molecules change into vapor.

This is why liquids boil faster at higher altitudes; the atmospheric pressure at higher altitudes is reduced, hence, the temperature at which liquid boils at this high altitude is normally lower than its known boiling point temperature.

It is also why food cooks to a temperature higher than the boiling point of water in a pressure cooker/pot. The added pressure ensures that the cooking water boils at temperatures higher than its boiling point; thereby exposing the cooking ingredients to a higher temperature, leading to faster cooking.

Hence, it is obvious why boiling is the answer to this question.

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