When does a substance reach its boiling point?

A substance reaches its boiling point when its vapor pressure equals the local atmospheric pressure. This is the defining characteristic of boiling: at the boiling point, the liquid's vapor pressure is sufficiently high that bubbles of vapor form within the liquid and can escape into the surrounding atmosphere.

As a liquid is heated, its vapor pressure increases. When this pressure matches the surrounding atmospheric pressure, the liquid can transition to the gas phase throughout the body of the liquid rather than just at the surface, allowing boiling to occur. This principle explains why boiling points vary with changes in atmospheric pressure—at higher altitudes, where atmospheric pressure is lower, liquids boil at lower temperatures.

In contrast, the other options describe conditions that don't accurately reflect the definition of boiling. For example, exceeding local atmospheric pressure or conditions related to freezing points do not pertain to the process of boiling, which fundamentally relies on the relationship between vapor pressure and atmospheric pressure.