What is indicated by the boiling point of a substance?

The boiling point of a substance is defined as the temperature at which its vapor pressure equals the surrounding atmospheric pressure. At this temperature, the liquid phase of the substance transitions to the gas phase, allowing bubbles of vapor to form within the liquid. This concept is crucial in understanding phase changes and is heavily relied upon in distillation and purification processes in organic chemistry.

The boiling point can vary with changes in atmospheric pressure, which is why the boiling point of a substance can differ at sea level compared to higher altitudes where atmospheric pressure is lower. Therefore, option C accurately captures this important relationship between vapor pressure and boiling point.