Under what condition will a liquid boil?

A liquid boils when its vapor pressure equals the atmospheric pressure. This phenomenon occurs because boiling is the process where molecules within the liquid gain enough energy to transition into the gas phase throughout the entire volume of the liquid, not just at the surface. When the vapor pressure reaches the external atmospheric pressure, bubbles of vapor can form within the liquid itself and rise to the surface, resulting in boiling.

The relationship between vapor pressure and atmospheric pressure is crucial; as the temperature of the liquid increases, its vapor pressure also increases. When these two pressures are equal, the liquid can no longer remain in the liquid state at that temperature and transitions to a gaseous state.

The other conditions mentioned do not facilitate boiling. For instance, simply having internal pressure exceeding atmospheric pressure does not guarantee boiling, as it does not address the vapor pressure of the liquid. Reaching the melting point involves a phase transition from solid to liquid, which does not pertain to boiling. Lastly, a decrease in viscosity may affect how easily a liquid moves but is not directly related to the boiling point or conditions required for boiling to occur.