Boiling point

The boiling point is defined as a thermodynamic state, associated to a determined temperature, called “boiling temperature”, and pressure, in correspondence of which the boiling process takes place.

In particular, the boiling temperature is the temperature at which the vapor pressure of a liquid equals the external pressure and the liquid begins to boil: it is a chemical-physical property of a pure substance or mixture and it is determined by the values of temperature and pressure in which coexist the liquid and aeriform phases.

When the temperature reaches the boiling point, aeriform bubbles begin to form spontaneously inside the liquid mass, which quickly rise to the surface releasing their contents in the environment above, shaking the liquid in a characteristic way: a boiling liquid is in fact the site of very strong convective motions due to the thrust of the bubbles to rise to the surface.

During the transition from liquid phase to aeriform phase, the substance or mixture absorbs a certain amount of heat to overcome the forces of attraction that hold together the atoms or molecules, and the temperature remains constant until the whole mass has made the change of state. This amount of heat has a characteristic value for each substance and represents the latent heat of vaporization. After that, as we continue to heat, the temperature begins to rise again.

Since the boiling temperature varies depending on the external pressure to which the substance is subjected, for example the statement that water boils at 100 °C is true only at sea level (pressure equal to about 1 atmosphere), while in the mountains boiling occurs at a lower temperature (for example, at 3.000 m above sea level it boils at about 90 °C).

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