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Knowledge database: Basics: Melting and boiling point

Each liquid has a specific pressure caused by its evaporation, called vapor pressure. The vapor pressure increases as the temperature increases. When the vapor pressure is equal to the external pressure, which in standard conditions equals 101325 Pa or 101.325 kPa, the system reaches its boiling point. Knowing this, one can easily conclude that the boiling point depends on the pressure acting on the system. At a lower external pressure, a lower temperature is required to reach boiling. That's why alpinists, at altitudes of several thousand meters above sea level, notice that water doesn't boil at 100 °C, but already at a lower temperature. This occurs because the atmospheric pressure is lower at higher altitudes.

Melting temperature is the temperature at which the substance goes from solid to liquid state. In the melting point, solid and liquid phases are in equilibrium, similar to liquid and gaseous phases at the boiling point. It is also important to mention the phenomenon that occurs at each stage of phase conversion. When one has a system that goes from one phase to another, for example, when ice turns into water, bringing more heat does not change the temperature. This occurs because the heat that is introduced in this period, doesn't rise the temperature of the water, as it is spent on tearing intermolecular bonds that hold the water molecules in the structure of ice. Similarly, this rule is valid in the opposite transitions too, and the transitions involving other phases (e.g. liquid to gas, etc).

chemistry tutorials - melting and boiling points

The range of boiling and melting point values in the PTE.


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