boiling point

Chemistry

2022

We explain what the boiling point is and how it is calculated. Examples of boiling point. Melting and freezing point.

At normal pressure (1 atm), the boiling point of water is 100 ° C.

What is the boiling point?

The boiling point is the temperature to which the Pressure steam from liquid (pressure exerted by the gas phase on the liquid phase in a closed system at a certain temperature) is equal to the pressure surrounding the liquid. When this happens, the liquid turns into gas.

The boiling point is a property that strongly depends on the ambient pressure. A liquid subjected to a very high pressure will have a higher boiling point than if it is subjected to lower pressures, that is, it will take longer to vaporize when it is subjected to high pressure. Due to these boiling point variations, the IUPAC defined the standard boiling point: it is the temperature at which a liquid turns into vapor at a pressure of 1 bar.

An important point is that the boiling point of a substance cannot be increased indefinitely. When we increase the temperature of a liquid to pass its boiling point and still continue to increase it, we reach a temperature called "critical temperature." The critical temperature is the temperature above which the gas cannot be transformed into a liquid by increasing pressure, that is, it cannot be liquefied. At this temperature, there is no defined liquid phase or vapor phase.

The boiling point is different for each substance. This property depends on the molecular mass of the substance and the type of intermolecular forces that it presents (hydrogen bonding, permanent dipole, induced dipole), which in turn depends on whether the substance is polar covalent or nonpolar covalent (nonpolar).

When the temperature of a substance is below its boiling point, only part of its molecules located on its surface will have Energy enough to break the surface tension of the liquid and escape into the vapor phase. On the other hand, when heat is supplied to the system, there is an increase in the entropy of the system (tendency to disorder of the particles of the system).

How is the boiling point calculated?

Using the Clausius-Clapeyron equation, the phase transitions of a system composed of a single component can be characterized. This equation can be used to calculate the boiling point of substances and is applied as follows:

Where:

P1 is the pressure equal to 1 bar, or in atmospheres (0.986923 atm)

T1 is the boiling temperature (boiling point) of the component, measured at a pressure of 1 bar (P1) and expressed in degrees Kelvin (K).

P2 is the vapor pressure of the component expressed in bar or in atm.

T2 is the component temperature (expressed in degrees Kelvin) at which the vapor pressure P2 is measured.

𝚫H is the enthalpy change of vaporization average over the temperature range that is being calculated. It is expressed in J / mol or equivalent units of energy.

R is the gas constant equivalent to 8.314 J / Kmol

ln is the natural logarithm

The boiling temperature (boiling point) T1 is cleared

Examples of boiling point

Some known and recorded boiling points under normal pressure conditions (1 atm) are as follows:

  • Water: 100 ºC
  • Helium: -268.9 ºC
  • Hydrogen: -252.8 ºC
  • Calcium: 1484 ºC
  • Beryllium: 2471 ºC
  • Silicon: 3265 ºC
  • Carbon in the form of graphite: 4827 ºC
  • Boron: 3927 ºC
  • Molybdenum: 4639 ºC
  • Osmium: 5012 ºC
  • Tungsten: 5930 ºC

Melting point

The melting point is the temperature at which a substance changes from a solid to a liquid state.

The temperature at which a solid turns into a liquid is called the melting point and during the solid-liquid phase transition the temperature is kept constant. In this case, heat is supplied to the system until its temperature rises sufficiently for the system to movement his particles in the solid structure is greater, which causes them to separate and flow towards the liquid phase.

Melting point also depends on pressure and is generally equal to the freezing point of matter (at which a liquid becomes solid when cooled enough) for most substances.

Freezing point

The freezing point is the opposite of the melting point, that is, the temperature at which a liquid contracts, its particles lose movement and acquire a structure stiffer, deformation resistant and shape memory (unique to the substances in solid state). That is, it is the temperature at which the liquid turns into a solid. The merger requires supplying caloric energy to the system, while freezing requires removing heat energy (cooling).

On the other hand, the freezing point also depends on the pressure. An example is what happens when water is cooled to a temperature of 0ºC to 1 atm, when it freezes and turns into ice. If it is cooled to a pressure very different from 1 atm, the result could be very different, for example, if the pressure is much higher, it could take time to freeze, as its freezing point decreases.

Melting point and boiling point of water

Water is often used as a standard when measuring the melting and boiling points of substances. In general terms, at normal pressure, its boiling point is 100ºC and its melting point is 0ºC (in the case of ice). This can vary greatly in cases where the Water have other substances dissolved in it, liquid or solid, such as sea water, rich in salts, which modifies its physical and chemical properties.

The impact of pressure is also very noticeable. It is known that at 1 atm the boiling point of water is 100 ºC, but taking it to 0.06 atm we would be surprised to notice that the boiling occurs at 0 ºC (instead of freezing).

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