We explain what the boiling point is and how it is calculated. Boiling point examples. Melting and freezing point.
What is the boiling point?
The boiling point is the temperature at which the vapor pressure of a liquid reaches that of the medium in which it is located, or in other words, the temperature at which a liquid leaves its state and passes into the gaseous state (vapor).
This point is reached by injecting heat to the physical system of the liquid, whose particles agitate more quickly and register a considerable increase in entropy (tendency to disorder the system), as particles located near the surface break the surface tension and They escape, turned into steam.
Different substances have different boiling points , since this depends on the molecular mass of the substance and the type of intermolecular forces it presents: covalent bonds (polar or non-polar) or hydrogen bonds, among others.
The speed with which this point is reached depends, in addition, on factors such as pressure, since the process is accelerated at higher pressures . On the other hand, the boiling point represents a maximum limit, since the temperature cannot increase beyond it, since there would be no more liquid to heat.
How is the boiling point calculated?
The formula for calculating the boiling point of substances is the Clausius-Clapeyron formula, which reads as follows:
T B = ([R ln (P 0 ) / ΔH vap ] + [1 / T 0 ]) -1
And where T B is the normal boiling point expressed in degrees Kelvin, R is the gas constant equivalent to 8.314 JK -1 .mol -1 , P 0 is the vapor pressure expressed in atmospheres, ΔH vap is the enthalpy of vaporization expressed in J / mol, T 0 is the temperature in degrees Kelvin at which the vapor pressure is measured and ln is the natural logarithm.
Boiling Point Examples
Some boiling points recorded and known under normal pressure conditions (1 atm) are as follows:
- Water: 100 ° C
- Helium: -268.9 ° C
- Hydrogen: -252.8 ° C
- Calcium: 1484 ° C
- Beryllium: 2471 ° C
- Silicon: 3265 ° C
- Carbon: 3825 ° C
- Boron: 4000 ° C
- Molybdenum: 4639 ° C
- Osmium: 5012 ° C
- Tungsten: 5555 ° C
The melting point is similarly the maximum temperature reached by a substance in a solid state before it enters the liquid state . It operates similarly to boiling: the system gains heat until the compact space of the solid structure is too small to contain the movement of the particles, which are held together but now with a unique laxity and fluidity of the liquids.
This point is also affected by the pressure at which the system is. This is what happens, for example, when melting ice.
The freezing point is the opposite of the melting point, that is, the temperature at which a liquid contracts , loses molecular movement and acquires a more rigid structure, resistant to deformation and with shape memory, unique to the substances in solid state . If the fusion requires injecting heat energy into the system, freezing requires removing heat energy (cooling).
Again, the pressure at which the substance is to be taken into account . This is what happens when you freeze water and get ice, for example.
Melting point and boiling water
The water is often used as a standard when measuring the melting points and boiling substances. And in general, at a normal pressure, its boiling point is 100 ° C and the melting point of 0 ° C (in the case of ice). This can vary greatly, however, in cases where the water has other substances dissolved in it, liquid or solid, as occurs with seawater, rich in salts, which modifies its physical and chemical properties.
The impact of the pressure is also very noticeable. It is known that at 1 atm the boiling point of water is 100 ° C, but taking it to 0.06 atmospheres we would be surprised to notice that the boiling occurs at 0 ° C (instead of freezing).