What is absolute temperature?

Absolute temperature is temperature measured in terms of an absolute scale, such as the Kelvin scale and the Rankine scale. This means that it is a scale that starts from zero (it does not admit negative values), in which value it is understood that there is an absence of temperature. In other words, absolute temperature is understood to be the temperature that begins to be measured from absolute zero, which is the lowest temperature that can be reached according to the laws of physics.

What is temperature?

Temperature can be defined in different ways. On the one hand, it is a property of matter that makes it possible to determine when two bodies are in thermal equilibrium with each other. By defining it in this way, it is possible to establish a relative scale of temperature, since what matters is what the temperature of a body or system is in relation to the other. This is the idea that allowed the development of common temperature scales, namely the Celsius or Centigrade scale and the Fahrenheit scale.

On the other hand, temperature is also a measure of the thermal agitation of the particles that make up a system. In fact, according to the kinetic-molecular model of gases, temperature is a direct measure of the average translational kinetic energy of the atoms and molecules that make up a gas.

Setting the absolute temperature scale

The absolute temperature was first determined by studying the behavior of gases. For example, the Lay of Charles and Gay Lussac states that there is a direct proportional relationship between the temperature and the volume of an ideal gas, as expressed by the following equation:

where K is a constant of proportionality. This equation has the form of an increasing linear function with slope K. It is experimentally observed that the slope increases with the number of moles of gas, and decreases with pressure, as schematized in the following image.

By extrapolating back these plots of relative temperature (in Celsius or in Fahrenheit ) versus volume for different initial pressures and for different initial amounts of gas, it can be seen that all lines intersect the temperature axis at the same point, regardless of the value of the slope. This point represents absolute zero, that is, the starting point of the absolute temperature and corresponds to a value of -273.15 °C or -459.67 °F.

More generally, temperature can be associated with the ideal gas equation of state, that is:

where T is the absolute temperature, P, V, and n are the pressure, volume, and number of moles, and R is the ideal gas constant. This law allows us to measure absolute temperature in different ways through the use of a gas thermometer.

Absolute temperature scales

Regardless of the units in which the absolute temperature is expressed, all the scales begin at the same point, that is, at absolute zero. This temperature is not expressed in any unit since the units are irrelevant when expressing the absence of a physical property. That is, the temperature at absolute zero is 0 (not 0K or 0°R). This is so for any absolute physical quantity, for example, it would be the same to say that the volume of a liquid is zero to say that it is zero liters, or zero cubic meters or zero cubic miles, so zero is preferred.

For all other temperatures, it becomes necessary to express them in suitable units. There are two absolute temperature scales in common use:

• The Kelvin scale.
• Rankine scale.

The Kelvin scale of temperature

We owe this scale to Lord Kelvin, formerly known as William Thomson, who, in 1848, designed a thermometer capable of measuring absolute temperature regardless of the gas from which it was made. This scale (named the thermodynamic scale of temperature, but later renamed to honor Lord Kelvin) turned out to be identical to the scale developed by extrapolation from the PT or VT curves.

The main feature of this scale is that the size of the unit (the kelvin or K) is exactly the same as the Celsius scale. In fact, the Kelvin temperature scale is nothing more than the Celsius scale moved 273.15 units to the right. The relationship between the Kelvin scale and the centigrade scale is therefore:

The kelvin scale is by far the most widely used absolute temperature scale in science and engineering.

Rankine temperature scale

This is the absolute temperature scale that has a degree size equal to a degree Fahrenheit. The zero of this scale is equivalent to -459.67 °F, so it represents the same Fahrenheit scale translated 459.67 units to the right. That is, the Rankine scale is related to the Fahrenheit scale through the following equation:

Relationship between the Kelvin scale and the Rankine scale

Since both the Rankine scale and the Kelvin scale are absolute temperature scales, they both start at the same point, so the only difference between the two is the size of the degree. The relationship between both scales is then the same relationship between the magnitude of a Celsius degree and a Fahrenheit degree. Since 1 °C is equal to 9/5 or 1.8 °F, then the relationship between °R and K is:

References

Atkins, P., & dePaula, J. (2010). Physical Chemistry (8th ed.). Panamerican Medical Editorial.

Chang, R., & Goldsby, K. (2013). Chemistry (11th ed.). McGraw-Hill Interamericana de España SL

Connor, N. (2020, January 16). What is the Kelvin scale? Absolute temperature: definition . Thermal Engineering. https://www.thermal-engineering.org/en/what-is-the-kelvin-scale-absolute-temperature-definition/

Odaris. (n.d.). Absolute temperature definition . deQuimica.Com. https://dequimica.com/glosario/504/Absolute-Temperature

Spiegato. (2021, July 14). What is absolute temperature? https://spiegato.com/en/what-is-absolute-temperature

Absolute temperature . (2010). ES-Academic. https://es-academic.com/dic.nsf/eswiki/440424

Kinetic theory of gases . (n.d.). Sc.Ehu.Es. http://www.sc.ehu.es/sbweb/fisica/estadistica/gasIdeal/gasIdeal.html