When a substance is pure? // Properties in their phase changes - Relationship with the food industry

It is important to study the properties of pure substances, since pure substances change phase altering their physical properties and states in which they can occur, it is also important the physical analysis performed to the processes in which phase changes for these pure substances. One of the most relevant analyzes are those made to the pressure - volume - temperature diagrams.

In the modern world there is a great application and use of certain components such as gas used in air conditioners, refrigerators, whose purpose is to function as a cooler in the conservation processes of some bodies of mass, these gases can enter the categorization of pure substances, so their study is of primary importance, and with the help provided by thermodynamics we have adopted optimized processes for the use of these pure substances.

Pure substance

In order to classify an element as a pure substance, it has to comply with a fixed chemical composition, pure substances are water, nitrogen, helium and carbon dioxide. Perhaps many of us dear steemians and lovers of thermodynamics and pure sciences think that a pure substance only has to be made up of a single chemical element or compound, and this is totally wrong, since a mixture of several chemical elements or chemical compounds It can also be a pure substance, the only requirement that has to meet to be a pure substance is that the resulting mixture has to be homogeneous. A clear example of pure substance that is a mixture of several compounds, but that is considered pure substance is air, air has a uniform chemical composition, however a mixture that we can not consider a pure substance is for example a mixture of water and oil, since this mixture makes the water not soluble in the oil, which makes it a non-homogeneous mixture therefore it is not considered a pure substance.

Image source Author: Markus Schweiss. Licensing: GNU Free Documentation

Image description: Water and water vapor are two phases of the same chemical substance: they differ in their state of aggregation (liquid and gas respectively).

The conclusion of the case for pure substances, is that they can present mixtures with different elements or chemical compounds, and because of this condition they are pure substances, the only indispensable condition is that they are homogeneous mixtures. However, under this point we ask ourselves the following:

If there was a substance formed by a single chemical element or compound, and that it was made up of two or more phases, would it be considered a pure substance?

The answer is yes, that is, a mixture of two or more phases is considered a pure substance as long as the chemical composition of the phases is the same. For this situation I will describe several examples so that we can describe if the following mixtures are pure substances.

In the case of water, although a mixture of two phases is shown, these two phases have the same chemical composition, that is to say, despite the fact that there are changes in properties such as pressure, volume and temperature so that these two phases can occur Water states do not change their chemical compositions, therefore this mixture of liquid water and vapor is a pure substance.

In this case the states: liquid and gas of the air present different chemical composition, reason why we can conclude that this mixture in spite of being the same component (air) the chemical compositions of the two phases are different.

Phases of a pure substance

Many of us have seen that in nature some compounds are presented in different phases, some of these elements retain their chemical properties, while in others they are altered. Such is the case of the two examples explained above, where the water is presented in a liquid state and in the form of vapor, without altering the chemical composition of the water, while in the other example it can be seen that the air does not it preserves the same chemical composition in liquid and gaseous state.

Not only are the examples cited above, we can also find at ambient temperature and pressure in solid state to copper, to mercury in liquid state and nitrogen in gaseous state. However, these three compounds could be presented in different phases if the pressure and temperature conditions are varied. A substance can have several phases within a main phase, for example in the case of water, it is being in the liquid state small gas phase can be found, it can occur in vice versa, that is to say while in the gaseous state small phases of gas can be found. liquid. Other cases are the following: carbon exists as graphite or diamond in the solid state, helium has two liquid phases, and iron three solid phases.

Image source Author: Diana Reyes Ayala. Licensing: Creative Commons Attribution-Share Alike 4.0

Image description: Heterogeneous mixture where two phases are distinguished.

The conclusion for this case of phase changes is that there are elements and compounds that do not stop changing their phase states while their environmental environment varies due to pressure and temperature. For example, helium subjected to high pressures can exist in seven different states.

A phase has a different molecular configuration, where it is homogeneous in all its parts, separating from the other phases by easily identifiable frontier surfaces.

Conclusions

Being able to study the different behaviors of the matter is of vital importance at present, I as a future food engineer need to know how to evaluate the different states of certain pure substances for the correct processing of certain foods that are presented in the food industry in solid states , liquid and gaseous.

Many foods need to be pressurized and subjected to various temperature changes, with the sole purpose of forcing them to change their physical condition, and in many cases their chemical composition, all with the intention of seeking the ideal conditions to preserve them for a long time. more time.

It is fundamental characteristics for our understanding in the profile of food engineers to be able to understand the physical and chemical changes of the most elementary pure substances that are commonly used in food industries, that is why I wanted to present this article, briefly highlighting some considerations theoretical about how to identify a pure substance from another that is not, with the primary objective that once the pure substance is identified, it will have a different treatment than the one that is not. It is also important to emphasize the importance of being able to know all the implements in order to be able to evaluate the pressures and temperatures to which certain substances have to be changing in order to reach the desired state. Such is the case in the dairy industries, once the liquid milk is obtained, a set of standardized pressure and temperature operations is necessary to process the milk until it reaches its solid state (commonly known as milk powder).

I hope that this article has the necessary utility for all those friends steemians passionate about science, engineering and technical applications.

Reference consulted

Thermodynamics book. Author: Yunus A Cengel. Michael A. Boles. 7th edition. Editorial Mc Graw Hill. Mexico 2011.

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