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Pure substances: concept and classification

From a chemical point of view, when studying matter and its composition, we say that a substance is pure if it is made up of only one type of atoms, molecules, or compounds , that is, if the chemical entities that make up matter are identical to each other. In other disciplines, the definition is extended to homogeneous mixtures, which do not vary even if the physical conditions in which they are found change.

Key facts:

1.- In chemistry, a substance is pure if it is composed of only one type of atoms or molecules.

2.- In a more general sense, a pure substance is any homogeneous mixture. That is, when matter appears uniform in appearance and composition, no matter how small the sample size.

3.- A chemically pure substance cannot be broken down into simpler substances using physical methods, such as settling or filtration.

4.- A pure substance has its own and clearly defined characteristics.

Examples and details to consider

1.- They are pure substances, among many others, tin, sulfur, diamond, water, pure sugar (sucrose), table salt (sodium chloride) and sodium bicarbonate. Crystals, in general, are pure substances.

2.- All elements are pure substances. Sugar, salt, and baking soda are pure substances that are compounds. On the other hand, examples of crystals are salt, diamond, certain proteins, and crystallized copper sulfate.

3.- Some homogeneous mixtures can be considered examples of pure substances, although from the strictly chemical point of view they are not. Examples of homogeneous mixtures include vegetable oil, honey, and air. While these substances contain multiple types of molecules, their composition is consistent across all samples. 

4.- Air is a homogeneous mixture that is often considered as a pure substance.

5.- By joining two or more pure substances, a mixture is obtained.

On the contrary, heterogeneous mixtures are not pure substances, such as gravel, a mixture of salt and sugar, etc.

How to recognize a pure substance?

If the chemical formula of a substance can be written or if it is a single element, we are dealing with a pure substance. The identification of a pure substance is based on the study of its chemical entities; if they are identical and cannot be separated by physical methods, we are dealing with a pure substance.

References

Barrera, R. (2017). Estimation of properties of pure substances and mixtures with Aspen Plus. University of Antioquia.

Easy Science for Everyone (2018) Pure substances. Available at https://www.youtube.com/watch?v=deV07dQWCTM&ab_channel=CienciaF%C3%A1cilParaTodos

Pure substances and mixtures (2019) Bioprofe. Available at https://bioprofe.com/sustancias-puras-y-mezclas/