Pure Substances and Mixtures

We differentiate a mixture of a pure substance normally by its physical constants, such as: boiling point (PE), melting point (PF), density (d) and solubility (solub). Pure substances maintain their constants during changes. different from mixtures. Example:

  • Pure water: - PE = 100 ° C; Mp = 0 ° C; d = 1g / cm 3.
  • Water and cooking salt (NaCl): do not have constants.
  • Pure Alcohol: PE = 78.5 ° C; Mp = -177 ° C; d = 0.79g / cm 3.
  • Alcohol and Water: do not have constants.


Mixture is the gathering of two or more substances without chemical reaction between them, and maintaining each their properties.

In the mixture of water and sugar, it is not possible to visually distinguish one component from another: sugar has disappeared into the water. We say this is a homogeneous mix.

Already in the mixture of water and talc, it is perfectly possible to visually distinguish the two substances. We then say that this is a heterogeneous mixture.

The definition of homogeneous and heterogeneous mixtures is linked to the way mixtures are perceived. Thus, a mixture that to the naked eye seems to us homogeneous, when viewed under the microscope may turn out to be heterogeneous. We then say that it is homogeneous from the macroscopic point of view and heterogeneous from the microscopic point of view.

Heterogeneous mixtures whose components can only be observed under a microscope as they are in the form of very small particles can be called colloids. Among others are colloids: foods like gelatin and mayonnaise; some pharmaceutical products such as ointments and creams; and the blood.

Heterogeneous mixtures have distinct phases. For example, a mixture of water and pieces of iron is a heterogeneous mixture in which two phases are clearly perceived, that is, two distinct components: water and iron. We then say that this is a biphasic mixture. In the same way we talk about three-phase, four-phase mixtures, etc.

Be careful not to confuse mixture with substance combination. If you add, for example, hydrochloric acid to magnesium, the result will be magnesium chloride and hydrogen, which comes off.

The result is not a homogeneous mixture, but a substance that did not exist before. It is possible to differentiate a mixture from a pure substance by observing its specific properties such as melting point and boiling point.

Pure water, for example, boils at 100 ° C (at sea level). This temperature (boiling point) remains constant throughout the boil. When boiling a mixture of salted water, the boiling point increases. The same goes for the melting point.