Pure Substances and Mixtures: Properties and Separation Methods

Posted on Jan 29, 2025 in Chemistry

Pure Substances and Mixtures

There are some characteristic properties that serve to identify substances (density, conductivity, color) and some general properties common to all types of matter (mass, volume, etc.). The main division of matter is into pure substances and mixtures. Pure substances have specific properties that characterize them, mixtures do not. The determination of any specific property of a substance is the best way to know if this substance is pure or not.

Pure Substances

Pure substances are those whose composition does not change whatever the physical conditions in which they are found. They cannot be decomposed into other substances using only simple physical procedures. Two types are distinguished:

  • Compounds: Pure substances that can be decomposed into other, simpler substances through a chemical process.
  • Elements: Pure substances that cannot be decomposed into simpler substances by any procedure.

Mixtures

Mixtures are combinations of several pure substances that can be separated using physical processes:

  • Heterogeneous mixture: A mixture in which it is possible to distinguish the components that form it by optical procedures (e.g., pizza).
  • Homogeneous Mixture/Solution: A mixture in which it is not possible to distinguish the components by conventional optical procedures (e.g., sugar water).
  • Solution: A homogeneous mixture that does not scatter light.
  • Colloid: A heterogeneous mixture that scatters light (Tyndall effect) (e.g., jelly, tomato sauce). These are heterogeneous mixtures in which there is a component in a higher proportion in which others are dispersed in a lesser ratio.
  • Emulsion: Particles that are in a lesser proportion remain scattered throughout a third substance called an emulsifier (e.g., mayonnaise is made with egg, oil, salt, and lemon).

Separation of Mixtures

The pure substances in a mixture can be separated using physical means. The method used depends on the particle size and the properties of the substances that we want to separate:

  • Screening: Ideal for separating mixtures in which a solid component present in the mixture has a particle size significantly different from the others.
  • Filtration: Indicated to separate an insoluble solid from a liquid with which it is in contact (e.g., to separate a mixture of copper dichromate, Cu(Cr2O4)2, and sulfur, based on the fact that dichromate is soluble in water and sulfur is not).
  • Crystallization: Suitable for separating a solid dissolved in a liquid (e.g., to recover solid copper dichromate, Cu(Cr2O4)2, from a blue solution, we evaporate the water from the solution, and new crystals of dichromate are obtained. Solid substances dissolve better in hot water than in cold water. If we dissolve as much solid as possible in hot water and let the solution cool, when the solvent evaporates, the dissolved solid forms crystals).
  • Magnetic separation: If a mixture contains a ferromagnetic metal (Fe, Ni, Co), it can be separated from the other components using a magnet (e.g., a mixture of sulfur and iron filings (Fe) can be separated with a magnet because of the magnetic properties of iron).
  • Decantation: This procedure is suitable for separating two immiscible liquids (that cannot be mixed) with different densities (e.g., oil and water). A special funnel called a separatory funnel is used, which has a valve at the bottom. When the two liquids are clearly separated, the valve is opened, and the denser liquid exits first. It is important to discard the border between the two liquids (interface).
  • Distillation: Indicated for separating two miscible liquids (that can be mixed) with very different boiling temperatures, or a liquid that has a solid dissolved in it. The mixture is introduced into a container and heated. When the temperature reaches the boiling point of the first liquid, it turns into steam, which passes through a cooling tube where it cools and condenses.
  • Chromatography: Used to separate the components of a mixture by exploiting their different affinities for a solvent. Chromatographic techniques are varied, but all involve a mobile phase, which can be a liquid or a gas, and a stationary phase, which is usually a solid. The inventor of this technique was the Russian botanist Mikhail Tswett. His first application was to separate the components of plant pigments, such as chlorophyll.