Classification of Matter
What Is a Mixture?
The Nature of Mixtures and Processes to Separate Them
Mixtures consist of two or more substances. Although mixed together, each substance retains its own physical and chemical properties. This allows a
mixtureglossary term (opens in a new window) to be separated into its component substances by physical means, such as evaporation, distillation, straining, or other physical processes.
For example, iron is a
pure substanceglossary term (opens in a new window). One of iron’s physical properties is its attraction to a magnet. Table salt is also a pure substance, and one of its physical properties is its lack of attraction to a magnet. If iron filings and grains of table salt are parts of a mixture, they can be separated by passing a magnet over the mixture. The iron filings will leave the mixture as they are picked up by the magnet. The grains of salt will remain behind.
Other physical properties of iron and table salt also can be used to separate them from a mixture. Table salt is soluble in water. Iron is not. So, if a mixture of iron filings and table salt is stirred in a beaker of water, the salt will dissolve in the water, but the filings will not. The
liquidglossary term (opens in a new window) can be poured off, leaving behind the iron filings.
This process, of course, produces another mixture, one that contains table salt dissolved in water. How can the components of this mixture be separated? Again, the physical difference of table salt and water can be used to get the job done. Water evaporates. Table salt does not. If the mixture is allowed to stand in a warm place, the water will evaporate, leaving behind crystals of salt. The process can be sped up by heating the water.
How might a mixture of two liquid substances be separated into its components? The two substances likely will have different boiling points. If so, then by slowly heating the mixture, the substance with the lower boiling point will boil off first, leaving behind the substance with the higher boiling point. The vapors of the substance with the lower boiling point can be condensed and captured by passing them through a long cooling tube. This process is called distillation.
How might a mixture of two or more gases be separated into its components? Air is such a mixture. It consists of about 78% nitrogen, 21% oxygen, and 1% other gases. Leaving aside the 1% other gases, how might the nitrogen and oxygen in air be separated? A clue to the answer lies again in the boiling points of nitrogen and oxygen. Nitrogen boils at -196 °C. That means it also changes from a gasglossary term (opens in a new window) to a liquid at this temperature. Oxygen boils at -183.0 °C. So, if a sample of air is slowly cooled, oxygen will become a liquid first. It can be collected, leaving the nitrogen behind.
Teacher Note: Misconception
Students may believe that the components of a mixture are chemically combined, and can be separated only by chemical means. In fact, the components of a mixture are physically combined, and although they may be in close proximity, they can be separated by physical means.
Teacher Note: Practices
Students must analyze data to identify design features of the components of a proposed process of separation of a mixture to optimize it such that the fractions are collected properly in this item, Separation of a Mixture. You may wish to review the technique of distillation with students before assigning them this item to make sure that they understand the design of a distillation apparatus and how it enables separation of a mixture. So as not to give away the answers to the assessment item, think about using the example of distillation of a salt solution that is composed of a volatile solvent and a non-volatile solute. You can explain the heating of the solution to vaporize the solvent, allowing it to distill while the non-volatile solute remains in the original vessel.
After students have completed this item, have a class discussion to discuss the separation of a mixture of liquids using distillation. Be sure that students do not have the misconception that distillation automatically and cleanly separates liquids into pure samples. Let students know that the vapor rising from a mixture of liquids typically is a mixture itself since each component vaporizes to some extent at temperatures below its boiling point. You may want to discuss the use of a fractional distillation column as a means for most effectively separating the individual components in a mixture over the course of a distillation procedure.
