Theoretical Background:
Mixtures form when two or more pure substances are combined physically, either evenly or
unevenly. Since the components do not chemically combine, each substance retains its identity.
Homogeneous mixtures have a uniform or โsameโ composition throughout, such that all
components of the mixture are evenly distributed throughout it. Therefore, taking any random
sample of a homogenous mixture should yield the same proportion of each component.
Solutions are homogenous mixtures and can consists of a variety of combined substances and
physical states. The one you are most familiar with is a liquid solution in which a solid has been
dissolved such as sea water or simple syrup. However, homogeneous mixture can consist of
varying physical states such as in diluted rubbing alcohol or a salicylic acid gel used to treat acne
or antibiotic ointment. Heterogeneous mixtures vary or are โdifferentโ in their composition
such that there is no uniformity. Taking multiple samples of a heterogenous mixture will yield a
different composition or proportion of components each time. Examples of heterogeneous
mixtures include salad dressing, slabs of marble, or milk.
Just as substances can combine physically to form a mixture, a mixture may also be broken
down into its original components, as long as it is done by physical means and does not disrupt
the identity of the substance. This simply means that you must recover the substance from the
mixture without altering it chemically.
There are many physical means by which we can separate components of a mixture. Which to
employ depends on the number of components and their physical properties, each of which
can be manipulated in order to separate it from the mixture without altering it or the other
substances. Therefore, only physical changes or means can be employed in this process.
Knowing the unique physical properties of each component will help you to separate it and it
alone. For example, if two components are water soluble, you would not choose that means to
separate them as they would both dissolve. The type of separation method can also inform
your methodology and sequence in the procedure. Iron filings, for example, are much easier to
remove when dry than wet; therefore, we would remove those using a magnet prior to adding
water for extraction of salt. The same could be said if you were attempting to sublimate
ammonium chloride (NH4Cl), solid carbon dioxide (CO2) otherwise known as dry ice, or
naphthalene (C10H8) by heating the mixture. If you have ever smelt the distinct odor of moth
balls (naphthalene), you would notice they remain in a solid state yet they have vaporized in
the air. You would need to ensure that the temperature needed to sublimate these chemicals is
not going to melt or burn the other components. Nor would you want to make them wet by
performing an extraction first and then attempting to sublime the solid.
Therefore there is a some research and detective work to do before you plan your procedure.
