Chemistry Reference
In-Depth Information
Homogeneous mixtures are also called solutions, and the uniformity
found in samples of homogeneous mixtures can be illustrated by a glass of
salt water. If you dissolve a spoonful of salt in a glass of water, the salt
becomes uniformly dispersed in the water. By the time the salt is fully dis-
solved, the concentration of the salt solution will be the same in each area
of the sample. It is not as if the top of the glass will contain only pure water
and the bottom of the glass will have really salty water; rather, the distribu-
tion of the salt will be uniform. Other common examples of solutions (ho-
mogeneous mixtures) would include metal alloys (such as bronze), the
mixture of gases that we breathe, carbonated water, and mixtures of water
and alcohol.
There is another important symbol that is used in chemistry that will
help you to recognize solutions. When a substance such as salt is dissolved
in water, its chemical formula will be followed by the subscript “(aq),” which
stands for aqueous. An aqueous substance is one that is dissolved in water
(in other words, a solution). You need to get in the habit of paying atten-
tion to the symbols, so that you can differentiate between the symbolic rep-
resentations of substances.
NaCl
(s)
— This formula represents a single substance,
the compound NaCl in its solid form.
NaCl
(aq)
— This formula represents a homogeneous
mixture of NaCl and water.
Another important way that mixtures can be differentiated from com-
pounds is that the actual composition of a mixture is not a fixed thing, al-
though the ratio by which elements combine in a compound is always the
same. For example, water is always made from two atoms of hydrogen and
one atom of oxygen; yet there are many ways to make a solution of salt
water. If you add a tiny amount of salt to water, you get a salt-water solu-
tion; if you add much more salt, you still have a salt-water solution. Mix-
tures come in varying concentrations.
If you drink coffee, you know the difference between a “strong” cup of
coffee and a “weak” cup of coffee. If you don't drink coffee, I am sure you
can recall having a glass of “weak” iced tea, or some other powdered drink
mix. When we talk about the “strength” of one of these drinks, we are really
talking about the concentration of a solution. We will be learning more about
concentration later on, but for now you should remember that you could
vary the concentration of a mixture but not of a compound.
