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Introduction to Solutions
A solution is a homogeneous mixture. That means the components of a
solution are so evenly
spread throughout the mixture that there are no perceivable differences in
composition. Solutions can
be formed by mixing two substances together such as sugar and water. If
you pour a packet of sugar
into a glass of water, initially you have a suspension as the sugar
crystals float about in the glass.
When you have stirred the sugar and water for long enough, you will
eventually get a clear, colorless
mixture. Some people, especially young children, can be fooled by such a
demonstration into
thinking that the sugar has "disappeared". However, as chemists, we know
better. The law of
conservation of matter states that the sugar can not just disappear, it
must have gone somewhere else.
That somewhere else is into solution. The sugar has become evenly
dispersed. In fact the sugar
molecules are so well spread out that we can no longer see a single sugar
crystals. However, if you
taste the water, you will find it to be sugary--confirming the presence of
sugar in the water. The
minor component of the solution is called the solute. In the present
example, sugar is the solute.
The major component of the solution is called the solvent. In this
case water is the solvent.
Solutions can also be formed by mixing together many different phases of
matter. For instance, air is
a solution. The solute gasses oxygen, carbon dioxide, argon, ozone, and
others are dissolved in the
solvent nitrogen gas. Another example is found in "gold" jewelry. Most of
the golden jewelry sold
in the world is not 24 karat (i.e. 100% pure gold) but rather it is a
solution of other metals,
commonly silver and copper, in a gold solvent. Such a solution of metal(s)
in another metal is called
an amalgam.
The Composition of Solutions
Perhaps the most important property of a solution is its concentration.
A dilute acetic acid
solution, also called vinegar, is used in cooking while a concentrated
solution of acetic acid
would kill you if ingested. The only difference between such solutions is
the concentration of the
solute. In order to quantify the concentrations of solutions, chemists
have devised many different
units of concentration each of which is useful for different purposes.
Molarity, the number of moles of solute per liter of solution, has the
units moles / L which are
abbreviated M. This unit is the most commonly used measure of
concentration. It is useful
when you would like to know the number of moles of solute when you know
both the molarity and
the volume of a solution. For example, it is easy to calculate the volume
of a 1.5 M solution
of HCl necessary to completely react with 0.32 moles of NaOH:
Normality, the number of molar equivalents of solute per liter of
solution, has the units
equivalents / L which are abbreviated N. To illustrate the
difference between molarity and
normality let's assume that we had used a 1.5 M solution of sulfuric
acid,
H2SO4, instead of a 1.5 M solution of
hydrochloric acid, HCl in
the above example. Because sulfuric acid can donate two protons to the
NaOH, as noted in the
, it will only take half as much sulfuric
acid as hydrochloric acid
to neutralize the sodium hydroxide.
In the present example, the 1.5 M solution of sulfuric acid reacts
like a 3.0 M
solution of hydrochloric acid because there are two equivalents of
H+ per mole of
sulfuric acid. Therefore, that solution of sulfuric acid is 3.0 N.
