Lesson 13
Make-up Lab
SOLUBILITY
By Chemtutor
For the best view of solubility, we will use
the examples of a solid solute dissolved into a liquid solvent. This does not
mean that other materials do not work in the same fashion.
The solubility of a solution is a measure of
how much of the solute can be dissolved into the solvent. The solution reaches
a point called the saturation point when no more solute will be accepted
by the solvent. Any further addition of solute will result in solid solute
mixed in with the saturated solution. Each solvent and solute pair has a
characteristic solubility at a given temperature. Usually as you increase the
temperature, an increased amount of solute will be able to dissolve.
Take a Pyrex measuring cup and put in exactly
a cup of table sugar. Heat water to boiling and pour in a small amount. Notice
what happens. The volume of material in the cup appears to shrink! Continue
adding boiling water until the level is back up to the 'one cup' mark. Notice
the temperature of the solution. It takes heat to dissolve sugar. Stir. You
should be able to almost dissolve all the sugar. The solution should be very
close to the saturation point at that temperature. The solution should end up
at about room temperature. Now add a few heaping tablespoons of sugar. Stir and
attempt to dissolve all the sugar. If you succeed, add another few tablespoonfuls
of sugar. Put the saturated solution with a lot of sugar, not dissolved, into
the microwave, and heat until all the sugar is dissolved. If you have a meat
thermometer, find the temperature of the boiling mixture. (Be careful. The
solution is VERY hot. Handle with something to insulate you from the heat.)
Observe the solution after you take it out of
the microwave and put it on the counter. Notice the temperature at which the
sugar crystals begin to form again.
If you have done the experiment just right,
you may see the crystals appearing at a temperature far below what you might
think. If you boil the solution enough in the microwave, you will dissolve all
traces of a seed crystal for the saturated solution to deposit sugar onto. At
one time your solution will be supersaturated, or beyond the normal
amount of solute in the solution. Supersaturation is
an unstable condition. If any crystal is presented to a supersaturated
solution, the crystallization of the solute onto it will occur fairly rapidly.
At home if you have done this demonstration
with only sugar and water in a clean cup, don't waste the sugar solution. A
little bit of maple flavoring will make it a fine
syrup for pancakes, or you can use it in the frosting of the chocolate cake I have published here on the
site. Do not eat any material made at school. Lab materials may contain traces
of contaminants. If you eat anything in the school laboratory, the school
lawyers will turn green and purple, have a conniption fit, and likely take
their discomfort out upon you.
Solubility of salts depends upon the type of
ions in the salt. There is a very great range of solubility of salts in water.
Even the most insoluble, such as silver chloride, have a very small but
detectable solubility. Some salts, called deliquescent salts, are so
soluble that they grab water molecules out of the air and can dissolve
themselves in this way.
Using the simplification of classifying
materials as either soluble or not in water at room temperature, there are some
nice easy general rules for predicting whether or not a salt will dissolve in
water. These rules are useful not just for predicting how to make solutions,
but ion reactions, such as a double displacement reaction, depend upon the
insolubility of a salt as a possible product for the reaction to happen.
Depending upon what your instructor suggests, it may be a good idea for you to
know the following rules:
(a) Almost all simple ionic compounds with Group I elements or ammonium ion,
(NH4)+, are soluble.
(b) All nitrates (NO4)-, most sulfates, (SO4)2-,
and most chlorides, Cl-, are
soluble. ** Notable exceptions to this rule are: barium sulfate, BaSO4)2-,
lead II sulfate, PbSO4)2-, and silver chloride, AgCl.
(c) Most hydroxides, (OH)-, carbonates, (CO3)2-
sulfides, S2-, and phosphates, (PO4)3-,
are insoluble except for the compounds of rule (a). Barium hydroxide, Ba(OH)2 is a
soluble exception to this rule.