THE SOLUBILITY OF A SALT IN WATER
AT VARIOUS TEMPERATURES
©2008, 2007, 1995, 1991 by David A. Katz.
All rights reserved.
Permission for academic use provided the original copyright is included.
To measure the solubility of a salt in water over a range of temperatures and to construct a graph
representing the salt solubility.
One of the most common forms of a homogeneous mixture is a solution.
The one component of a
solution, which is usually present in the greatestr proportion, is called the
The other components,
present on a smaller scale, called
, are considered to be dissolved in the solvent.
There are a
number of different kinds of solutions: gases in gases (example: air), liquid in liquids (example: gasoline),
gases in liquids (example: carbonated soft drinks), solids in solids (example: alloys such as brass), and
solids in liquids (example: salt water).
This experiment will involve a solution formed with a solid solute
(a chemical salt) and a liquid solvent (water).
If the properties of a solution remain constant, the system of solute and solvent is considered to be at
Obviously, if the solid is disappearing into the solvent, the system is not at equilibrium.
Solubility of a solid in a liquid is dependent on temperature, thus, at a given temperature, only a certain
maximum amount of solute will dissolve in a given amount of solvent.
Beyond that amount of solute, no
more will dissolve and excess solute will remain in the solid form, settling to the bottom of the solution
This maximum amount of dissolved solute, expressed quantitatively, is given in units of grams
of solute/100 g of solvent.
Such a solution is termed a
, since it is holding all the solute
it can hold at that temperature.
Experiments show that when excess solute is in contact with a saturated
solution, an equilibrium is established in which solute is continually dissolving in amounts just equal to
the solute separating from solution (crystallization) (see Figure 1).
When saturated solutions of solid solutes are prepared at elevated temperatures and then permitted to
cool, the excess solute usually separates from the solution by crystallizing.
However, if a saturated
solution is prepared at an elevated temperature and any excess, undissolved solute is removed,
crystallization often does not take place when the solution is allowed to cool undisturbed.
can contain more of the solute than normally is held in equilibrium with the solid state.
are said to be
A supersaturated solution is a system in a metastable (unstable) condition.
Agitation of the solution or the addition of a seed crystal of the solute may start crystallization of the
After crystallization, a saturated solution remains.