BACKGROUND:
A mixture is a combination of two or more materials in
which each material is identifiable. For example salt combined with pepper
is a mixture. A solution is a combination of two or more materials in which
one is dissolved in the other; the materials cannot be recognized. In a
solution of sugar and water you cannot distinguish between the water and the
sugar.
A solution consists of a solute (the dissolved
material) and a solvent (the substance in which the solute is dissolved).
The solute is present in a smaller quantity than the solvent. The amount of
solute in the solvent is called the concentration. As the amount of solute
in a solution increases, the solution becomes more concentrated. At the
point of maximum concentration, the solution is saturated. Any additional
solute added to a saturated solution will precipitate and drop to the bottom
of the container.
In general, increasing the temperature of a solution
will increase how much solute can be dissolved in it (solubility). Once
solvent temperature has been increased, more solute has been added, but no
more will dissolve, the solution is termed supersaturated.
PROCEDURE:
- Discuss the differences between mixtures and solutions.
- Have the students experiment with saturated and supersaturated
solutions. If the materials are not available, the exercise can be done
as a homework assignment.
- Have the students prepare a saturated solution of Kool-aid or an
equivalent powdered drink mix. Have them follow these directions:
- Make Kool-aid according to the package directions. Use a pan that
can be heated on a stove top.
- Add sugar to the Kool-aid, and dissolve it by stirring.
- Keep adding sugar until no more will not dissolve, and instead it
begins to settle at the bottom of the container. This produces a
saturated solution.
- Have the students experiment and see what happens if they swirl
the mix around. Does more sugar dissolve? Ask them to carefully
observe what happens as more sugar is added.
- Heat the saturated solution and observe what happens. Make sure that
the students are careful not to stick their hands in the hot solution.
The students should see that sugar accumulated on the bottom of the
container has dissolved. Ask the students where the sugar went, and why.
Have them keep adding sugar again, until no more will dissolve. Explain
that they have prepared a supersaturated solution. Have them taste the
solution and determine if it is sweeter than the original or the
saturated solution.
