Experiment 4-1

Dissolving and Concentration

Background Information

It is a universal experience on this water planet that some materials seem to disappear when stirred into water.  We know from experiment 2-1 that the mass does not change during this process, so apparently the this disappearing materials does not merely vanish, but becomes an invisible part of the liquid.

In order to discuss this process, some vocabulary is useful:
Solvent is the material such as the water that absorbs the second material.  The solvent is most often a liquid, but there are situations where gases may be used as solvents and after combining a liquid might freeze as in the formation of an alloy.

Solute is the second material being absorbed, often a solid, but the term also applies to gases and liquids.

The combination of solvent and solute is commonly referred to as a solution.

Experiment

Purpose:  In this experiment we wish to investigate the effect of the amounts of the solvent and solute on the properties of the resulting solution.

Materials needed:
• A colored solute, perhaps a powdered beverage mix.
• A solvent, preferably water.
• five identical small containers for the solutions and allow simultaneous comparisons of the colors.
• A stirring stick.

Procedure:

1. Put an equal amount (a few mL) of solvent (water) in each of the five containers.
2. Add a small, equal amount (fraction of a mL) of solute to each container.
3. To the second and third container, add a second, equal portion of the solute.
4. To the fourth container add a large amount of solute, approximately double the amount of solvent.
5. To the third and fifth container, double the amount of solvent.
6. Stir to dissolve, but not longer than a couple minutes.
7. Compare the colors of the solutions. noting which are paler, which are equal color, and which are darker.
8. Note if the color of EACH solution is uniform.  Also note if any solute remains undissolved in any container.

Record your results in your science journal.  Write a Formal Report if you need to earn credit.

Discussion of Observations and Conclusion:  The solute obviously did not vanish, but rather became part of the solution.  In the case of a colored solute, the intensity of the color is a property related to what is called the concentration of the solution.  What does the uniformity of color tell you about the concentration at different locations in the solution?  What do the different intensities of color tell you about how concentration varies depending on amount of solute (compare containers #1, 2 and 4) and the amount of solvent (compare containers #1 and 5)?  Account for why containers #1 and 3 have about the same color intensity.  Did anything happen in container #4 different than the rest?  Why?  It is possible for some materials that the concentration of solute exceeds the capacity for the solvent to dissolve it.  In such a situation the solution is describes as being saturated.

10/27/2002
revised 10/27/2002
by D Trapp