Experiment 12

Solutions and Solubility

In this experiment, you will perform experiments designed to understand solutions and factors which affect solubility.

Be certain to record all data in your laboratory notebook.  Keep good notes, and draw conclusions at the end of the experiment.

The subject of solutions is a complex one.  It involves such phenomena as the process of dissolving, solubilty, and factors that affect solubility.  The rate of dissolving depends on a number of conditions.  The properties of the solvent are affected by the quantity of solute that is present.  In this part of the experiment, we will qualitatively observe some of these phenomena, and we will learn some of the common terms used in connection with solutions.  Observations of solution color can best be made against a white background.

Discover the Effect of Temperature on the Rate of Solution

Place one gram of powdered copper(II) sulfate pentahydrate (CuSO₄·5H₂O) in each of two test tubes.  To one add 5 mL of cold distilled water, and to the other add 5 mL of distilled water that has previously been heated to boiling.  Suspend the second tube in a beaker of boiling water.  Compare the rates of solution in these two test tubes.  Record your observations.  Keep these solutions.

Which sample dissolves faster?  What is the explanation?

Discover the Effect of Temperature on the Solubility of a Solute

Add a few drops of sulfuric acid to each of the above solutions.  Now weigh out several approximately one-half gram portons of copper(II) pentahydrate on weighing papers.  Add one of the portions to each of the test tubes and mix the contents.  Continue heating the hot tube while the other remains at room temperature.  Repeat the addition of one-half gram samples of copper(II) sulfate pentahydrate to each tube until it becomes impossible to dissolve any more.  Record the weight of CuSO₄·5H₂O dissolved in each tube at the two different temperatures.

What effect does the acid have? (You might need to find out from your instructor).  Which tube becomes saturated first?  Is this what you would expect?  What happens if you place to saturated CuSO₄·5H₂O solution from the hot temperature in a bucket of ice?  Can you explain what supersaturation is?

Discover the Effect of Particle Size on the Rate of Solution

Weigh a single crystal of CuSO₄·5H₂O that is pea-sized or larger.  On a separate piece of weighing paper, weight about the same amount of powdered CuSO₄·5H₂O. Place each in a test tube and add 5 mL of distilled water to each.  Shake each tube gently for two minutes.  Contrast the amount of solute that has gone into solution in the two test tubes.  State your conclusions in your notebook.

Why does one dissolve faster than the other?  

To Prepare a Supersaturated Solution

A solution that is in equilibrium with undissolved solute is called a saturated solution (there is not net change in the amount of solute that dissolves).  Under certain circumstances a solution may be prepared that is more concentrated than a saturated solution of the solute.  This is called a supersaturated solution.  This condition may occur when the solubility of the solute is gradually decreased by lowering the temperature.  A supersaturated solution is not in equilibrium with the solute, and the solute quickly begins to crystallize if the solution is distrubed, or if a crystal of the solute is introduced.

Sodium thiosulfate: In a clean test tube, place 4 mL of distilled water and 15 grams of sodium thiosulfate, Na₂S₂O₃·5H₂O (sodium acetate may be substituted for the sodium thiosulfate; see procedure for sodium acetate below).  Heat the contents of the test tube gently until all of the sodium thiosulfate has dissolved, but do not boil the solution.  Set the tube and contents aside where it will not be disturbed, and allow it to cool to room temperature.

At the end of the laboratory period, see whether any crystallization has taken place.  If not crystals have formed, drop a single small crystal from the stock bottle of sodium thiosulfate into the solution. What happens?  Feel the test tube and not whether any temperature change has occurred.  

Sodium acetate: Clean a large test tube thoroughly with detergent and a test tube brush. Rinse the test tube several times with tap water and then two or three times with distilled water. Fill the test tube about one-half full of solid sodium acetate and then add about 2 mL of distilled water (no more). Warm the tube as much as necessary to dissolve all of the solid. Do not boil the solution and avoid shaking or tipping the tube. Set the tube carefully in a beaker of cool water.

Examine the contents of the tube. None of the salt should have crystallized from solution. Remove the tube from the water and add one tiny crystal of solid sodium acetate. Watch for the formation of more crystals. Feel the tube as these crystals form. Is this process exothermic or endothermic?

Disposal of chemicals:  Place all liquid waste in the liquid waste container.  Be sure to clean up any spills on the lab bench and work area.

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