The Effect of Temperature on Solubility

 Theory:  Hot tea can dissolve more sugar than iced tea, and warm water dissolves less oxygen than cold water.  The maximum amount of any solute that can dissolve in a given amount of solvent is called its solubility, and this depends on the temperature.

            The solubilities of gases  always decreases with increasing temperature.  For liquids and solids, solubilities generally increase with increasing temperature, as is the case with sugar in tea.  However, there are a number of exceptions to this, two examples being cerium sulfate, Ce₂(SO₄)₃, and lithium carbonate, Li₂CO₃   For these ionic solids, solubility decreases with increasing temperature.

            In this activity, you are to determine the effect of temperature on the solubility of potassium chloride, KCl, in water.  After collecting data at various temperatures, you will plot the data to produce a solubility curve showing how the solubility of potassium chloride varies with temperature.  This, then, can be used to predict the solubility at other temperatures.

Objective:  How does the solubility of KCl in water vary with temperature?

Procedure:

1.     Label test tubes 1 through 5, respectively.

  2.     As accurately as possible, determine the mass of a weighing pan.  Record.  Use this to measure out KCl.

 3.  To each of the test tubes, 1 through 5, you want to add samples of KCl having the following masses:     2.50g, 2.30 g, 2.00 g, 1.80 g and 1.50 g.  It is not necessary that your samples be exactly these masses; it is necessary that the values be within 0.05 grams, and that you determine the masses as accurately as possible.  Use a spatula to add or remove small amounts of the solid from the weighing pan.  If you need to remove some KCl, do not return it to the original container.  Use a secondary weighing pan and give me the excess KCl to use as demonstration material.

 4.  Fill the large beaker about ¾ full of water to use as a hot-water bath.  Place the beaker on a piece of wire gauze on the ring or hot plate (depending on which we use).  Place a thermometer in the water and begin heating while you proceed with step 5.  The goal is to heat the water to 90 ºC.  

 5.  Put 5.1 mL of distilled water in a 10 mL graduated cylinder.  Use a medicine dropper to bring the bottom of the meniscus right to the 5.1 mL line.  The extra 0.1 mL is to correct for drops left behind when the water is transferred to a test tube.  Add this water to test tube 1.  Repeat this process, adding 5 mL of water to each of the test tubes.  (hint:  we may use pipets instead of graduated cylinders)

 6.  Place test tube 1 in the hot-water bath.  With a glass stirring rod, stir the KCl mixture until all the solid has dissolved.  At this point, remove the stirring rod and rinse it.  Take the thermometer out the hot water bath, quickly wipe any excess drops of water from it, and place it into the test tube while it is still warm.

 7.  Carefully hold the test tube up to a light.  Watch for the very first signs of crystallization, and then immediately note the temperature.  If crystallization begins too quickly, or you do not catch it the instant it begins, redissolve the precipitate and repeat until you are able to determine the temperature at which the crystallization begins.  Record this in the data table.  If you are working with a partner, the second person should be setting up test tube 2.

 8.  Repeat steps 6 and 7 for all five test tubes.  For test tube 5, it will be necessary to use an ice bath to effect crystallization.  Simply place 3 or 4 pieces of ice in a beaker and fill half way with water.

Sample data table:

Mass of weighing pan:   _______________

 Test              Mass of sample                    Mass of sample                   g/100 mL                Crystal
tube               and pan                                in 5.0 mL H₂O                      of H₂O                     temp (ºC)

1                      _____                                      _____                                      _____                          ______

2                      _____                                      _____                                      _____                          ______

3                      _____                                      _____                                      _____                          ______

4                      _____                                      _____                                      _____                          ______

5                      _____                                      _____                                      _____                          ______

Calculations:

 1.  Plot the solubility of KCl on the y axis in g/100 mL H₂O versus the temperature in ºC on the x axis.  DO NOT connect the dots, but instead draw a smooth curve passing through or coming as close as possible to your data points.

 2.  Bonus:  Look up the value of the solubility of potassium chloride.  The temperature at which the measurement was made is usually given.   From your graph, determine the value at the same temperature as you found.  Determine a percent error by dividing the difference between the book value and your value by the book value, and then multiplying by 100 %.

 3.  Bonus:  the dissolved oxygen content of water is very critical to marine life.  Investigate the solubility of oxygen in water as a function of temperature, and couple that with an investigation of the oxygen needs of various kinds of aquatic animals.  Such studies relate to “thermal pollution,” a serious concern near power plants and other industries that release heated water into streams and rivers.  Write a report on this.  Pay particular attention to any such problems in your area.

 

Alternate Solubility Lab