The Effect of Temperature on Solubility

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.  Remember, if you get 2.53 grams for the first test tube, that's perfectly OK.....what is CRITICAL is that you write down your EXACT measurement for each sample!  

 4.  Fill the large beaker about ¾ full of tap 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 and to dissolve all the KCl.  Remember, however, when you read the thermometer, hold it upright and NOT touching the beaker (otherwise, you will be measuring the temperature of the beaker glass, not the water within it).

 5.  Put 5.1 mL of distilled water in a 10 mL graduated cylinder.  Use a plastic 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 5.0 mL of water to each of the test tubes.

 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.  Be sure to RECORD YOUR VALUE TO THE CORRECT NUMBER OF SIG FIGS.  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 and be sure to record your temperature to the correct number of sig figs.  If you are working with a partner, the second person should be setting up test tube 2.

NOTE: If you label your test tubes, you can be heating several of them up at one time in the water bath.  You may be wise to get the measurement of the first test tube (so you are familiar with the crystallization process) and then do the others more together.  However, see next step for test tube #5.

 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 tap water..

Sample data table:  Click to print off actual data table

Calculations:

 1.  Using Microsoft Excel, plot the solubility of KCl on the y axis in g/100 mL H₂O versus the temperature in ºC on the x axis. 

• Be sure to include both vertical and horizontal grid lines.

• Be sure to label the axes showing the temperature on the X axis and the solubility on the Y axis.

• DO NOT connect the dots using the computer, but instead, using a ruler and a pen, mark in a smooth and straight line (called a "line of best fit") passing through or coming as close as possible to all of your data points.

• IMPORTANT NOTE: The solubility of this particular compound (KCl) happens to have a "straight line" solubility curve in the range of temperatures you are testing.  Most compounds actually have a real curve to the line - you will see this as you continue on to question two below.