Constructing a Solubility Curve of Potassium Nitrate
Introduction:
A key factor affecting the solubility of a substance is the temperature. For most substances increasing the temperature will
increase the solubility—more solute will be able to dissolve in the same amount of solvent. A solubility curve illustrates how the
solubility of a substance varies with the temperature. By determining the mass of solute that that can be dissolved in a volume of
solvent under a variety of temperatures, we can easily construct a solubility curve. In this lab exercise you will create a solubility
curve for potassium nitrate, KNO3. You will then be able to calculate the solubility of this substance at various temperatures.
Safety:
1. Use caution when using and preparing the hot water bath. Steam will burn as will the hot plate.
Procedure:
1. Prepare a hot water bath by filling a 600mL beaker ½ full of water. Place the beaker containing the water on a hot plate.
2. Using a thermometer, monitor the temperature of the water at 95C. DO NOT LET THE WATER BOIL!! If the water becomes
too hot, remove from the hot plate until cooled to temperature.
3. While the water is heating, label 4 clean test tubes 1, 2, 3, and 4.
4. Accurately measure out the masses of potassium nitrate, KNO3, placing the salt in the appropriate test tube. It is not necessary
that you measure out exactly the masses given below, but you must record the precise masses you actually use to the tenths place.
 Test tube 1: 2 g KNO3
 Test tube 2: 4 g KNO3
 Test tube 3: 6 g KNO3
 Test tube 4: 8 g KNO3
5. Add exactly 5.0mL of distilled water to each of the test tubes.
6. Place each of the test tubes into the 95C water bath in order to dissolve the KNO3 in each tube. Stir using a glass stirring rod to
speed up the dissolving process.
7. Remove test tube 1 from the hot water bath once the KNO3 has fully dissolved and place a thermometer in the tube. Watch the
solution carefully. Record the temperature as soon as you see crystals forming within the tube. You may have to wait awhile for
the crystals to form in test tube 1.
8. Repeat step 7 for the other test tubes. It is important that the KNO3 be completely dissolved before removing it from the water
bath.
9. Be prepared to act quickly for test tube 4 as crystallization may occur quickly. If it occurs too fast, place the tube back into the
water bath to re-dissolve the crystals and try again.
10. All solutions should be disposed of as directed by your teacher.
11. Clean up your lab station and have your teacher check you out of lab.
Data Table:
Test Tube
Mass KNO3 used,
g
Volume H20,
mL
Conversion,
g/100mL
Saturation Temp,
C
1
2
3
4
Calculations/Analysis:
1. Convert the mass/volume ratio you used for each test tube into mass/100mL ratio. Place calculations in your data table.
2. Using graph paper, construct a solubility curve for KNO3 based on your data.
Add the additional data to your graph:
Solubility at 0C is 13g/100mL
Solubility at 100C is 247g/100mL
Questions:
1. Based on your solubility curve, predict the solubility of KNO3 at the following temperatures:
a. 50C
b. 70C
2. Based on your solubility curve, would you best describe the following solutions as unsaturated, saturated, or supersaturated?
a. 70g/100mL H20 at 45C
b. 80g/100mL H20 at 70C
c. 80g/100mL H20 at 30C