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Summative Lab: Solubility Curve of Potassium Nitrate in Water
Background:
Potassium nitrate is an ionic compound that is soluble in water. As the temperature of water increases, more potassium
nitrate can be dissolved in a given amount of water. A solubility curve is a graph showing the amount of potassium
nitrate that can be completely dissolved in water at different temperatures. In today’s lab, we are going to dissolved
different masses of potassium nitrate in water, and then we are going to measure the temperature at which
crystallization begins as the water cools.
Pre-Lab Questions:
1. Define the following termsSolutions: ____________________________________________________________________________
Unsaturated Solution: __________________________________________________________________
Solubility: ____________________________________________________________________________
2. Explain how water separates ions in ionic compounds.
3. What factors influence the rate at which a substance dissolves?
Materials:
Balance
Spatula
Test tube holder
Potassium nitrate
Thermometer
Stirring rod
Test tube rack
Distilled water
Hot plate
Test tubes
400-mL beaker
10-mL graduated cylinder
Safety:
- Always use test tube clamp to remove test tubes from the hot water bath
- Do not let the thermometer touch the bottom of the beaker for accurate measurements.
- Let the water bath cool on hot plate
Procedure:
1. Using the graduated cylinder to measure the distilled water and a balance to measure the KNO3, prepare your
group’s test tube according to the data below. Combine the water and KNO3 in the test tube.
Test tube#
1
2
3
4
5
6
grams of KNO3
1.5
3.0
4.5
6.0
7.5
9.0
ml of distilled H2O
5
5
5
5
5
5
2. Fill a 400 ml beaker about ¾ full of water to be used as a hot water bath. Place the water bath on the hot plate.
3. Place the test tube into the hot water bath. Stir the KNO3 water solution with a stirring rod until the KNO3 is
completely dissolved. Once it’s all dissolved place a thermometer into the test tube. Once the temperature
reaches 90⁰C use the test tube holder to remove the test tube from the hot water bath.
4. Hold the test tube up to the light (or place it in a cold water bath) and watch for the first signs of crystallization
in the solution. Be very careful not to spill the solution!Record the temperature immediately as crystallization
begins in the class data table on the board.
5. Repeat steps 1-4 with the same amount of KNO3 for trial 2.
6. Clean up! The contents of the test can be rinsed down the drain. Wipe down your lab table and make sure there
is no trash in your sink or under your table. Be sure to push in all stools.
7. Solubility graphs usually show how many grams can dissolve in 100 mL water. What do we need to multiply our
5 mL of water by in order to have 100 mL water? ___________ Scale up all the grams of KNO3 by the same
number in order to calculate how many grams would have dissolved in 100mL instead of 5mL/
Record all the class data in your group data table. Using the scaled measurements, graph the data on the next page.
INDEPENDENT variable should be on the x-axis. DEPENDENT variable should be on the y-axis. Make sure to correctly
label your graph
Data Table:
Grams of KNO3 in 5mL
Temperature at which crystallization
first occurred (Celsius)
Trial 1
Trial 2
Average Temp
Scaled Grams KNO3 in 100 ml water
1.5g
3.0g
4.5g
6.0g
7.5g
9.0g
Qualitative observations:
Write down observations here during the experiment.
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Graph:
Discussion Questions:
Answer the questions below in 2-3 complete sentences.
1. Compare your graph to the published solubility graph for potassium nitrate
below. What were possible errors?
2. Do you think that if we continued to heat the water bath we could have dissolved infinitely more KNO3?
Why or Why not?
3. According to your graph, what mass of potassium nitrate will dissolve in 250.0 g of water at 60°C?
Conclusion Question
A
B
C
Prompt: When mixing 100 g of potassium nitrate in 100 mL of water, the resulting solution can be
unsaturated, saturated, or supersaturated. Using the graph above along with known rules of solubility,
explain how temperature affects the solubility and saturation level of KNO3 in H2O while using the points A, B,
and C to help explain your reasoning. Use clear claim, evidence, and reasoning to justify your response.
Claim:
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Evidence:
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Reasoning:
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