1.1, 1.2 Effect of Temperature on Solubility of a Salt-Teachers Notes
COS Objectives:
Standard 4: Solubility
Standard 4: Bullet 1: Solubility Curves
Standard 4: Bullet 4: Rate of solution
ENGAGE
Super Saturation
Teacher Notes
COS Objectives:
Standard 4: Solubility
Standard 4: Bullet 1: Solubility Curves
Standard 4: Bullet 4: Rate of solution
ENGAGE
Demonstration of rapid precipitation from a Reusable Hand Warmer
(1) Explain the concepts of saturation, supersaturation, crystallization, and seed crystals
(2) Hand each pair of students one of the Reusable Hand Warmers.
(3) Have one student Flex the metal disc back and forth until crystals start to form. Once crystals
start to farm gently knead the pouch to intensify the heat.
Materials for Engagement activity
Reusable Hand Warmer
Hot Plate
Large Beaker
Water
Tongs
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Chemistry Module
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Restoring to liquid form (using a hotplate)
(1) In a large beaker boil water.
(2) Hold the Reusable Hand Warmer with the Tongs in the boiling water (be sure to not let it touch
the bottom of the beaker or the plastic will melt!
If you get tired of holding the Reusable Hand Warmer with the Tongs, an alternative is to wrap it
in a heavy cloth towel. The cloth towel should protect the plastic from melting against the
bottom of the beaker. Then you can let the protected (cloth wrapped) Reusable Hand Warmer
drop to the bottom of the beaker.
(3) Remove the Reusable Hand Warmer when ALL of the crystals are gone (approx. 7-10 minutes)
(4) Let it cool before use (the plastic is wearer when hot and the metal disc may slice it open).
NOTE: If you want to use a microwave to Restore the Hand Warmer watch the video at:
http://beprepared.com/article.asp?ai=725&
EXPLORE
See Student Handout
Sample Data
Solubility of KNO3 (per 100 g H2O) vs. Temperature
Trial
Solubility
(g / 100 g H2O)
Temp
(°C)
1
40.0
25.5
2
80.0
47.2
3
120.0
64.0
4
160.0
76.4
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Chemistry Module
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EXPLAIN
Answers to Questions
1. According to your data, how is solubility of KNO3 affected by an increase in temperature of the
solvent?
Solubility of KNO3 increases with increasing temperature.
2. Using your printed graph, tell if each of these solutions would be saturated or unsaturated:
a. 110 g of KNO3 in 100 g of water at 40°C _____Saturated________________
b. 60 g of KNO3 in 100 g of water at 70°C _____Unsaturated______________
c. 140 g of KNO3 in 200 g of water at 60°C _____Unsaturated______________
3. According to your graph, will 50 g of KNO3 completely dissolve in 100 g of water at 50°C? Explain.
Yes, it will dissolve, since this point is below the solubility curve.
4. According to your graph, will 80 g of KNO3 completely dissolve in 100 g of water at 30°C? Explain.
No, it will not dissolve, since this point is above the solubility curve.
5. At 30°C, approximately how many grams of KNO3 would dissolve in 100 g of water?
At 30°C, ~50 g of KNO3 would dissolve in 100 g of water.
EXTEND
1. One way to extend this is to have students use the following graph to make comparisons between
different salts or between salts and the gas SO2.
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Chemistry Module
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Note that the solubility of the gas decreases with an increase in temperature which is opposite that of
the general trend for salts.
You can ask students to figure out how much more solution NaCl is in boiling water than room
temperature water and compare that to KCl. Then ask the students why do they think we used KCl in
the experiment? Answer: It’s solubility changes considerably more than salt in the temperature range
that we use in the lab.
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Chemistry Module
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2.
Visit
NASA’s
website
http://nasascience.nasa.gov/earth-science/oceanography/physicalocean/salinity. Answer the following questions:
 Where are the highest salinity and the lowest salinity of oceans found?
Highest salinity of seawater is in the mid-Atlantic Ocean
Lowest salinity is found near the Arctic and Antarctic
 Why higher seawater salinity is found in the middle of oceans near the equator?
High salinity is usually in the center of the ocean basins away from the mouths of rivers, which
input fresh water. High salinity near equator is due to intensive evaporation.
 Why lower seawater salinity is found near the Arctic and Antarctic areas?
Due to glaciers melting
 Describe two techniques used for measuring ocean water salinity. Which method can provide a
complete global data with relatively short periods (a few days) of time?
Salinity data can be collected by satellites (using radiometers) or in situ measurement (by ships,
buoys, drifters, and moorings). Satellite data collection covers the entire globe and can be done
in a few days.
Troubleshooting & Safety
1. Have 20 g of solid KNO3 available per student lab team. HAZARD ALERT: Potassium nitrate is a
strong oxidant; fire and explosion risk when heated or in contact with organic material; skin irritant.
Hazard Code: B—Hazardous.
The hazard information reference is: Flinn Scientific, Inc., Chemical & Biological Catalog Reference
Manual, 2000, P.O. Box 219, Batavia, IL 60510. See Appendix F of this book, Chemistry with
Calculators, for more information.
2. Students should be well-prepared for this experiment in order to comfortably complete it during a
50-minute class period. Remind them of the tips in Step 11 of the student procedure. Starting hotwater baths promptly will save time. The 2-g and 4-g samples do not need to be heated above 60°C
in order to have all of the solid dissolve; thus, the water bath does not need to be at 90°C for these
two trials. Toward the end of the class period, some students may have to use cool-water baths,
especially for the 2-g and 4-g samples.
3. When using hot plates, caution students to keep the temperature probe wire well away from the
hot plate, as mentioned in Step 8.
4. The Vernier temperature calibrations that are stored in the DataMate data-collection program will
work fine for this experiment.
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Chemistry Module
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