Teacher’s Guide by: Ian Kihara (Pocatello High School)
Lesson Overview: Laboratory experiment: survey of “Bubble-O-Metric” titrations of water hardness causing divalent cations.
Subject area(s): Chemistry and
Environmental Science
National Science Education Standards Addressed:
1.
?
Materials:
 0.01 M aqueous CaCl
2
 2% tincture of green soap in water
 various water samples
 de-ionized water
 dropper pipettes
 small vials or test tubes
Learning Objectives:
Topic: water chemistry
Audience: All levels of High school chemistry
Suggested time: 50 minutes
1.
10 min pre-lab discussion
2.
20 min lab work
3.
20 min post-lab discussion
Resources:
The Reactions of Divalent Cations With
Soap; Soap Titrations
Students will:
1.
Calculate Titration equivalence of the soap solution.
2.
Calculate hardness level semi-quantitatively of several water samples.
Background:
1.
Soap and hard water react to form soap scum.
2.
Water hardness is formed by divalent cations, primarily Ca 2+ and Mg 2+ .
3.
Many designer waters are extremely hard.
Assessment Strategy:
1.
Are students able to correlate calculated numbers to relative water hardness?
2.
Can students discuss differences in results between groups and identify possible sources of errors?
Teaching Tips:
1.
Thoroughly agitate samples between drops of soap.
2.
Make sure that students are dropping soap solution and not soap bubbles.
Extension(s):
Where might a natural abundance of “hard” water be found?-
Natural springs and geysers
Why can’t other soaps be used?- They contain chelates like EDTA and NTA
References:
Adapted from:
The Reaction of Divalent Cations with Soap; Soap Titrations. Lab Experiments: The Chemistry of Natural
Waters. Idaho State University, Chemistry Department.
This material was developed with support from the National Science Foundation’s Graduate Teaching Fellows in K-12 Education (GK-12) program
(DGE-0338184) and Idaho State University. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF.
Idaho State University’s GK-12 Project http://www.isu.edu/%7Esanghann/Index.html
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Goal: To examine the effect of divalent cations, such as Ca2+ and Mg2+, on typical soap
solutions.
Discussion: The reaction may actually be carried out quantitatively in the form of a dropwise titration. A standard soap solution is added dropwise to the natural water sample containing Ca 2+ and Mg 2+ . The solution is shaken vigorously between soap drops. The end point is suds formation!
Experimental Steps: 1. Wash 2 glass vials and caps with distilled water.
2. Add distilled water to each vial to about 1 cm in height.
3. Use a microburet to deliver 10 drops of the 10 -3 of the vials.
M
calcium standard to one
4. Fill another microburet with 2% soap solution.
NOTE: You can use a different microburet here because this titration is only semiquantitative.
5. Deliver 2 drops of soap to the vial containing only distilled water.
6. Replace the cap and shake vigorously.
• What happens at the surface?
7. Titrate the solution in the other vial by adding soap solution dropwise until you obtain suds formation. Use the first vial for comparison.
• Look carefully at the solutions in the two vials against a black background. What do you see?
• What is the chemical composition of the precipitate?
• How much water hardness is titrated by 1 drop of 2% soap solution?
(i.e. create a conversion factor of X
M
Ca +2 /1 drop soap.)
8. Repeat this titration using two other water samples.
This material was developed with support from the National Science Foundation’s Graduate Teaching Fellows in K-12 Education (GK-12) program
(DGE-0338184) and Idaho State University. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF.
Idaho State University’s GK-12 Project http://www.isu.edu/%7Esanghann/Index.html
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