Note to Teachers: This lab is an inquiry-based lab. The
idea is for students to come up with their own procedure. They must decide
what will be held constant, what will be varied, and how to make their
measurements. The notion is to have students learn science by doing science
the way scientists do it. Students may either write narrative answers to
the questions as posed in the objective section (complete with graphs) or
write a full lab report. I expect that it will require anywhere from 1 to 3
full hours of classroom time, depending on the level of complexity you
require from the students. Students may extend their investigations at home
in order to get a better grade. If you try it, please get in touch (my
email address is on the home page) and let me know how it worked for you.
Objective
Hard water is water containing higher-than-normal concentrations of metal
ions. These ions form waxy insoluble salts with soap molecules, creating the
“soap scum” so dreaded in bathroom cleanser commercials. Because
this happens, soap is a less effective cleanser in hard water. In this lab you
will explore the effect hard water has on the ability of soap to make lasting
bubbles and also do some research into the following questions:
The structure of a type of soap molecule.
What is the difference between hard water and soft water (water without
metal ions) when it comes to the amount of soap bubbles that can be
formed?
Hard water can be counteracted simply by using more soap. Quantify how
much more soap is needed by counting the number of extra drops of soap it
requires to get the same amount of bubbles as you get with soft water. Calculate how many more times the amount needed for soft water is needed in hard water.
Hard water can also be counteracted by putting something else besides
soap into the water to react with the metal ions. These substances are called water softeners. How much Washing Soda (in grams) does it take to make hard
water soft? Calculate the number of grams required for a liter of hard water and report the result in g/L. Try to find the smallest effective amount of washing soda since that would minimize costs.
Materials
several 250-mL flasks w/stoppers
50 mL graduated cylinder
lab balance
soft water
prepared hard water solution
several droppers
ruler
lab spatula
liquid soap (diluted to ½-strength)
Washing Soda
Note to Teachers: The prepared hard water solution is
made by mixing 10 g of Epsom salt (magnesium sulfate heptahydrate) per
liter of distilled (or tap) water. Or mix whatever fraction of a liter is
appropriate. The proportion given makes a 0.04 molar solution of magnesium
sulfate.
I originally used a solution with ten times this concentration but I
found that no amount of washing soda had any effect on the amount of suds. This
small concentration of Mg2+ is
still plenty to show a noticeable difference in the amount of soap bubbles
when compared with distilled water or soft tap water.
By the way, distilled water costs money and tap water costs you nothing.
As long as the tap water is reasonably soft you can use that. Do a test as
suggested for the student’s first demonstration to find out. If the water turns out not to be soft, simply pre-treat it with sodium carbonate. Add a little at a time until the shake test shows much higher bubbles than the prepared hard water. This is ultimately cheaper (probably) than buying all the distilled water this lab would require.
Finally, take a look at this crude figure that can be used as an overhead or handout. It shows a molecular model of magnesium sulfate dissolved in water (showing the dissociation of the ions) and a model for the reaction between magnesium ions and soap molecules.
Background
Water is an excellent solvent and even very pure natural waters contain
some dissolved minerals. The ground water which is used to supply water to the
tap will often contain enough minerals that it is called “hard
water”. Hard water is water that has metal ions dissolved in it such as
magnesium (Mg2+), calcium
(Ca2+), and/or iron (Fe3+). These ions are generally harmless and can
be consumed without ill effects. Even so, hard water has the disadvantage of
making washing up harder to do. In hard water soap forms an insoluble salt
with the metal ions and is not available to do the work of washing. Instead
it leaves an unsightly scum behind both in the sink and on the things being
washed. Hard water also leaves a white flaky substance called scale inside
pots and appliances used to heat water (such as coffee machines and tea
kettles).
Soft water is water with much lower amounts of dissolved minerals. Soft
water is ideal for washing since soap works best when there are only small
amounts of dissolved minerals. Soap can create far more bubbles in soft water
than it can in hard water.
By using more soap it is possible to get the same cleaning power and
sudsing ability in hard water that much smaller amounts of soap provide in
soft water. This is possible because all of the minerals in the hard water
are used up in combining with the extra soap and more soap is available for
cleaning. This means that it costs more to do the washing and that more soap
scum clings to the things being washed. This results in dingy clothes and
dull dishes. Not to mention dull and lifeless hair.
Washing Soda (sodium carbonate, Na2CO3)
is a water softener. When dissolved in hard water it makes soaps work better
by reacting with the metal ions. It reacts to make insoluble white solids such as magnesium carbonate and calcium carbonate. Since the metal ions are busy reacting with
the washing soda they are not available to react with the soap. This takes
care of both low suds and soap scum.
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Procedure
The procedure for this lab is mostly up to you. After your teacher gives
you an introduction to the lab it is your decision about how to proceed.
For each objective answer the following questions
before beginning your work. If you do then
your work will make more sense and be more efficient.
What experiments are you going to perform?
In your experiments, what are you going to hold constant?
What are you going to change?
Before you set up the experiments try to decide what you think
is going to happen. Write this down and refer back to it as you work.
Modify further experiments as needed on the basis of your results.
How are you going to decide when you have answered the questions?
Set up the data table you will need before beginning to
collect data. This will keep your work organized and help you to see the
big picture. You can use the data you collect to make sure you are on the
right track.
While you are working use the data you collect to decide whether you are
on the right track. Modify future experiments based on the data you collect.
Be reflective and think about what your results mean before proceeding. Do
not work to just get it all done as quickly as possible. Finally, be creative! Solving problems in science requires a different kind of creativity from that needed to create art or music but it is just as important in science as in the arts.
Objective One
Establishing the Difference between Hard and Soft Water
Suggested
procedure:
Soft Water
Put 50 mL of soft water in a flask
Add ten small drops of liquid soap; try to make them all the same
size
Stopper the flask and shake it
Let it rest for about 15 or 20 seconds and then measure the height
the bubbles in the flask; always use the same measuring
technique throughout the lab
Hard Water
Put 50 mL of hard water in a flask
Follow the same procedure as with soft water
Describe the experiments you will perform:
What are you holding constant in your
experiments?
What will you change (vary) in your
experiments?
What do you think the results of the experiments
will be?
How are you going to decide when you have
completed the objective?
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Use the following data table:
Soft Water
Hard Water
Sample Number
Height of Suds (cm)
Sample Number
Height of Suds (cm)
1
1
2
2
3
3
Average
Average
Check-in ______
When you have completed your preparations for this section check in with your teacher. This is
a required part of the lab and your teacher’s initials are required
before you can start your experiments.
Summary for Objective One
Write a brief summary of your experiments and the results in the space
below. In other words, answer the question(s) in the objective. Use your
experimental results to justify your conclusions.
Check-in ______
When you have completed this objective check in with your teacher. This is
a required part of the lab and your teacher’s initials are required
before you can move on to the next objective.
Objective Two
Counteracting Hard Water with More Soap
Describe the experiments you will perform:
What are you holding constant in your
experiments?
What will you change (vary) in your
experiments?
What do you think the results of the experiments
will be?
How are you going to decide when you have
completed the objective?
Design your own data table for this part of the lab. Be sure to plan carefully and leave room
for multiple trials of the same experimental set-up. Also, remember to leave
room to enter an average value for each experimental set-up. You may find it
helpful to use a ruler. Sketch your data table on a separate piece of
paper and show it to your teacher for approval before putting the final
version here. You must obtain your teacher’s initials before putting
your data table here.
Check-in ______
Summary for Objective Two
Write a brief summary of your experiments and the results in the space
below. In other words, answer the question(s) in the objective. Use your
experimental results to justify your conclusions.
Check-in ______
When you have completed this objective check in with your teacher. This is
a required part of the lab and your teacher’s initials are required
before you can move on to the next objective.
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Objective Three
Counteracting Hard Water with Washing Soda
Hint: You will need between 0.1 and 2.0 g of washing soda.
Describe the experiments you will perform:
What are you holding constant in your
experiments?
What will you change (vary) in your
experiments?
What do you think the results of the experiments
will be?
How are you going to decide when you have
completed the objective?
Design your own data table for this part of the lab. Be sure to plan carefully and leave room
for multiple trials of the same experimental set-up. Also, remember to leave
room to enter an average value for each experimental set-up. You may find it
helpful to use a ruler. Sketch your data table on a separate piece of
paper and show it to your teacher for approval before putting the final
version here. You must obtain your teacher’s initials before putting
your data table here.
Check-in ______
page break
Summary for Objective Three
Write a brief summary of your experiments and the results in the space
below. In other words, answer the question(s) in the objective. Use your
experimental results to justify your conclusions.
Check-in ______
When you have completed this objective check in with your teacher. This is
a required part of the lab and your teacher’s initials are required
before you can move on to answer the lab questions.
Lab Questions
There will be no formal lab report required for this lab. Instead, answer
the following questions using your experimental data. Justify your answers
using the standards
for explanations provided at the beginning of the year.
Additional research is required to write good answers to some of the
questions, especially the ones that concern atomic-level explanations. Start
with Wikipedia.org and http://sci-toys.com/ingredients/ingredients.html. Provide
a bibliography at the end of this packet for sources that you consult.
What makes hard water different from soft water
at the atomic level?
How does hard water affect the performance of
soap? What evidence do you have for your answer?
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Does adding more soap make up for the effects of
hard water? Can you add enough to make the water act like soft water?
Provide data showing how additional soap changes the amount of
sudsing.
How does adding more soap counteract hard water
at the atomic level?
Can Washing Soda counteract the effects of hard
water? Provide data to support your answer.
How does adding a water softener like washing
soda counteract hard water at the atomic level?
Science is a method for generating knowledge about the world. Sometimes a great deal of creativity is needed to be able to solve problems. How did you demonstrate creativity in problem solving in this lab?
page break
Why do you need to hold some things constant when
you are setting up experiments? Give examples from your own experiments of things you held constant. Finally, what would be the effect of changing your constants while still changing the variable (such as the amount of soap)?
Did you use data you collected to help determine
how to run upcoming trials while you worked? What was it that you did and
why did you do it?
Bibliography (give specific page addresses or book references):
This page owes its inspiration to two sources. First, to Bassam Z.
Shakhashiri of the University of Wisconsin, Madison. His site describing home chemistry projects includes
one involving hard water. Second, the book A+
Projects in Chemistry by Janice VanCleave has a wealth of ideas
including several borrowed for this lab.
Last updated: Jan 22, 2009
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