## Hard Water

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.
1. 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?
2. 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.
3. 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

• 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:
1. Soft Water
1. Put 50 mL of soft water in a flask
2. Add ten small drops of liquid soap; try to make them all the same size
3. Stopper the flask and shake it
4. 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
2. Hard Water
1. Put 50 mL of hard water in a flask
2. 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.

#### 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.

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#### 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.

1. What makes hard water different from soft water at the atomic level?
2. How does hard water affect the performance of soap? What evidence do you have for your answer?

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1. 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.
2. How does adding more soap counteract hard water at the atomic level?
3. Can Washing Soda counteract the effects of hard water? Provide data to support your answer.
4. How does adding a water softener like washing soda counteract hard water at the atomic level?
5. 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?

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1. 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)?
2. 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?
3. 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 Home