Introduction

Water that contains calcium ions, Ca2+, and magnesium ions, Mg2+, is said to be hard water. These ions are leached from ground water flowing over rock formations containing limestone and other minerals. Hard water interferes with the cleaning action of soaps.

When soap is added to hard water, insoluble compounds form which appear as sticky scum. This scum leaves a deposit on clothes, skin, and hair. You could have ring around the collar!

When boiled, hard water leaves a deposit of calcium carbonate, CaCO3. This scale builds up in tea kettles and inside hot water heaters.

Detergents have replaced soap for many cleaning jobs around the home.

The development of synthetic detergents by chemists was a great advantage for people with relatively hard tap water in their homes.

Do you know whether the tap water used in your home is soft or hard? How could you test it to find out? Why do you use detergents for many household cleaning jobs? These are some of the questions you will be able to answer after completing this laboratory investigation.

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Purpose

To investigate the chemical action of soap vs. detergents in hard water and the use of a precipitation reaction to soften hard water.

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General Safety Considerations

Wear protective glasses and an apron at all times. Avoid skin contact with solids and solutions. Dispose of all solutions in the containers provided by your teacher. Wash your hands before leaving the laboratory.

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Procedure

Part I: Soap vs. Detergent in Hard Water

  1. Add 100 mL of distilled (soft) water to a 250 mL beaker. Then add 2.0 g of magnesium sulfate, MgSO4, and stir. Label this beaker hard water. You now have prepared a stock solution of hard water.
  2. Arrange three test tubes in a test-tube rack. Half-fill the first test tube with distilled water (soft water). Half-fill the second tube with tap water. Half-fill the third with hard water from the stock solution.
  3. Using a metal spatula, add a pea-sized sample of solid soap from a bar of commercial hand soap to each of the three test tubes. (Caution: Remember after shaking several times to stop, release the pressure, re-stopper, and shake.) Stopper and shake the first test tube. Ten shakes is adequate. On your data sheet record the height of suds in the test tube as measured with a centimeter scale. Draw a picture of the test tube; record all your observations in the picture.
  4. Repeat Step 3 for the other two test tubes. Shake each test tube in an identical manner.
  5. Dump and rinse the test tube containing the soap.
  6. Repeat Steps 2 and 3, but use a pea-sized sample of a commercial detergent instead of the soap.
  7. Dump and rinse the test tubes containing the detergent.
  8. Repeat Steps 2 and 3 using 20 drops (approximately 1.0 mL) of a commercial hair shampoo in each of the three water samples.
  9. Dump and rinse the test tubes containing the shampoo.

Part II: Emulsifying Test

  1. Place four test tubes in a test-tube rack. Half-fill each with hard water.
  2. Add 3.0 mL of cooking oil to each of the four tubes.
  3. Add nothing to Tube 1, add a pea-sized sample of hand soap to Tube 2, a pea-sized sample of detergent to Tube 3, and 20 drops of shampoo to Tube 4.
  4. Stopper and shake all tubes in an identical manner. Return the tubes to the test tube rack.
  5. On your data sheet draw a picture of the four test tubes; record all your observations in the picture.
  6. Dump and wash out the test tubes as directed by your teacher.

Part III: Softening Hard Water by Precipitation

  1. Obtain a 2.0 g sample of sodium carbonate, Na2CO3. Add it to the hard water remaining in the 250 mL beaker.
  2. Place the beaker and contents on a hot plate. Heat and stir for 5 min.
  3. Remove the beaker and allow it to cool.
  4. When the beaker is cool enough to handle, separate the solid precipitate from the liquid using a filtering apparatus or centrifuge, as directed by your teacher.
  5. Half-fill a clean test tube with the clear filtrate. Add a pea-sized sample of solid hand soap to the test tube, stopper, and shake.
  6. Record your observations.
  7. Dispose of the filter paper and solid as directed by your teacher.
  8. Clean up your bench area and return all materials to their proper places.
  9. Wash hands thoroughly before leaving the laboratory.

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Data Analysis and Concept Development

  1. Name the cation and anion added to the distilled water in Part I.
  2. Which ion is responsible for creating hard water in this activity? Support your answer with observations you recorded throughout the activity and those demonstrated in class.
  3. Did the hard water look different than the soft water?
  4. Based on your data, does soap work better in soft or hard water?
  5. Based on your observations, which works better in hard water -- soap or detergent?
  6. From your data, would you conclude that shampoo is more like a soap or a detergent? Explain your answer.
  7. According to this investigation, would the tap water used be classified as soft or hard water? Justify your answer.
    1. What effect did addition of sodium carbonate (washing soda) have on the hardness of the water? Explain.
    2. Was the filtrate hard or soft?
    3. Based on class demonstration and your data, complete the following chemical equation for the reaction between magnesium sulfate and sodium carbonate solutions.
      MgSO4(aq) + Na2CO3(aq)-->
      If you need help, ask your teacher for assistance. (Data Check: Obtain your teacher's initials.)
    4. What is the solid in the filter paper? Identify it by name and formula.
  9. Which cleaning agent worked best to disperse (emulsify) the oil in the hard water? Justify your answer.
  10. What was the most interesting thing you learned in this activity?

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Implications and Applications

  1. What advantage do detergents have over soaps as cleaning agents?
  2. Examine the chemical formulas for soap and detergent shown in the Introduction. How are they chemically alike? How are they chemically different?
    1. What chemicals are used to synthesize soaps?
    2. What chemicals are used to synthesize detergents?
  4. Why don't soaps create suds in hard water?
  5. Why do you think detergents create suds in hard water, while soaps don't?
  6. Do you have a water softener in your home?
  7. How could you test the tap water in your home to decide whether you would benefit from a water softener?
  8. What do you think are some advantages and disadvantages of having a water softener?
  9. How do you think a water softener works?
  10. List two questions about something you observed during this laboratory activity you did not understand or would like explained by your teacher.

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Teachers Guide

Preparing for the Laboratory Activity

Conducting the Laboratory Activity

Assessing the Laboratory Learning

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Preparing for the Laboratory Activity

Major Chemical Concept

Soaps and detergents behave differently in hard water, based on differences in their molecular structures. Precipitation reactions can be used to soften hard water.

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Level

Appropriate for all levels.

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Expected Student Background

Students should be able to:

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Time

45 min

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Safety

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Materials

Non-consumables (per lab team)

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Consumables (per lab team)

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Advance Preparation

  1. Provide bars of hand soap, detergents, and shampoos for students who do not bring samples from home on the day of the activity.
  2. Prepare one liter of hard water by adding 10 g of magnesium sulfate, MgSO4, per liter of solution. Keep for demonstration use and for students who might spill their beaker of hard water during the activity.

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Conducting the Laboratory Activity

Pre-Lab Discussion

Discuss the chemical synthesis of soap starting with fats or oils.

Fat + Lye --> Soap + Glycerine

Discuss the history of early soap synthesis in the home using fats from animals and lye leached from fireplace ashes.

Demonstrate to students that:

  1. presence of sulfate ion is not responsible for hard water behavior. Use sodium sulfate, sodium chloride, and maybe another sulfate solution.
  2. solubility of sodium sulfate to show that it is not precipitating when magnesium sulfate and sodium carbonate solutions are mixed.

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Optional

You may decide to prepare some homemade soap before this discussion. Explain how you completed the synthesis and show students your product. Compare the product with commercial bars of soap brought from home. Synthesis of soap requires working with hot fats and high concentrations of caustic lye solutions. This activity is best done by you, not the students.

Discuss the chemical nature of a soap and detergent molecule. Write the chemical formulas on the overhead or board. Note that most detergents are made starting with hydrocarbons separated from crude oil.

Place a pea-sized sample of soap in a large test tube of distilled water, stopper, and shake. Demonstrate the technique of shaking and releasing the pressure. Point out the importance of protective glasses when shaking materials in test tubes. Ask students to explain the sudsing action of the soap in the test tube when agitated.

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Teacher/Student Interaction

Ask students to list on the board or overhead the commercial brand names of hand soaps, detergents, and shampoos they brought from home. Have students read the information provided by the manufacturer of each product. Tell them to look for chemical information regarding active ingredients in the products.

  1. Ask students why they prefer the product they brought versus other brands.
  2. Ask students to compare the sudsing action of their detergent with detergents brought by other students.
  3. As you walk around the laboratory, ask students what they think is happening in the various test tubes.

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Anticipated Student Results

  1. Sudsing action with bar of hand soap: Students will observe 3 to 5 cm of sudsing in the soft water. The sudsing in the tap water will vary depending on the hardness of the water in your area. Little or no sudsing will be observed in the hard water; the production of a scum at the top of the test tube will be very evident.
  2. Sudsing action with the detergent: Results will vary depending on the brand of detergent the students bring. Some may bring low-sudsing detergents. You may want to point out to students that sudsing action is not the only factor in judging the cleansing action of a product. Students will observe sudsing in all three test tubes. The sudsing of the detergent in the hard water is the most noteworthy observation.
  3. Sudsing action with the shampoo: The results will vary depending on the brand of shampoo the students bring. Students will observe sudsing in all three test tubes, and in most cases more sudsing than observed with the detergent. The sudsing of the shampoo in the hard water is the key observation.
  4. Emulsification Test
    Tube 1 - Control
    Tube 2 - Little or not sudsing action with the soap in the oil and hard water mixture.
    Tube 3 - 2 to 3 cm of sudsing action with the detergent
    Tube 4 - 4 to 5 cm of sudsing action with the shampoo
    The shampoo will be the best emulsify agent. You might point out to the students that shampoos are designed for that very purpose.
  5. Softening the hard water by the addition of sodium carbonate.

Student should have 25 to 30 mL of the hard water remaining in the 250 mL beaker for this investigation. When the sodium carbonate is added to the beaker they will observe the immediate formation of a cloudy white precipitate, magnesium carbonate, MgCO3(s ). Heating the mixture helps coagulate the precipitate, giving a clear filtrate.

The chemical reaction observed:

MgSO4(aq) + Na2CO3(aq) --> MgCO3(s) + Na2SO4(aq)

When the clear filtrate is checked for sudsing action using the soap, students will observe some sudsing. This indicates the hard water sample has been softened by addition of sodium carbonate.

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Answers to Data Analysis and Concept Development

  1. The cation is Mg2+, the anion is SO42-.
  2. The magnesium ion is responsible for creating hard water. Basis: Sulfate demonstration and the Introduction to the activity.
  3. No, they looked the same.
  4. Soap suds best in soft water.
  5. Detergents will work either in soft or hard water.
  6. The shampoo suds either in soft or hard water, suggesting that shampoos have chemical formulations similar to those found in detergents.
  7. This answer will vary depending on the hardness of tap water in your area. In most cases the sudsing action observed by students will indicate some degree of hardness.
    1. The sodium carbonate precipitated the magnesium ions in the hard water sample. The sudsing of the soap in the filtrate provides evidence of the softening.
    2. Filtrate was soft
    3. Products: MgCO3(s) + Na2SO4(aq)
    4. Magnesium carbonate, MgCO3
  9. The shampoo was the best emulsifying agent.
  10. These answers will obviously vary.

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Answers to Implications and Applications

  1. They will "suds" in hard water.
    1. Both have polar and nonpolar areas in the molecule.
    2. Detergents contain different elements in the polar group.
  3. .
    1. Lye and fats or oils
    2. Crude oil by-products and inorganic acids
  4. They form precipitates in the presence of magnesium and/or calcium ions.
  5. They do not precipitate in the presence of magnesium and/or calcium ions.
  6. These answers will vary.
  7. By testing the tap water with soap to see if it will suds.
  8. Advantage: Less soap used because it will not be precipitated by magnesium and/or calcium ions.
    Disadvantage: The cost of operating the water softener. The addition of sodium ions to the water.
  9. They remove magnesium and/or calcium ions.
  10. These answers will vary.

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Post-Lab Discussion

Discuss the chemical precipitation reaction of the soap molecule with calcium or magnesium ions to produce the sticky scum ("ring around the collar"), as shown in the following equation:

Reaction of Soap with Hard Water

Discuss the chemical modifications in the detergent molecule which allow these molecules to retain their chemical activity, and not produce precipitates in hard water.

Discuss the functions of the polar and nonpolar regions in soap and detergent molecules in acting as cleaning agents.

Discuss the advantages and disadvantages of having a home water softener.

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Possible Extensions

  1. Have students call the 800 numbers provided on some products, or write to some of the commercial manufacturers of detergents, seeking technical information on their products.
  2. Discuss how home water softeners work.
  3. Discuss what is meant by biodegradable detergents.
  4. Explore with students whether drinking hard water is hazardous to ones health.
  5. Have students collect advertising claims of various cleaning agents. Design experiments to check these claims.

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Assessing the Laboratory Learning

Laboratory Practical Items

Four different lab practical activities are described below. Select one or more of these to assess the laboratory activity.

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1. Group Assessment

Separate the class into groups of four. Give each group four unmarked test tubes: one filled with distilled (soft) water, one with hard water, one with tap water, and the last with alcohol. Tell them that their problem is to develop a laboratory procedure to identify which liquid is in each test tube, using their data sheets and the chemicals used in the this laboratory activity. After developing the procedure they will run the analysis in the laboratory and turn in one group report.

Advance Preparation

Prepare one set of four test tubes marked A, B, C, and D containing the following:

Safety

Anticipated Student Results

Students should be able to identify the four test tubes based on the extent of sudsing with soap and detergent.

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2. Individual Qualitative Analysis Activity

Give each student a numbered test tube filled with either soft, hard, or tap water. Tell them they must design a laboratory procedure using soap and detergent to identify which type of water is in their test tube.

Advance Preparation

Fill numbered test tubes with distilled (soft), hard, or tap water. (One test tube for each student.)

Materials

Fill some with distilled, some with tap, and some with hard water. Prepare the hard water by dissolving 10 g magnesium sulfate MgSO4, per liter of solution.

Anticipated Student Results

Students should be able to identify the unknowns by the sudsing action of the soap and detergent.

Soft Water - sudsing with both soap and detergent

Hard Water - sudsing only with the detergent

Tap Water - will vary depending on the hardness some sudsing with soap and detergent

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3. Identify an Unknown Cleaning Agent

Give each student a zip-closure bag containing a white solid. Have the student design a laboratory procedure to determine whether the bag contains a soap or detergent.

Advance Preparation

Fill numbered plastic zip-closure bags with powdered soap or a white powdered detergent.

Materials

Anticipated Student Results

The soap product will not suds in the hard water; the detergent will suds both in the hard and the soft.

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4. Pair Learning Activity - Finding a Water Softener

Students working in pairs are asked to design an experiment to determine which of the following household chemicals could be used to soften hard water by precipitating calcium and/or magnesium ions.

Household Chemicals

Advance Preparation

Students will need their data sheets and/or lab reports to complete this activity. Ask each pair of students to design a procedure to determine which household chemicals could be used to soften hard water.

Materials

Anticipated Student Results

Chemicals which soften the water:

Chemicals which do not soften the water:

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Paper/Pencil Items

1. Formal Lab Report

The report should include the following:

2. Written Assignment

Using your data sheets and/or lab report, answer these questions:

  1. What is hard water?
  2. Discuss the sudsing action of soap and detergents in hard water.
  3. How could you test the hardness of tap water in your home?
  4. Many people add the de-icer calcium chloride, CaCl2, to their sidewalks and driveways in the winter. When this substance enters the river or ground water it produces hard water. How could it be removed from river water?
  5. On the molecular level explain and/or draw pictures of the process involved in removing magnesium ions from hard water using sodium carbonate.

3. Post-Laboratory Quiz

Quiz for the Soap vs Detergent Laboratory Activity

Directions:

Using the code letters in the Figure, identify the correct responses to each question. There may be more than one answer for some questions, or none of the above.

  1. Which, if added to water, would produce hard water? Answer : B, C, and G
  2. Which is a soap? Answer: A only
  3. Which is a detergent? Answer: E and F
  4. Which can be added to hard water to soften the water? Answer: D only
  5. Which will produce suds in hard water? Answer: F and E
  6. Which will not produce suds in hard water?
    Answer: A only
  7. Which is not water soluble? Answer: H only
  8. Which will produce an insoluble precipitate when combined with magnesium ions? Answer: A and D
  9. Which is made from heating lye and animal fats or oils? Answer: A only
  10. Which is hazardous to your health in very small quantities? Answer: None

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Student Survey

Ask students to answer these questions:

  1. What was the most important thing you learned in this laboratory activity?
  2. What did you enjoy most in this activity?
  3. What question would you like answered about this activity?
  4. What will you remember about this activity?
  5. List two essay questions which might appear on an exam over this activity. Write the answers for each of your questions.
  6. Write two multiple-choice questions which might appear on an exam over this activity. Circle the correct answers
  7. What new laboratory technique did you learn in this activity?
  8. What new chemical terms did you learn in this activity?
  9. What additional investigations or extensions would you like to do on this activity?
  10. What additional or extra activities did you do related to this activity?
  11. What could be done to improve this activity?
  12. Circle the grade you would give this activity:

A B C D F

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