Does Soap Float?

What You Need


  • 4 bars of different brands of soap (one should be Ivory and one should be a store brand)
  • Bowl of water deep enough for soap to float or sink
  • Paper plates
  • Paper towels
  • Microwave oven
Does Soap Float? Photo Credit: Clipart.com


To carry out an investigation in order to answer a central question: Does soap float?


In this lesson, students will form hypotheses and carry out an investigation in order to answer a central question: Does soap float?

According to Benchmarks, "Children’s strategies for finding out more and more about their surroundings improve as they gain experience in conducting simple investigations of their own and working in small groups. They should be encouraged to observe more and more carefully, measure things with increasing accuracy (where the nature of the investigations involves measurement), record data clearly in logs and journals, and communicate their results in charts and simple graphs as well as in prose." (Benchmarks for Science Literacy, p.11 .)

The focus of this lesson is on scientific inquiry, but it incorporates scientific topics such as sinking and floating so it is recommended that students have at least some prior experience with these concepts. For a Science NetLinks lesson addressing sinking and floating, see Sink It.

The term "density" is not appropriate for all students at this grade level; however, this lesson includes activities that will help lay the groundwork for future study of this topic.

Planning Ahead

You could read these resources for background information:


Have students begin to think about whether soap floats. Show them the four brands of soap and ask questions such as:

  • In general, what do you think makes something float?
  • Do you think an empty paper cup would float? Why or why not?
  • Do you think any of these four brands of soap will float? Why or why not?


Part I
Have students record their hypotheses about whether the specific brands of soap in this activity will float on the Does Soap Float? student sheet.

Note: Distribute only the first page of the student sheet at this time.

Then ask questions such as:

  • What characteristics of the soap do you think determine whether it will float?
  • Do you think the texture of the soap affects whether it will float?

Now have students place each bar of soap (one at a time) in a bowl of water and record their observations on the student sheet.

Note: Each bar should be placed in the water individually. There should never be more than one bar of soap in the water at a time.

Ask questions such as:

  • Was your hypothesis correct for each of the soaps?
  • Did the bar of (brand x) soap float? Were the results the same for all groups of students?

Students should have noticed that only the Ivory soap floated. Discuss why they think that was.

Ask students:

  • Why do you think Ivory soap floated while the other brands didn't?
  • How is the appearance (size, texture, etc.) of Ivory soap different from the others? Do you think that has something to do with why it floats?
  • Read the ingredients on the soap label. Is there something in Ivory soap that isn’t in the other soaps? If so, would that make it float?

Now have students read a little about the background of Ivory soap. Have them read Ivory Soap Comes Clean on Floating, from CBS News.

Once students have read this information, ask questions such as:

  • Describe how Ivory soap's most famous feature—its ability to float—came about.
    • (It came about by having air whipped into the soap.)
  • Do you think the fact that Ivory soap floated had anything to do with its physical characteristics such as color and odor?
    • (Students should realize that it had nothing to do with things such as color and odor, but that it had to do with the air bubbles [density].)
  • What about Ivory soap makes it float? 
    • (The air bubbles, which make Ivory less dense than water.)
  • What other objects float? A sponge? A Styrofoam cup? A brick?
    • (Answers may vary. Encourage your students to explain their answers.)
  • Why do you think these objects float? 
    • (Although they shouldn't be expected to use the term "density," students should begin to understand related concepts. Students should demonstrate understanding that those objects more dense than water will sink and those less dense than water will float.)

Now have students use what they have learned about Ivory soap to think of ways to make the other brands of soap float. Eventually lead them to a discussion of making the soaps less dense than water by "adding" air bubbles to them by heating them.

Ask questions such as:

  • Based on what you learned about why Ivory soap floats, what do you think we could do to make the other brands of soap float?
  • What would happen if we heated or cooled the soaps?
  • How could we do this? (Lead students to the idea of heating soap in the microwave.)

Part II
Now distribute the second page of the Does Soap Float? student sheet and have students microwave each bar of soap. You could have students do this in their groups, or you could microwave the soaps for the class while they record their observations.

Note: Heat the soap on a paper plate. The soap should be watched very carefully and heated for only one minute. It will become very hot, so you should let it cool for at least 3-5 minutes before handling it.

The heated soap should expand. It should look like "marshmallow fluff" and feel a little slippery (like foamy plastic). After it cools for 3-5 minutes, the soap should become hard and stiff like Styrofoam.

After the bars of soap has been heated and cooled, students should predict whether they will float and record their hypotheses on the student sheets. Next, students should place the soaps in a bowl of water (one at a time) and record their observations.

Ask questions such as:

  • How is the soap different than before it was heated?
    • (Answers may vary. Encourage students to explain their answers.)
  • Did the soap float after it was heated? Why or why not?
    • (All bars should float after being heated.)
  • How is the heated soap more like Ivory soap?
    • (Students should be able to describe in their own words that heating the soaps made them less dense, therefore they floated.)


Students could present their findings in an oral presentation, a written report, or on a science board.

Note: Consider developing a rubric for assessment of this activity. There are several resources on the Internet that describe the use of rubrics in the K-12 classroom, a few of which are highlighted here.

To learn more about rubrics in general, see Make Room for Rubrics on the Scholastic site.

For specific examples of rubrics, more information, and links to other resources, check out the following sites:

Finally, you can go to Teacher Rubric Makers on the Teach-nology.com website to create your own rubrics. At this site you can fill out forms to create rubrics suitable for your particular students, and then print them instantly from your computer.

Have students answer questions such as the following:

  • Were there any differences between the brand name soaps and the store brand soap?
  • What were the differences before heating them? After heating them?
  • What happened to the soap once it was heated (e.g., changes in appearance, color, smell)?
  • What do you think would happen if we heat the soap in a different way? What if we soak the soap in hot water for a couple of minutes? (You could supply a bowl of hot water for exploration.)
  • Why did heating soaps cause them to float?
  • What did you learn about sinking and floating in this activity?
  • What happened in the activity that you didn't expect?


For an exploration of buoyancy, see the Science Netlinks lesson Buoyant Boats.

For an excellent activity on sinking and floating, go to Zippy Zappy Boats activity on the Science for Kids website.

You could extend the soap activity in this lesson in many ways. For example, you could:

  • Have students write a complete lab report referring to steps of the scientific method (e.g., Problem, Materials, Hypothesis, Procedure, Observations, and Conclusion).
  • Have students collect data on the cost of soaps versus the density. Is there a correlation? Do less (or more) expensive soaps have more air in them after being heated?
  • Have students write the various soap companies and discuss the results. You could use this as a language arts extension.
  • Have students think of other objects that may float differently when heated and design an experiment to test their ideas.
  • Use this lesson as a connection to social studies and language arts. Have students think about what ingredients they would use for a soap they invented, and why. Have them name their new soap and come up with a marketing and advertising plan for it.
  • Have students explore other serendipitous scientific discoveries.

To learn more about soap, specifically that soap and water wipe grease and grime away because of the special characteristics of soap molecules, go to Meet Molly Cule at The Magic School Bus website.

For information on bubbles, go to The Art and Science of Bubbles at The American Cleaning Institute.

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Lesson Details

Grades Themes Type Project 2061 Benchmarks