Charles' Law and Ivory Soap

Nancy Thill Forest Elementary School Crystal, Mn
Adapted from experiment #53 and Foss Water unit
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With this activity students will observe and record the properties of various types of bar soap. They will discover that due to the ingredients some soaps are denser than others, and ivory soap will actually float. Students will see a model of an atom and a molecule, and act out how molecules behave as they are being heated. They will learn about Charles' Law which states that as the temperature of a gas increases, the volume increases as well. This law will be demonstrated by heating ivory soap in a microwave oven.

Learning Goals

Students will use observation skills to determine properties of various soaps. They will review the terms density, variable, atom, and molecule. Students will develop a way to record their data in a way that can be later displayed and shared. They will be able to demonstrate or make a diagram to explain what happens as molecules are heated.

Context for Use

This activity will best suit students in grades 4-6. A class of 28 could be divided into 7 groups of 4 students each. Students should be familiar with cooperative group skills and have some background knowledge of creating a graph or other type of data collection sheet. Various bar soaps should be available with wrappers to verify ingredients if possible, plastic cups, thermometers, rulers, magnets, water, a microwave safe plate, and a microwave oven. This activity can very easily be used in other settings. The full lesson should take 1-2 class periods.

Subject: Chemistry:General Chemistry:Thermodynamics:Heat, Chemistry:General Chemistry:Thermodynamics, Bonding & Molecules, Properties of Matter
Resource Type: Activities:Lab Activity
Grade Level: Intermediate (3-5)

Description and Teaching Materials

This activity will be used as an extension with the Foss Water unit that is taught. It would be best to teach the lesson on density in the Foss kit (investigation 2 part 2) so the students would have some prior knowledge about density and also some experience with various record sheets. Introduce the activity by reviewing properties and what tools may be useful when gathering information. List ideas on the board so that all students can refer to others' ideas. Remind the students about safety precautions (not to taste anything unless they are given permission, care of equipment, general rules for procedures, etc...) Have students in groups of 4 develop a table or graph to record properties of the various soaps. At the materials station have clear cups, various bars of soap, rulers, gram balances, thermometers, magnets, pitchers of water, and a tray to gather the items. These things should be covered so that when students are offering answers for tools to gather information, they will not have preconceived ideas. Once recording sheets have been established, have the groups send a getter to gather materials in order to collect data about properties of the soaps. Allow time to measure, weigh, and experiment with the items. Have the groups gather together as a whole group to discuss results. Determine similarities and differences that may have been found and talk about why having multiple measurements on the same item is important. Also review how to determine a "constant" is, and how to change only one variable.
During the next part of this activity remind students about the experiment from the Foss unit that discussed density. "What happened to molecules of water that were cold?" "What happened as water molecules were heated?" Have students act out or draw diagrams to show models of how the molecules behave at different temperatures. Propose the idea of heating the bars of soap in a microwave oven. This part could be done as a whole class. Students could make their own predictions of what will happen for each bar of soap, and the teacher could microwave one type of soap at a time and have the group record their observations. Discuss Charles' Law and why the ivory soap expanded so much. The closing strategy would be to have the students change one variable and develop a new experiment (to test another material or to heat the soaps in another way.) This activity was adapted from experiment #53 on Steve Spangler's web site . There are also other sites that have information about the "ivory soap in the microwave", but the Spangler site has a great explanation.

Teaching Notes and Tips

Do not use a whole bar of soap in the microwave. Either cut about 1/3 of a bar, or use the small hotel sized bars when doing this activity. If the ingredients of the soap are not listed on the labels, many common brands have the ingredients listed on a web site. It may be fun for the students to know the history of ivory soap. It was actually created due to a mistake. The students could research other items that were discovered by mistake, but are now widely used (i.e.: scotch tape or silly putty) and discuss how mistakes are not always a bad thing! The ivory soap that has been micro waved can still be used as soap. Another idea is to tie the "cloud shaped soap" into a unit with weather and climate to show cumulus clouds.


The recording sheets that the students develop will be used as an assessment. The graph or table should be easy to understand and the tools used should include weight, length, width, and what happened when the soap was put into water. Other notes may be included as well. An informal assessment should be done with the models for molecules.

Standards Measure temperature, volume, weight and length using appropriate tools and units. Describe how the states of matter change as a result of heating and cooling. Recognize that when scientific investigations are replicated they generally produce the same results, and when results differ significantly, it is important to investigate what may have caused such differences.

References and Resources