Ivory Soap in Microwave Oven
Lesson 4 of 5
Objective: Students will able to work in groups to determine why ivory soap expands into a puffed structure when heated in a microwave.
MS-PS1-4: Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
DCI: PS1.A - The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter.
Science and Engineering Practice 2: Developing and using models
CCC: Cause and effect relationships may be used to predict phenomena in natural or designed systems.
Students place a bar of Ivory Soap in the microwave and watch how the soap bar expands into a fluffy white structure. As a group they discuss how that structure was created. Along they way, they are required to create a drawing that adequately explains the process and serves as a model for their discussion (MS-PS1-4, PS1.A, S&E Practice 2: Developing and using models).
The microwave oven causes the air molecules embedded in a bar of Ivory Soap to become excited ( heated). These excited air molecules push with greater internal pressure on the inside cavity of the air pocket and cause the now warm bar of soap to expand outward. It is the students task to explain the effect of the foaming structure (CCC: Cause and Effect).
Background science: Ivory Soap is unique in the soap business as being the only soap that can float in a bathtub of water. This is achieved by whipping the soap material, forcing air into the mixture, before it is cooled down and poured into soap bar molds. A microwave oven heats the air bubbles, which causes the air to expand. The microwave also softens the soap material. The expanding air bubbles are able to cause the soap to expand into a fluffy mess.
I start this activity with background knowledge about the three phases of matter (solid, liquid, gas) and move into gasses, touching on the Combined Gas Law
(P1V1)/T1 = (P2V2)/T2
REFLECTION: Many kids think that microwave ovens work by magic or nuclear radiation. I make sure to explain that microwaves cause molecules, specifically water, to vibrate faster. This faster particle motion translates into heat. I explain that microwave ovens heat food by cheating - they don't impart any heat into the food, they cause particles to move faster which in turn produces heat.
I also have a class discussion about why Ivory Soap is unique. I guide the discussion until students have identified that Ivory Soap is able to float.
As part of this initial discussion I use the PhET Simulation for ‘Phase Changes’ to show the relationship to pressure, temperature, and pressure.
I project the simulation and ask the students what phase of matter the material is in. Hopefully they answer that it is a solid because the particles are locked together in a fixed pattern. I then manipulate the ‘heat’ option located at the bottom of the screen and cause the temperature to rise, which sends the particles into the ‘liquid’ then the ‘gas’ phase. I point out that the simulation has instruments to measure temperature and pressure. I remind them that temperature is actually measuring the motion of the particles and pressure measures the force the particles have on the inside of the chamber. I can also manipulate the simulation by adding more particles (pumping the bicycle pump) and change the volume of the chamber (pull the lid).
The fun begins when I switch the material to ‘water’ (upper right corner). I point out that when the water molecules are in the solid state they form rings that occupy more space then when they were in the liquid state. This explains why water expands then it becomes ice. By decreasing the volume of the chamber I am able to demonstrate that temperature can be affected without adding any heat, which is one of the principles of the Combined Gas Law’
Their activity will be to: 1) observe what happens to Ivory Soap in a microwave, and 2) describe how the structure of the bar changed and why they think it happened.
To help students understand the "what" that makes Ivory Soap different from other bar soap, I have my students read the Ivory Soap Origin article from Snoops.com.
I then give them half a bar of Ivory Soap and instruct them to place it on a paper plate then in the microwave for 45 seconds.
TIP: To ease congestion at the microwave (I only have one), I assign half the class the article to read and the other class can observe the Ivory Soap in the microwave. After about 10 minute, I switch the groups. I have also temporarily borrowed other teacher’s microwaves and had them on my counters.
Caution – soap may be very HOT. It cools down quickly. My students are allowed to touch the soap only if they agree to use the soap to wash their hands at the end of the period.
While students are examining the foamy Ivory Soap they are to start devising an understanding of what is occurring that produces the foamy structure and how it changed.
To complete this assignment each student must provide a detailed drawing of what is occurring to the Ivory Soap and write, at least, five sentences describing the drawing. I require that they use a minimum of five colors and labels with arrows to explain the drawing. The explanation must use scientifically accurate vocabulary. At the end of each sentence the students must number each sentence and highlight that number - this makes it easier to spot check for fluency. If they have achieved my requirements the assignment receives a 'stamp' (I go back and count stamps at the end of a chapter to assign points). If they have only done half the work the stamp is marked with 1/2 next to it. If they did such a bad job that they need to redo the assignment, a slash is placed over the stamp and they will receive zero credit. As I am spot checking the assignment, I provide immediate feedback on what they have to do to improve the assignment. The whole process takes a few minutes at the beginning of class and provides valuable insight that is both positive and meaningful to my students.
OUTCOME: The students should explain that the air molecules inside the Ivory Soap (exclusive to Ivory Soap) are being excited by the microwaves and they are moving about with greater energy. These excited air molecules are becoming warmer and melting the solid Ivory Soap into a liquid. The expanding gases (air molecules) are pushing outward with greater pressure causing the soap to expand into the foamy shape observed. When the microwaves are no longer acting on the air molecules the soap structure cools back into a solid.
To extend this lessons, students could determine the overall density (Density=mass/Volume) of a single bar of Ivory Soap by weighing the bar on a triple-beam balance scale (mass), then use a ruler to measure the length, width, and height (Volume=lwh).
To calculate density the students must divide the mass by the volume, using the label g/cm3. As long as the overall density is less than 1 g/cm3 (density of water) then the bar will float. (Practice 5 Using Mathematics and Computational Thinking.)