Exploring density and Charles’ Law Using Ivory Soap

Ivory soap is has some interesting characteristics that we can discover through some simple experiments.

When I get the chance, I’ll figure out how to upload a downloadable data recording sheet to go along with this experiment.

Experiment 1:  Does soap sink or float?


  • bar of Ivory Soap
  • 2-3 different brands of soap, maybe even one that is a smallish sliver of soap.
  • large bowl or tub
  • water
  • paper towels


Step 1.  Fill bowl or tub with tap water.

Step 2:  If you have a kitchen scale, weigh each bar of soap. Record the results.

When we weighed our soaps, these were our results

Ivory (whole bar) – 4.5 oz.
Dial (whole bar) – 3.75 oz
Dial (sliver) – 0.75 oz
Dove (whole bar) – 2.25 oz

Step 3.  Predict whether bars will float or sink

Step 4.  Carefully, one at a time, add bars to water.


Step 5.  Record observations
What happens to the bar of Ivory Soap? It floats
What happens to the other brands of soap? They don’t float


Brand 1 Brand 2 Brand 3 (large) Brand 4 (small)
Brand soap Ivory Dove Dial Dial
Prediction Float Sink Sink Float
Actual Float Sink Sink Sink
Weight  (oz) 4 ½ 3 ¾ 2 ¼ ¾

Interestingly enough, the bar of Ivory soap was actually the HEAVIEST bar of soap, and yet it was the only one that floated.

Reason:  In order for something to float in water, it must mean that it is less dense than water. Density is NOT the same as weight. Density has to do with how tightly packed the molecules are. In the soaps that sink, the soap molecules are densely packed. In the Ivory soap that floats, the soap molecules are less tightly packed.

So what is taking up the space if not soap molecules?

It turns out, in the manufacturing process, air is pumped into the soap as it is being made.    It effect, the soap is whipped like whip cream before it is formed into bars.  It’s the only bar on the market that floats.

In order for us to test this effect, we actually did another experiment in conjunction with the soap.

We actually WHIPPED cream to really see how if we could test this out.

Experiment 2: Making a solid less dense than a liquid


  • Heavy whipping cream
  • bowl
  • whisk or electric beater
  • clear glass cup or bowl
  • water
  • small container for transferring (optional)


Step 1. Pour a small amount of un-whipped whipping cream into a clear bowl or cup containing tap water.


Step 2. Observe what happens


After a pretty swirling pattern occurs, the cream will eventually spread out to fill the container.  (Eventually, if you wait long enough, the cream might rise to the top, but I’m not sure because we dumped it out before I thought to check that out, but for all intents and purposes, we deemed the cream to sink).


Step 4. Take the remainder of the heavy cream and whip a very, long time.  Observe the air bubbles being created by the process of whipping.
Be careful not to overwhip or you’ll make butter. If you overwhip the butter, you’ll have a mess.


Step 5. Take a dollop of whip cream and put it into a fresh cup or bowl of water and observe.

What happens?

The cream floats on top because we whipped air into the cream, making it less dense (there are less cream molecules because the air molecules, which are less dense than water, take up some of the spaces, giving the cream buoyancy).


(Optional step 6:  Use the leftover whip cream for punkin’, apple or pecan pie.

Add cinnamon and honey to the rest of the whip cream and whip a little longer. You just made and make cinnamon honey butter)

Step 7: Now, break open the bar of soap.

Do you see the air pockets inside?


So how can we tell this bar of soap is whipped?

Well, you can HEAT it up and see what happens.

Experiment 3:  What happens when you heat Ivory soap in the microwave?

Caution:  Kids, do NOT use a microwave without adult supervision.


  • 1 bar of ivory soap
  • glass plate
  • Microwave oven


Step 1:  Place one bar of soap (or a cut up bar of soap) onto one glass plate.

Step 2:  Set the microwave for 2 minutes.   (note, stop it before the soap spills over the plate).

Step 3: Watch the magic happen


So what is happening?

When the soap is heated, the molecules of air in the soap move faster causing them to move far away from each other. This causes the soap to blow up like a balloon. Charles’ Law states that as the temperature of a gas increases so does its volume. Other brands of soap without whipped air tend to heat up and melt in the microwave.

For more information regarding Charles’ Law, click HERE.

For more information regarding density, click here to go to KidsGeo.com.
So, what can you do with the leftover Ivory soap after you blew it up?

5 Orange potatoes has a great idea for making soap sculptures and making soap balls.

For a free printable method and recording sheet, click on the download or print button on this image.

This entry was posted in Density, Physics, Properties of matter. Bookmark the permalink.

4 Responses to Exploring density and Charles’ Law Using Ivory Soap

  1. Natalie says:

    Wow – this is awesome! I have never heard that Ivory soap is different than other soaps. I just learned something new. Question: does microwave gets all splattered with soap and does it smell afterwards?

  2. theexplorationstation says:

    Oh, that reminds me to put a note in the directions. No, the soap doesn’t make a mess in the microwave, IF you either get a very large plate or if you stop it before it flows over your plate.

    And no there is no lasting soap smell in the microwave.

    What makes more of a mess is if you let the kids play with the soap puff afterward – it will crumble and get flaky, so you’ll want to cover your table. And make sure your daughter doesn’t rub her eyes while playing with the soap.

  3. jenn says:

    Great job. I love it.

  4. theexplorationstation says:

    Thanks Jenn!

    It took me a few hours to work on this last night, but I think it turned out well.

    I’ll be adding more printable method/results sheets for the other experiments.

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