Which Soap Floats?

Ivory soap floats.

"The Nursery" by Alice Barber Stephe...

“The Nursery” by Alice Barber Stephens, 1898 Ivory Soap advertisement. (Photo credit: Wikipedia)

An accident at the Ivory soap manufacturing plant made consumers start contacting the company to get more of “the soap that floats.”

Without their realizing it, the accident had introduced air into the bars of soap, making them, suddenly, less dense than the bath water they were used in.

Now, the company whips air into the soap as part of the manufacturing process.

It is this air inside the bar of soap that makes it less dense than the surrounding water, so it floats.

Can You Do This with Any Bar of Soap?

Yes, if you heat a bar of soap in the microwave for a minute or two, the soap will froth, incorporating air into the edges of the bar.

It will then float.

Let’s try it.


  • One bar of Ivory soap
  • One bar of another soap (I used Dial)
  • Three bowls, including one glass bowl, suitable to be used in a microwave oven
  • Water
  • Measuring cup
  • Microwave


  • Put four cups of water in each of two bowls
  • Add one bar of soap to each bowl of water
  • The Ivory soap should float
  • The other soap should sink
  • Put the bar of soap that sank into an empty glass bowl
  • Put the glass bowl with soap in it into the microwave
  • Turn the microwave on high for one minute; take the bowl out of the microwave
  • Allow the soap in the bowl to rest for 3-5 minutes; it will be very hot
  • Put the microwaved bar of soap in a bowl of water.

What Should Happen?

The non-Ivory bar of soap should float.

In my case, only one end of the bar of soap floated.

I did two things:

1. I left the bars of soap to dry out overnight, to see if they would still float in the morning.

The bar of Ivory still did. The bar of Dial did not.

So, incorporating the air into the bar of Dial soap did not last.

2. Second, I put the bar of Dial back into the microwave, in a glass bowl, and put the microwave on High for two minutes.

The bar of Dial floated the same as the bar of Ivory.

Why Is This Happening?

Air is less dense than water.

What does that mean?

When you incorporate air into an object that would otherwise sink, you are changing the density of the object.

Density is the ratio of mass to volume.

So, we are changing the mass of the soap by substituting some of it for air when we use heat to make some of the soap foam.

Foam is just little films of soap with air bubbles inside.

We are stretching the soap around pockets of air, changing how much actual soap is in the same size place, and substituting it for air.

Just like boats or inner tubes float because of the air in the hull or inside the tire.

Thanks to sciencenetlinks.com for this activity.

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Carol Covin, Granny-Guru

Author, “Who Gets to Name Grandma? The Wisdom of Mothers and Grandmothers


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