Ivory Soap in the Microwave - Heat Makes Foam

Person holding soap foam on a paper plate.
This soap sculpture resulted from a small piece of Ivory soap. Anne Helmenstine

If you unwrap a bar of Ivory™ soap and microwave it, the soap will expand into a foam that is more than six times the size of the original bar. It's a fun trick that won't hurt either your microwave or the soap. This soap trick can be used to demonstrate closed-cell foam formation, physical change, and Charles' Law.

Soap Trick Materials

  • Bar of Ivory™ soap
  • Paper towel or microwave-safe dish
  • Microwave oven
  • Other brands of soap for comparison (optional)

Perform the Soap Trick

  1. Unwrap a bar of Ivory™ soap.
  2. Place the bar of soap on a paper towel or microwave-safe dish.
  3. Microwave your soap. Watch the soap closely to see what happens.
  4. Depending on microwave power, your soap will reach its maximum volume within 90 seconds to 2 minutes. If you microwave the soap longer (we went up to 6 minutes) nothing bad will happen, but the soap won't continue to grow.
  5. Allow the soap to cool for a minute or two before touching it.
  6. The soap will feel brittle and flaky, but it's still soap, with the same cleaning power as before. Go ahead and get it wet and you'll see it lathers the same as ever.

About Foams

A foam is any material that traps a gas inside a cell-like structure. Examples of foams include shaving cream, whipped cream, Styrofoam™, and even bone. Foams can be fluid or solid, squishy or rigid. Many foams are polymers, but the type of molecule isn't what defines whether or not something is a foam.

How the Soap Trick Works

Two processes occur when you microwave the soap. First, you are heating the soap, which softens it. Second, you are heating the air and water trapped inside the soap, causing the water to vaporize and the air to expand. The expanding gasses push on the softened soap, causing it to expand and become a foam. Popping popcorn works in much the same way.

When you microwave Ivory™, the appearance of the soap is changed, but no chemical reaction occurs. This is an example of a physical change. It also demonstrates Charles' Law, which states the volume of a gas increases with its temperature. The microwaves impart energy into the soap, water, and air molecules, causing them to move faster and further away from each other. The result is that the soap puffs up. Other brands of soap don't contain as much whipped air and simply melt in the microwave.

Things to Try

  • Place a bar of Ivory™ in a bowl of water. Does it float? Try this with other brands of soap. Do they float or sink?
  • Cut or break off a piece of Ivory™ and examine it. Do you see pockets of air? The air that causes Ivory™ to be less dense than water has been whipped into the soap, so you won't see bubbles or pockets of air. This is important because it's the reason why the soap trick works.
  • Try microwaving other brands of soap.

Soap Trick Safety

  • Don't leave the microwave unattended while microwaving soap.
  • Don't place metal in the microwave.
  • Be aware that while microwaving soap will not harm either your microwave or the soap, it will cause your microwave to smell floral and pretty for several hours.
  • Wash your hands after playing with soap so that you don't accidentally eat it (though it's not toxic) or get it in your eyes (which would burn).