Soap to Foam

    Kids, what could make a piece of soap change to a ball of foam without using any liquid? Small pieces of Ivory™ soap, when microwaved, will expand into a foam that is more than six times their original size! It's a fun trick that won't hurt either your microwave or the soap. The causes are from closed-cell foam formation, physical change, and Charles' Law.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    To try it yourself you’ll need a bar of Ivory™ soap, a paper plate or microwave-safe dish, and a microwave oven. Unwrap a bar of Ivory™ soap and have an adult partner break it into small pieces. Place a small piece on a paper plate. Microwave your soap at normal power and watch to see what happens. Depending on microwave power, the soap will reach maximum volume within 30 seconds to 2 minutes depending on the size of the pieces. If you microwave the soap longer nothing bad will happen, but the soap won't continue to grow. Allow the soap to cool for two minutes before handling. It may 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.

    What’s happening? A foam is a material that traps a gas inside a cell-like structure. Examples include shaving cream, whipped cream, Styrofoam™, and even bone. Foams can be fluid or solid, squishy or rigid.

    Two processes occur here. First, heating the soap 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 gases push on the softened soap, causing it to expand and become a foam. Popping popcorn works in much the same way. 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.

    Other Things to Try. (a) Cut 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 actually see bubbles or pockets of air with your eyes but this is the reason why the soap trick works. (b) Try microwaving other brands of soap. (c) 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? 


    Kathleen Carrado Gregar, PhD, Argonne National Labs
    December 2008


    Reference:  Dr. Anne Marie Helmenstine at: