soapWhen you warm up leftovers, have you ever wondered why the microwave heats the food and not the plate? (Well, some plates, anyway.) It has to do with the way microwaves work.

Microwaves generate high energy electromagnetic waves that when aimed at water molecules, makes these molecules get super-excited and start bouncing around a lot.

We see this happen when we heat water in a pot on the stove. When you add energy to the pot (by turning on the stove), the water molecules start vibrating and moving around faster and faster the more heat you add. Eventually, when the pot of water boils, the top layer of molecules are so excited they vibrate free and float up as steam.

When you add more energy to the water molecule, either by using your stove top or your nearest microwave,  you cause those water molecules to vibrate faster. We detect these faster vibrations by measuring an increase in the temperature of the water molecules (or in the food containing water). Which is why it’s dangerous to heat anything not containing water in your microwave, as there’s nowhere for that energy to go, since the electromagnetic radiation is tuned to excite water molecules.

To explain this to younger kids (who might confuse radio waves with sounds waves) you might try this:

There’s light everywhere, some of which you can see (like rainbows) and others that you can’t see (like the infrared beam coming from your TV remote, or the UV rays from the sun that give you a sunburn). The microwave shoots invisible light beams at your food that are tuned to heat up the water molecule.

The microwave radiation emitted by the microwave oven can also excite other polarized molecules in addition to the water molecule, which is why some plates also get hot. The soap in this experiment below will show you how a bar of Ivory soap contains air, and that air contains water vapor which will get heated by the microwave radiation and expand. Are you ready?

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182 Responses to “Microwaving Soap”

  1. honeymilkforever says:

    Sweet!!! Can’t wait to try this tomorrow. We have a bunch of this soap for soap carving. 🙂 Thank you for this experiment.

  2. Ivory is a brand of soap made by Proctor & Gamble. This soap is manufactured in a way that creates lots of air bubbles inside. In fact, there used be commercials promoting the fact that it floats! If you can find another brand of soap that floats (indicating that there are lots of air bubbles inside), you could give it a try.

  3. shehla_baig says:

    what is an ivory soap? Not sure if will be able to find it here in my country. Is there an alternative one.

  4. Yes, it is best to use Ivory soap because it has a lot of air whipped into it at the factory.

  5. monicabohndb says:

    does it have to be ivory soap?

  6. Hmm, something doesn’t sound right with your account – I’ll have Tonya connect with you and make sure everything is set up correctly.

  7. rlipford1021 says:

    I can’t seem to find this experiment. I purchased the Ultimate Science Curriculum last week and a lot of the links end up with a message about not having access. I thought I was purchasing access to 5 years of science and all content. ??? Confused….

  8. lissoneal says:

    Hi ! How can I watch the soap video?

  9. Anna Boussaidi says:

    its a little hot and the insides of the soap just open up.

  10. Ivan Kusakovic says:

    Now there’s more soap 😀

  11. oops I meant Mrs. Aurora

  12. thanks Aurora ;D

  13. Oh no, Tyler! At least it smells “kinda good?” 🙂

    We recommend soap that floats on water because that has a lot of air trapped inside. The air is important so that the water vapor molecules will have lots of space to expand once the microwaves get them moving.

    However, it really sounds like an unscented or lightly scented bar would be best as well. And LOTS of ventilation!

  14. so we use Kirkland soap from Costco and my mom says it’ll make our trash can smell nice :D…..30 seconds later our entire house smells like Kirkland soap 😮 not recommended (cough, cough) smells kinda good and kinda bad

  15. do we have to us ivory soap and how much less the fluffiness of irish soap cause that’s all we have that might be close to ivory soap… right ;D I definitely DO NOT want burnt soap smell like the other person did 😮

  16. David Harn says:

    We didn’t have any Dove soap around, so we tried it with Irish Spring. It doesn’t come out near as “fluffy”, but it works, although it produces a less than desirable aroma. (Could be the perfume or the dye. I’m not sure which.) My eight year old daughter complained that she couldn’t see the action through the shielding in the door glass, and it was rather hard for me to see, as well, but the result was clearly in line with what we expected. We’ll get a bar of Dove and see if the results are more to her liking. (More fluffy and less bad smell.) Thanks for the great experiments. My kids are in love with the program and every project we’ve done so far.

  17. Meg Kupsh says:

    We didn’t have Ivory soap so we used Zest soap, not recommended. Our entire house smells Zestfully “Burnt” Clean. We still loved the experiment though. Thanks!

  18. Hi Jazz,

    Great idea trying the different soaps! I am guessing some soaps had more water content than others… the puffed ones had more than the melted ones. Color isn’t usually a factor in experiments like this. Enjoy!

  19. Student Science says:

    Hi Aurora

    Hello all the way from London England. We have done the experiment but because we are from England we could not get Ivory soap. so we decided to base our experiment on which type of soap works the best we used, Dove, Sainsbury Basic and Pears. Sainsburys worked the best it just went in to a big lump of puff it was not as big as Ivory but it was still big and we think we put it in a bit too long but it was still very cool. The Dove was looking good but when we took it out it just deflated and still had the lumpy puffy kind of texture. However, the Pears soap just melted but Pears was seethrough and was not white it was orange. Do you think the colour and the fact that it was seethrough changed the results? what do think Aurora.

    from Jazz (aged 10)

  20. Sherry Quackenbush says:


  21. Possibly, depending on how large the soap got (did it touch the sides of the microwave?) and how long the soap was in there for. 🙂

  22. Carla Bettles says:

    After the experiment will food cooked in the microwave taste soapy?

  23. Thanks for the tip – and yes, the water vapor in the air had time to escape, making it burn. Definitely do this experiment with a new bar… and it sounds like you really learned something!

  24. Ronald Hamilton says:

    Just telling you, we tried some really old soap and it didn’t work out too well. It started to burn before it blew up into a cloud at all. It should be newer soap so that the experiment will work right! When we got some new soap, it worked the right way. My theory is that the air escaped from the soap over time. Do you think that’s what it might be?

  25. Michelle Stevens says:

    Me and my sister used food dye and it looked great!

  26. Michelle Stevens says:

    Mine filled up the whole microwave!!!

  27. Physical – the water (gas) in the bar of soap is expanding. There’s no chemical reaction going on in side the soap to make it expand. Good question!

  28. Margaret Bish says:

    Hi Aurora,

    Is this a physical or chemical change?


  29. We got ours from a thrift/second-hand store. 🙂

  30. Laina Wilburn says:

    Hi Aurora where can I get a cheap microwave and where can I get some Ivory soap

    Thanks Chloe:)

  31. Amanda McKee says:

    We wanted to tell you that this experiment worked for us! My daughter measured the final size of the soap- it expanded to 10 inches! They were very excited to complete this experiment!
    Amanda McKee, mom of Ellie, age 7 and Ethan, age 5

  32. You need a bar of soap that floats in water. It needs to have a good amount of air mixed in side in order for the water vapor molecules to expand.

  33. Amanda McKee says:

    Do you have to use ivory soap? Ellie age 7

  34. Angela Hillier says:

    We just finished up our science experiments with Ivory soap. Wow! We had fun! We tried many different variations and even re-wetted the soap, formed it back into a bar and re-heated it. My son was very excited about how much larger it puffed up the second time around. Never had this much fun doing science. Thanks!

  35. What web browser (and version) are you using? And are you logged in to the program?

  36. Caroline Pestka says:

    I can’t seem to click on any of the student sheets or worksheets on any experiment. Help!

  37. Give it a try! Most microwaves will work. 🙂

  38. Andrea Stenger says:

    It was so awesome please mail me some more videos cause it was amazing

  39. Andrea Stenger says:

    My type of microwave is whirlpool will it work

  40. I am so sorry you are having trouble! I’ll have my team connect with you right away.

  41. Lucrecia Choto says:

    Although I was sent an email to access this experiment, the page states that, “Sorry, but it looks like you don`t have access to the rest of this content. Each month, you`ll be given access to the next 1-2 units, so you will probably have access in the future.” Can you please tell me how to access this experiment?


  42. Great job observing what worked or didn’t work! 🙂 Keep it up – that’s exactly what a real scientist would do. Let me know how it goes!

  43. Sarah Glidewell says:

    Our experiment didn’t work as mapped out, but we figured that was because our bars of Ivory soap were found in the depths of a cabinet (ie: old soap). We moved from a humid climate to a very dry climate, and figure that our bars were dried out. The soap did crack and expand a bit in the middle, showing us that whatever moisture was left inside was just that: inside. The rest of the bar looked the same. We actually didn’t mind this outcome, as it gave my 12yo son and I a chance to talk about what might have skewed our result.

    We will try it again with new bars of soap and report back!

  44. You’ll want to look over the notes and comments to see what others have experienced with this experiment. For the soap experiment, it’s usually okay, but the plasma grape experiment has more of a possibility of damaging the microwave.

  45. Andrea Stenger says:

    Does it damage the microwave?

  46. Chikako Wade says:

    Can we use the soap in the shower after it has expanded?

  47. I am constantly adding new experiments! There are a dozen or so new ones in Lasers (Unit 9), and there’s a new set under “Advanced Physics” for HS students. More coming in e-Camp next week also! Keep an eye out for it!!

  48. EJ & Anika Byers says:

    When are you going to make new videos for new science experiments? I really liked the microwave soap experiment! It was so awesome and I want to do it over and over again! I also liked the airplane one, too. We made a game to throw the airplane and try to get to the end of our sidewalk and whoever does, wins. It’s awesome! I feel great that I get to use this program. I just want to do all the experiments right now!

    Anika 🙂
    (age 6)

  49. Suzanne Stafford says:

    “I thought this was neat because it was puffing up.” 8 year old.

    Mom perspective: I do suggest trying bowl to contain soap as ours went all over but I forgot to take out the rotating plate. Just had to clean the microwave. It was interesting to see the volume difference when we tried different portions of soap (1 bar, 1/2, 1/4 and 1/8). Will have to weigh and measure the resulting foam.

  50. Yes because you are basically short-circuiting the microwave, you should not try this with a new one. It’s not a power thing… just a “I don’t want to break my new microwave” thing. 🙂

  51. Herb Richards says:

    Why do you add to the instructions, not to use a new or expensive microwave? (We just remodeled and got rid of our old microwave that looks like the one you use in the video.) Does it harm the microwave, or is it that the newer microwaves are too strong for the experiment?

    Thanks in advance for the insight.

  52. You are probably right, an older bar of soap will have less water vapor in the air pockets, which is why the bar expands in the microwave. When you get another bar, get more than one… your kids will love it!

  53. Melanie Hovancik says:

    We tried this today with an older bar of Ivory soap, it was probably a few years old. The soap did not expand the way it did in the video. It kind of cracked into 4 pieces. I am wondering if this could be because the bar of soap was old.

  54. Yes, however most of the ones you’ll find on an internet search don’t tell you about the toxic fumes they give off, or how they can kill your microwave. Stick to the grape, soap, and cherry tomatoes and you’ll be fine. 🙂

  55. Kerry Marsh says:

    We figured out that marshmallows expand in the microwave for much the same reason (although they shrink back down after the microwave shuts off). Is there anything else that we can try to blow up in the microwave? Thanks!

  56. You tell me! 🙂 (Hint – look at some of the other comments here…)

  57. Anonymous says:

    It’s awesome how it does that

    How does it do that


  58. timothy boyce says:

    we do this when i need to make new laundry soap. it crumbles so much that i don’t need to shred the bar of soap. saves on time and the kids are learning. double bonus for us.

  59. Anonymous says:

    to cool

  60. We did the Ivory soap experiment … and my kids were so excited they did it again. They didn’t want to waste the soap so we put it in a pretty jar by the bathroom sink. It looks like cotton.


    We have also made the paper airplanes which turned out great, we keep them in a bag & take them to the park. They are the best ones we have ever made. Thank you!

  61. Janet Cotner says:

    “This experiment ROCKS.” -8yo
    “I definitely want to do THIS program – it’s awesome!” -11yo, evaluating the trial period
    Thanks so much. -me

  62. Heidi Robinson says:

    We had so much fun when we did this that we wanted to do it repeatedly without wasting too much soap. So, we decided to try and condense the microwaved soap back down (by adding water and squishing it together) and microwave it again. It expanded again in the microwave but when we opened the microwave door, it shrank to a little messy soap puddle. So our question is: “Why does it shrink when we open the microwave door?”

  63. ou can use it just like the pre-microwaved soap… wouldn’t it be fun to stick it in the shower before an unsuspecting grown up got in?

  64. Ashanta Ambush says:

    cool what can i use the soap for

  65. If you’re having account trouble, contact us through the “Contact” tab and we’ll be able to help you out quickly. 🙂

  66. Becky Brooks says:

    hi, can we get this experiment now? thanks!

  67. Bonnie Cusatis says:

    My 7 year old son loved this!! Mom & Dad both think it is pretty cool too. It looks like growing crystals or cotton candy. He can hardly wait to take a bath tonight to try our the blob!! Thanks

  68. Terri Cleary says:

    Before microwaving the soap, we put a bar of Ivory and another brand into a sink partially filled with water. We observed the Ivory floating, and the other soap sank. From earlier lessons we realized that Ivory is less dense. Now we know that is because of the high amount of air trapped inside.

    After microwaving the soap, we repeated the experiment with the other bar for comparison. The more dense soap still had some expansion meaning some trapped water molecules, but not as impressive as the Ivory.

    “It was totally huge!” – Xander (10)

  69. Lisa West says:

    The boys loved this experiment. We also microwaved some marshmellows and had them predict what they thought would happen and why. They noticed that the house began to smell like ivory all over and it opened up a great discussion about molecules and diffusion. Thanks again.

  70. We have enjoyed science so much! We have been in the midst of a move this month (we moved out of our sold home but our new home isn’t finished being built yet), and science has been like a break to us. I allow the children to choose the experiments and then we just do them. I’ve attached some pictures of the children gathered around the microwave to watch the ivory soap experiment. We actually did this experiment at a friend’s house (since all of our belongings are in storage & we don’t have access to a microwave) and their children were so interested, they joined in our science lesson.Sorry about the picture quality, I took this picture with my phone. I want you to know that these pictures are NOT staged! They were really as excited as they look in the pictures.

    I also wanted to thank you for such a wonderful program. Last year my children groaned when it was time for science. We have only been escience members a few weeks, and the attitude toward science has improved. Thank

    null null

  71. Gennifer Hogan says:

    We are going to put this in a freezer to see what happens next

  72. Ruthann Carlson-Smith says:

    cool! =D

  73. It’s difficult to video a microwave in operation, especially with the shielding in place. Hopefully you get the idea of how it works so you can do this yourself! 🙂


  74. Pamela Francisco says:

    The video was black during the microwave portion….the most interesting part!

  75. James Hale says:

    That’s awesome!! Use a clear glass bowl, like pyrex, to contain it!! I can probably convince my boys to bathe with this tonight, lol.

  76. James Hale says:

    Sara, the video is also REALLY dark, we could see a couple of seconds of it, but could mainly hear it.

  77. Unfortunately, yes. I recommend bringing a couple bars of soap to a friend’s house… 🙂

  78. Pamela Church says:

    does it have to be a microwave to do this? we do not own one. thanks

  79. The videos are in flash, so you need a regular computer (not a cell phone or iPad) to watch the videos. You’ll want to install a flash player if you don’t already have one installed – search for one on the internet that is compatible with your system.

  80. sara harper says:

    Hi, Aurora,
    We just signed up for e-science and cannot see/view the videos–just a blank space where the video should be. We have a mac computer so I’m guessing we need a plug-in of some kind? Appreciate any advice! Thanks!