Marshmallow Smashies

Age

Elementary School, Middle School, High School

Format

Stage Show, Hands-on

Materials

   Büchner flask (vacuum flask)
   Marshmallows
   Vacuum pump
   Rubber racquet ball
   Partially inflated balloon 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.

Nothing really dangerous about this demo, just don’t let kids eat the marshmallows

Preparation

Make sure you have a balloon that is partially inflated and that the pump is working. It tends to leak once it’s turned off so keep this in mind.

Demonstration

This demo demonstrates how pressure pushes in and out on things. The first part of the demo involves blowing up a balloon by removing the air around it. Place the partially inflated balloon in the flask and (with the rubber ball over the top) vacuum the air out of the flask and watch as the balloon inflates. Be careful not to let the balloon cover the hole that the air is being sucked through as the balloon can burst there. Let the air back in to bring the balloon back to its normal state. The second part of the demo involves marshmallows instead of a balloon. Place a few marshmallows in the flask and repeat. You can let the kids draw faces and what have you on the marshmallows if the demo is hands-on as opposed to being part of a stage show. The marshmallows will inflate just as the balloon did. Once they stop inflating, let the air back in and watch as they shrink up into little shriveled blobs. Try to vacuum the air out again after they’ve shrunk to show that they stay the same size.

What to Say

Most kids have blown a balloon up or at least seen it done. Ask them if they know why the balloon gets bigger. If no one can answer, explain that when you put air into the balloon you’re increasing the pressure inside and it expands.

Now might be a good time to introduce air pressure into the equation. The air pressure outside of the balloon is pushing in on the balloon while the pressure of the air inside is pushing out. The pressures will try to balance each other out. This is why the balloon expands when the pressure inside increases.

Ask if there would be any other way to make the balloon expand. If no one comes up with the idea to remove the pressure outside, perhaps hint at it by asking if you could simply take away the air around the balloon.

Place the balloon in the flask and suck the air out of the flask. Before actually doing this, it might be a good idea to explain what the pump will be doing (sucking the air out of the flask) so they know what’s actually going on. Be sure to have the audience observe what happens to the balloon.

Once the balloon has increased its size significantly, shut off the pump. See if the audience can figure out what will happen to the balloon if the air were to be let back into the flask. Let the air back in and see if they can explain what happened. If they’re unable, try to help them along. When the air was let back in, it just pushed back on the balloon until the pressures were balanced again.

Now for the marshmallows. Marshmallows are made of mainly sugar and air. Marshmallows are really just dozens of tiny little balloons just like the one that was just put in the flask. It might be a good idea to have a marshmallow that’s ripped in half to show the bubbles inside. If the air is removed again, shouldn’t the same thing happen to them? Put the marshmallows in the flask and repeat the process mentioned above. Behold! The marshmallows expand just like the balloon!

Make sure the audience pays close attention to what happens. At a certain point, the marshmallows stop expanding and seem to shrink slightly. What happened was the balloons inside the marshmallow burst after being blown much too large. See if they can figure out what happened on their own, but remember, the marshmallows will now begin shrinking over time so don’t hesitate too long for the finale! Hinting at what happened by asking what happens to a balloon if you blow it up too big usually works well.

And now the finale! See if the audience thinks the marshmallows will go back to their original size , stay the same or something else! Remove the rubber ball to let the air back in and wait for surprised gasps.

It’s likely no one will know exactly what happened to the marshmallows. What happens is the little balloons get too big and burst, allowing the air to escape. Once the air is let back in, the stretchy marshmallow wants to shrink up and the air pressure wants to push the remaining bubbles back to their normal size. The bubbles that burst now have no air to stop them from shrinking and the marshmallow shrivels up into a little blob.

Try putting them back in and see if anyone can guess what will happen. As you may have guessed, they stay the same size (they may grow very slightly, but nothing like they did originally).

Ask for any questions and do your best to answer them. If you aren’t sure about a question, don’t be afraid to say you don’t know or to ask someone else!

Real Life Examples

Blowing up balloons