Microwave Hot Spots With Marshmallows

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★☆☆ Can be easily done by most teenagers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Kitchen Chemistry, Electromagnetic Spectrum and Waves, Food Science and Nutrition

Alternative titles: Create a Marshmallow Map of Your Microwave, Microwave Wavelength Demonstration

Summary

Marshmallows are heated in a microwave with and without the rotating tray to reveal the uneven distribution of energy inside the oven. This demonstrates how microwaves create hot and cold spots due to their wave pattern.

Procedure

Butter two microwave-safe casserole dishes.
Place marshmallows in neat rows across the first dish.
Put the dish on the rotating tray inside the microwave.
Microwave on low power for about one minute.
Carefully remove the dish and observe which marshmallows softened or melted more.
Repeat with the second dish, but first remove the rotating tray so the dish stays still.
Microwave for one minute, then carefully remove the dish and check for melted spots.
Compare the results: note patterns of melted and unmelted marshmallows, and measure the distance between melted areas if desired.

Links

Marshmallow Test for Hot Spots - Nebraska Extension:

📄 Create a Marshmallow Map of Your Microwave Oven! - Science Buddies & Megan Arnett: https://www.scientificamerican.com/article/create-a-marshmallow-map-of-your-microwave-oven/

Variations

Try using chocolate chips instead of marshmallows.
Use a ruler to measure the wavelength distance between melted spots and calculate the speed of light.
Test different microwave power settings or times to see if the pattern changes.

Safety Precautions

Always use oven mitts when handling hot dishes.
Ask an adult for help with removing the rotating tray.
Take care not to burn yourself on hot marshmallows or containers.

Questions to Consider

Why do microwaves create hot and cold spots inside the oven? (Because microwaves form peaks and valleys in their wave patterns, concentrating energy in some spots and leaving others with less.)
How does the rotating tray help food cook more evenly? (It moves the food through different parts of the wave pattern so all areas receive heat.)
What would happen if microwaves had a shorter or longer wavelength? (The spacing between hot and cold spots would change, affecting heating patterns.)
Could you use this experiment to estimate the wavelength of microwaves? (Yes—by measuring the distance between melted spots and multiplying by two, since this represents half a wavelength.)