Key concepts
Physics
Heat
Temperature
Thermal insulator

Introduction
Have you ever thought about surprising your classmates with ice cream—but wondered how to get it to school before it melted? An insulated bag or a cooler filled with ice might keep your treat cool just long enough. Using the same principles, it is even possible to bake ice cream in a hot oven and have it come out still frozen! This activity will show you how.

Background
Heat is the energy that automatically flows from a higher to a lower temperature. This energy can flow in three ways: Sometimes it is carried by a movement of fluids (liquids or gases) known as convection. An example of this is when hot air in your oven flows to cooler areas. Other times radiation carries this energy. You might have experienced this when the grill in your oven glows red and you can feel it radiate heat. Direct contact is the third way this energy can flow from one place or object to another—you know this when you touch something hot and burn yourself!

We can prevent this energy flow by using thermal insulators, which are made of materials that hinder heat flow. An insulator placed in a cooler or insulated bag can keep heat out—so can a thermos.

To keep ice cream solid inside a hot oven, you need to protect it with thermal insulators. Luckily, some baked desserts are excellent insulators. Sponge cake and meringue are good examples. Can you find out why? Similarities between these baked goods and other thermal insulators such as cardboard cups, down feathers or layered clothing can lead you in the right direction!

Materials

• Two round bowls, preferably identical, that can be placed in the freezer
• Plastic film or plastic wrap
• Ice cream
• Spoon
• Freezer
• Sponge cake, about one inch larger in diameter than that of the bowl
• Three egg whites at room temperature
• One cup of white sugar
• Half teaspoon of cream of tartar
• Handheld electric mixer
• Mixing bowl
• Spatula
• Two cake pans, one at least the size of the sponge cake
• Oven
• Oven mitts
• Knife to cut the cake
• Adult helper (Some steps in this activity such as operating an electric mixer and using a hot oven require adult help.)

Preparation

• To create an ice cream mold, cover the inside of a round bowl with two layers of plastic film or plastic wrap and fold the excess over the rim. The plastic will make it easy to remove the ice cream from the bowl.
• Repeat with the second bowl.
• Fill the molds with ice cream. If your ice cream is too hard, leave it out for a few minutes and try again. Press the ice cream down so there are no air pockets left inside and level the top surface.
• Place the molds filled with ice cream in the freezer for at least one hour. This ice cream will be placed in a hot oven later in the activity.

Procedure

• Preheat the oven to 400 degrees Fahrenheit. Do you have any ideas on how you can place ice cream in an oven heated to 400 degrees F without it melting?
• Pour the egg whites into a mixing bowl and ask an adult to help you whip them with an electric mixer at medium speed. Watch how the translucent liquid egg white becomes a white foam. Did you see how the volume of the egg whites changed in the process? Why would this be?
• Add the cream of tartar, and ask the adult to keep whipping until soft peaks form when you lift up the beaters. The peaks will curl over, which is fine.
• Add a small amount of sugar, and ask an adult to whip it into the foam using the high-speed setting. Repeat this until all the sugar has been added and is dissolved into the egg whites. If the mixture looks shiny and peaks stay standing up when the beaters are taken out, you have done it right! This mixture is called meringue.
• Ask the adult to unplug the electric mixer and remove the beaters. You can wash the beaters with the other tools later.
• In a moment you will assemble the cake. Start this about 20 minutes before you plan to eat it. Check if your oven has preheated before you start assembling.
• To assemble the cake place your sponge cake in the middle of your cake pan.
• Take one ice cream–filled mold out of the freezer and use the plastic to take the ice cream out of the mold. Turn the ice cream over and place it, flat side down, in the middle of the sponge cake. Remove the plastic.
• Use a spatula to cover the ice cream and the visible parts of the sponge cake with a thick layer of meringue.
• Leave it in for about 10 minutes, until the outermost layer of meringue turns brown.
• What do you expect to happen to frozen ice cream when it is placed in a hot oven?
• To test your prediction, take the second ice cream–filled mold out of the freezer, remove the mold and turn the ice cream over onto a second cake pan. Remove the plastic and ask an adult to place this pan in the oven. Observe what happens.
• When the cake is done, ask an adult to take it out of the oven. You can leave the pan with just ice cream in a little longer because it was put in later. Turn the oven off.
• Cut a quarter of your cake and look inside. Is the ice cream still cold and hard or did it melt into a liquid? How is it different compared with the ice cream still in the hot oven? What do you think kept the ice cream in the center of the cake cold? Why did it keep the ice cream cold?
• Extra: Do you think you can make small ice cream–filled cakes with cookies on the bottom? What type of cookie would you use? Try it out and see if it works!
• Extra: Why would the recipe call for baked cake as bottom layer? Would putting the ice cream on batter and baking the batter and the meringue all at the same time work as well? If you feel unsure, try it out!
• Extra: Use items you find around the house to make an insulator that can keep a cup of ice cream cold for 20 minutes. To measure the performance of your creation, take two identical ice cubes and place one inside and one outside the insulator. Wait 10 or 20 minutes and then compare how much the ice cubes weigh—or how much melted water they provide. The more your ice cube weighs compared with the one that was not insulated, or the less water it provides, the better your creation works!

Observations and results
Was the ice cream in the center of the cake still hard and frozen? Did the uncovered ice cream melt quickly when placed in a hot oven?

Sponge cake and the meringue both have plenty of air bubbles trapped in them. This layer of stationary air keeps the heat out. It is a good thermal insulator, and as such it does not easily let heat flow through it. In this way it protects the cold ice cream underneath—at least for some time. If you left the cake in the hot oven for too long, however, the ice cream center would eventually warm up and melt. As you probably observed, without the protection of sponge cake and meringue this happens way faster.

Insulated bags or coolers often use a stationary layer of air to keep heat from flowing through the walls, much like the cake and meringue!

Cleanup
Clean your work area and wash all the tools and containers with soapy water.

More to explore
Heat Transfer—for Kids, from Real World Physics Problems
How Do Arctic Animals Stay Warm?, by Scientific American
Stay Warm with Thermal Insulation, by Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies