Key Concepts
Physics
Heat
Temperature
Insulation

Introduction
How does a cooler keep things cold? Which material makes the best insulation? Try this project to find out how long you can keep an ice cube from melting once it's out of the freezer!

Background
Have you used an insulated lunch box or bag to pack lunch for school or a cooler to pack food for a picnic? Why not just throw the food in a paper bag or plain box? What exactly does the cooler do that's so special? You'll find out in this project—but first let's learn a little bit about heat transfer.

Heat naturally flows from hot to cold. You can observe this by watching (and feeling) a hot drink. A cup of hot chocolate will eventually cool down and reach room temperature. In this case heat flows from the hot drink to the cooler environment. A cup of cold water will also eventually reach room temperature. In this case heat flows from the warmer environment to the cooler cup.

Heat is the vibrations and collisions between the molecules in a substance. There are three different ways heat can move between objects: conduction, convection and radiation. Conduction occurs when objects are in direct contact with one another. You feel conduction when you touch, for example, an ice cube or a hot mug. Convection occurs when a fluid (such as air or water) flows over an object. Convection can be natural (for instance, a hot object, such as a pan on the stove, will warm the air around it, causing this warmer air to rise) or forced (such as by a fan in an air conditioner). You feel convection when you stand outside in a chilly winter wind and feel cold. Finally, heat can be transferred by electromagnetic radiation. Most notably, you can feel this when you stand outside in the sun. You can feel the warmth of the sun on your skin—even though the sun is millions of miles away with the vacuum of space between you and it (so there is no matter that allows for conduction or convection)!

That's a lot of different ways for heat to flow from one place to another. That makes keeping food cold (or hot) a challenge. How do you stop heat transfer if it's happening naturally all the time? Or how can you at least slow it down so your food doesn't spoil? The key is insulation! Thermal insulators are materials that help reduce the rate of heat transfer. Some materials are poor conductors of heat so they make good insulators to slow down heat conduction. Other materials can block radiation in visible or infrared light. In this project you will build your own cooler and explore how insulation can help keep an ice cube from melting (for a while, at least).

Materials

• Two equally sized plastic food storage containers (one will need to have a tight-fitting lid).
• Ice cubes (at least two of the same size)
• Two plastic zip-top bags or matching smaller food storage containers/cups that fit inside the larger ones
• A work surface where both of your containers will have equal exposure to external sources of heat (such as sunlight or a vent). (To speed up the activity put your containers in the sun and/or take them outside on a warm day.)
• An insulating material such as cotton balls, bubble wrap, crumpled paper, and so on (more than one is optional)—enough to mostly fill one of your large food containers

Preparation

• Mostly fill the inside of one of your plastic containers with insulating material. This is your "cooler."
• Place a zip-top bag or smaller plastic container inside each of the larger containers (make sure that the lid on the insulated container will be able to close all of the way; remove any insulation needed until the lid can close; leave it open for now).

Procedure

• Place one ice cube in each of your smaller containers or bags.
• Quickly close the lid on the "insulated" cooler container. (Do not put a lid on the other larger container.) Which ice cube do you think will last the longest?
• If your workspace isn't very warm, the ice cubes might take a while to melt. Go do something else for 15 minutes, and then come back. How big is the ice cube in the open container now?
• Depending on how fast it is melting, continue to check the ice cube periodically. Keep checking until it has melted completely.
• As soon as that ice cube finishes melting, open your cooler and check on that ice cube. Has it also fully melted? If not, how big is it?
• Extra: Try making multiple coolers with different insulating materials—or even combine layers of multiple insulating materials inside a single cooler. Which material(s) keep the ice cube from melting the longest? Why do you think that is?
• Extra: Here's something we haven't considered yet in this project: room! It would probably be nice if you had room for food and drinks in your cooler—instead of filling it mostly with insulation. Try the activity again, but tightly pack the insulation against the walls to leave more room in the container. Does this change how quickly the ice cube melts? Can you optimize this design with different materials?

Observations and Results
When the ice cube in your open container finally melted, you might have discovered that the ice cube in the insulated container was still almost half its original size! Putting the ice cube in a sealed, insulated container dramatically reduces the amount of heat it can gain by conduction (because the insulating material is not a good conductor of heat) and convection (because the lid is sealed extra air cannot flow around the ice cube). If your insulation is opaque, it also reduces the amount of heat transferred by radiation—although this is a much bigger factor outside in the sun than if you're doing the activity indoors.

Think about what you learned in this activity and how it relates to real coolers. For example, have you ever seen a transparent cooler? Why would that be less desirable? If you want to keep something cold, is it better to leave the cooler lid open or closed? If you need to keep something cold for a long time, would you use a lightweight, thin-walled cooler or a cooler with thicker walls? How does what you learned in this activity impact how you might pack your lunch or a picnic?

More to Explore
Energy Transfer, from Physics4Kids
Stay Warm with Thermal Insulation, from Scientific American
How Animals Stay Warm with Blubber, from Scientific American
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies