The Ice Cube Insulation Experiment: Which Material Keeps Ice Frozen Longest?

Which keeps an ice cube frozen longest: a wool sock, aluminum foil, bubble wrap or nothing at all? This is one of the most popular elementary and middle school experiments, and for good reason. It’s cheap, it has a clear measurement, and it surprises people.

Here’s how to run it as a proper fair test, what to expect and how to explain the result.

What you need

Ice cubes, all from the same tray
Small zip-top bags or identical cups
Insulation materials to compare: a wool sock, newspaper, cotton balls, felt or fleece, bubble wrap, aluminum foil, cardboard
A small measuring cup with millilitre markings, or a kitchen scale
Tape and a timer

The method

Predict. Ask each student or team which material will keep the ice frozen longest, and why.
Set up. Put one ice cube in each bag. Wrap each in a different material, and leave one unwrapped as the control.
Keep it fair. Same size cubes, same amount of wrapping, same place, same start time.
Wait. Leave them for 30 to 60 minutes.
Measure. Unwrap each cube, pour off the meltwater and measure it in millilitres. Less meltwater means better insulation.
Repeat. Run it at least three times and compare.

If you use a design challenge, give each team a budget and let them build their own ice cube keeper. Our lesson Can you keep an ice cube frozen the longest? does exactly this, with a class leaderboard.

What usually happens

Materials that trap air, such as wool, cotton balls, bubble wrap and foam, tend to do well. Plain aluminum foil on its own often does badly, because metal conducts heat easily. Foil can help when it’s part of a layered design, reflecting radiant heat on the outside of an air-trapping layer. The unwrapped cube usually melts fastest, though not by as much as students predict, since the surroundings, not the cube, set the pace.

Results will vary with your room and your materials, so tell students that is expected. An honest “this time, wool won” is better science than a result everyone guesses.

Why it works: heat moves, cold doesn’t

The biggest mix-up in this experiment is the idea that wool “keeps the cold in.” There is no such thing as a flow of cold. Heat flows from warmer things to colder things. The ice melts because heat from the warm room flows into it. An insulator is a material that slows that flow.

Still air is a very poor conductor of heat. Wool, cotton, bubble wrap and foam are mostly tiny pockets of trapped air, which is why they insulate well. Metal’s loosely held electrons carry heat quickly, so metals feel cold to the touch and make poor insulators.

That’s also why a wool sock can keep an ice cube cold and a hand warm. It does not make heat. It slows heat leaving your hand, or entering the ice.

Variations

Drinks instead of ice. Wrap jars of chilled water, read a thermometer every five minutes and graph the temperature. See Which wrap keeps a drink coldest?
Hot instead of cold. Which wrap keeps hot water warm longest? The reasoning is the same, with the heat flowing out. See How do you keep a drink hot for the longest?
Change one design feature. Thickness, layers or a lid on the bag. Test one at a time.

Questions to ask afterwards

Which result surprised you?
If you could improve your design, what would you change, and why?
What does a cooler or a winter jacket have in common with your best design?

Keep going

These lessons come from the Hot and Cold and Keep It Cool units. Sign up for three free lessons, or browse by grade in the library.