Why did the cotton ball insulated cup freeze the slowest?

[TooManyHours]

My son in grade 5 had to do an experiment to make insulated cups using different materials. He made one using insulated Styrofoam pieces from ripped up cups and one using cotton balls. He packed Styrofoam pieces into a large cup and placed a smaller cup inside the larger cup and packed more Styrofoam between the small cup and the large cup. He did the same with 2 other cups and cotton balls.

He then poured equal amounts of water into each cup and placed them in the freezer and timed how long it took for the cups to completely freeze.

The one that seemed to work the best was the cotton ball cup (i.e. it took the longest to freeze).

But, he has to explain why? Is it because it is easier to pack cotton balls than Styrofoam (i.e. less air gets in around the outside edge of the Styrofoam cup)?

 

[Severian596]

You know, I'm going to go out on a limb and provide some feedback based largely on my intuition. If anyone has a more professional or experimental reply to debunk mine please feel free to naysay me, I won't mind.

My assumption: Stable (non moving) air is a better insulator than matter, and a vacuum is a better insulator than matter OR gas.

If we assume this is true, then I think the better of the two insulators (styrofoam or cotton) would provide more stable air around the liquid with less matter. That is, the less dense of the two solids would be the better insulator. I believe the cotton is less dense than styrofoam because of its nature (thin strands of material all knotted together, as opposed to a bubble-based solid that has many more tightly-packed molecules), and therefore provides better insulation because it "packs" more stable air in the space than the more dense styrofoam.

What do you think?

 

[TooManyHours]

I don't know - that's why I'm asking...

 

[Severian596]

I don't know - that's why I'm asking...

This is why I asked, "What do you think?" If you knew, why ask?

 

[NateTG]

Heat transfer in fluids (liquids and gasses) is a big nasty complicated affair that is not actually all that well understood. (For an example of the kind of wierdness that can occur, Google "Mpemba effect".)

Regarding the hypotheses of air movement and packing: You should be able to think of easy experiments to test both of them, so if you have the time you can simply do that instead of getting an answer from the book.

That said, convection causes fluids to conduct heat more efficiently in many circumstances, and air is quite a good insulator, but even in an experiment as simple as yours, other factors - for example, the inital temperature of the water, the materials that the cups are made out of, how well the styrofoam and cotton are packed, the shape of the containers, and the location of the containers in the refrigerator could easily have a larger effect than the experiment gives them credit for. (A great experiment would be to get some thermometers and put them in different locations in the refrigerator overnight, and see what numbers they come up with...)

 

[Severian596]

Well said. Air from the freezer fan blowing over one cup and not the other would have a large impact on their freezing times. Maybe it would be more accurate to simply find a place in your home that has no air flow (like under a cardboard box), and insert the thermometer into each cup in turn, then place the cup under the box. After X minutes, remove the box and take a reading, and repeat for a few more intervals of X just to get an accurate heat dissipation rating for each material.

I think because it's a 5th grade experiment everything's to be taken with a huge grain of salt, but it never hurts to try for perfection!