Science Buddies: "Ask an Expert"

I am doing my project on cooling a soda. I am comparing aluminum cans, plastic bottles, and glass bottles (in addition to styrofoam coolers and thermal coolers, refridgeration, ice, water, ice/water, ice/water/salt, and dry ice). I am working on the research part and have found information comparing aluminum cans to plastic bottles and aluminum cans to glass bottles, but I cannot find any information comparing plastic bottles to glass bottles. Can you enlighten me on this? Thank you.

Hi animalexpert,

Welcome to the forum! Can you tell us what your hypothesis is? Also: are you more interested in cooling or insulation?

When two objects at different temperatures contact one another, heat (thermal energy) flows from the warmer object to the cooler one. Thermal insulators (make sure you aren't reading about electrical or other insulation during your research) slow the flow of heat, but they cannot reverse it -- for that you need an active device that consumes energy, like a refrigerator or an air conditioner.

I don't think that soda containers are primarily designed to function as insulators -- really, they just need to keep the liquid inside, not chemically react with it, resist breaking during shipping, and be thin and light so that they take up less room and don't require as much energy to transport. But yes, metals are better conductors (= worse insulators) than plastics or glasses. I expect you will find that most plastics and glasses have thermal conductivities that are very similar to one another when compared to the difference between their thermal conductivities and those of metals. My best guess would be that glasses are somewhat better thermal insulators than plastics (they also tend to be thicker in soda containers, which helps with insulation) -- we have plastics that are transparent to visible light and less brittle than glasses, yet we still use glass windows in houses (which need to be insulated from outside temperatures most of the time). It will help if you can find out the chemical composition of the type of plastic and glass you are interested in -- for example, the name of the plastic is probably something like polycarbonate, polystyrene, etc.

If you want to compare the insulating properties of different soda containers, you will want to keep everything the same except for the type of material. This means the shape and size of the container (and, properly speaking, its thickness) -- as well as the method you use to measure how long it takes for the temperature to rise.

So, you're talking about using two independent variables: container type (3 kinds) and cooling environment (8 kinds). Do you realize that this is 3*8 = 24 experiments you're talking about? If you choose to vary only one of these, then you have either 3 or 8 experiments. In general it's also good to do multiple trials of each experiment to get an idea of the reproducibility of your results -- this could take a lot of time with 24 experiments.

When you place a bottle of soda into an environment -- ice water, a refrigerator, etc -- after a long time its limiting temperature will be the same as that of the environment, which is called a "reservoir" or "bath" in many discussions. This is assuming that you keep the environment at the same temperature -- e.g., keep the refrigerator door closed, don't let all the ice melt and not replace it, etc. The rate of heat flow can depend on a number of things. One of them is how effective an insulator the container is. Another is whether the environment is liquid (ice water) or gas (air in a refrigerator). Molecules in a liquid are packed more densely, so there are more collisions between them and your container, and it's during these collisions of molecules that heat is exchanged. (This is why many thermoses have double walls with vacuum in between them.) You might really have a problem with dry ice, because it will be sublimating (evaporating directly from a solid to a gas) the whole time, and it will be hard to keep track of how much contact the soda has with each phase. It's also important to realize that if you place a room-temperature bottle of soda into a cooler and leave that cooler out at room temperature, the soda is not going to get any colder.

This discussion probably contains some terms you aren't familiar with as well as some that are being used in a more technical sense than you might be used to. I recommend searching Wikipedia as well as the whole internet for more information. For starters:

http://en.wikipedia.org/wiki/Heat_transfer
http://en.wikipedia.org/wiki/Temperature
http://en.wikipedia.org/wiki/Thermal_insulation
http://www.infinitepower.org/pdf/09-Lesson-Plan.pdf

Our project guide: https://www.sciencebuddies.org/science- ... ndex.shtml

Some of the information you find is likely to seem a little technical, so feel free to ask us questions about it. You might have noticed that I've been careful to refer to heat flow, rather than cold flow; this should be a feature of any good information that you find. Hope that helps -- if you can tell us more about your hypothesis and the reasoning behind it, I'm sure we can provide more advice.

Amanda

There is another aspect to the comparison of heat conduction through container walls that involves "thermal mass". Glass containers are typically heavier (have more mass) than plastic containers that hold the same amount of liquid. This means that there is more mass that has to be heated or cooled in order to change temperature.

For example, glass has a thermal conductivity coefficient (big words for something that descibes how fast heat can travel through some unit of area and thickness) of 1.05 at typical room temperature while typical plastics range from 0.1 to 0.5 and aluminum is 250.

Because the materials aren't the same thickness and they don't have similar mass, you have a problem that can't be easily broken down to make any predictions/calculations. Since aluminum soda cans are typically 12 fluid ounces and glass and plastic bottles typically hold more liquid, you will have a hard time comparing their performance in any scientific way because there are multiple differences.