Science Buddies: "Ask an Expert"

I am doing my science fair using a homemade calorimeter. In this project I measured how much chemical energy is stored in different types of food by burning/oxidizing pieces of food and measuring the heat energy released by the measurement of increased temperatures in a water reservoir. Basically "the increase in the temperature (in C degrees) times the mass of the water (in grams) will give you the amt of energy captured by the calorimeter, in calories". The formula that we used gave us quite large numbers that did not seem to even be close to the actual calories of the item being oxidized. For instance, I burned a standard size marshmallow. My distilled water (of 100g) began at 20 degrees Celsius. The water after being heated by the burning marshmallow was 27 degrees Celsius. When I plugged this in to the formula, my answer came out to 700 kcalories. I tried this several times and still came out with large numbers that did not seem reasonable. I am wondering what it is that I am doing wrong. There is no way that one marshmallow is 700 kcalories. Any direction that you could give me would be so appreciated. thank you

Hi bella14,

One of the awkward things about calories is that 1 Calorie (with a capital C) = 1 kilocalorie (lowercase C). Calories (capitalized) are what get reported on nutrition facts. So, your answer of 700 kcal is equal to 700 Calories. Now, 700 Calories is still about 30 times higher than I would expect for a single marshmallow (~23 Calories), so that can't be the only problem.

I crunched your numbers and got a different answer. I think it's a units issue. (100 grams)*(7 Kelvin)*(4.18 J/[g K]) = 2925 joules. I calorie = 4.18 J, so the enthalpy of combustion is 700 calories - with a lowercase c. That is 0.7 Calories (capital C). That's still ~a factor of 30 off from what I would expect, though. How did you insulate your calorimeter? What kind of thermometer did you use and how far was it immersed in the water? Any other details of your experimental setup would be very helpful for troubleshooting your project.

Well, I used two tin cans(one fitting inside of the other)the smaller one containing 100 grams of distilled water, which the starting temperature was 20 degrees Celsius, and a cork and a needle that would hold the piece of food. I allowed the marshmallow to burn and heat the water in the smaller can until the flame went out. The thermometer was not touching the bottom of the can and was about halfway into the water. I waited for the temperature of the water to stop at about the degrees of the heated water and it came out to 27 degrees Celsius. The flame of the marshmallow burned for no more than 20 to 30 seconds.

Thank you for your quick reply,
bella14

Hi bella14,

Thank you for these helpful details!

It sounds like your thermometer placement is good. I suspect that part of the challenge is how much water you are using. The energy released is, as you say, mass*change in temperature*specific heat. Theoretically, the mass of the water shouldn't matter because if you have more water, the temperature will not increase as much. However, in practice, I suspect your temperature measurement is less accurate than your mass measurement. It also takes time for the heat released but the marshmallow's combustion to transfer to the water, and from the water into the thermometer. Since the marshmallow doesn't burn for very long, you want to capture as much heat as you can in a short amount of time. If you use a smaller amount of water (25 grams, for example), the temperature of the water should increase more. You may get a more accurate measure of the change in temperature. If you decrease the volume of water, you will probably need to adjust the position of your thermometer so that it is sufficiently submerged. Without seeing your setup, I can't say if decreasing the volume of water will work, but it's one thing that comes to mind.

Another thing to consider is how completely your marshmallow is burning. Your calculation will only tell you how much energy was released by the part of the marshmallow that burned. If a large portion of the marshmallow isn't burning, then that may be part of the reason for the low energy you measured. Can air circulate efficiently into where the marshmallow is burning? (The marshmallow could stop burning early if there were a lack of oxygen.)

Finally, how close is the burning marshmallow to the bottom of the container holding the water? The closer the flame is to the water container, the more efficiently it will transfer its heat. The heat from combustion has to be conducted through the metal into the water, but it will also be advected away by the air around the calorimeter. Your calculation will only account for the heat that goes into the water. So, you want the flame close to the tin can holding your water, to reduce the fraction of heat getting advected away. But, you don't want the can so close that there isn't enough oxygen to allow complete combustion. There is a tradeoff between heat transfer and combustion efficiency in simple calorimeters.

A couple of things to highlight here: Your calculation only tells you the amount of energy released by the part of the marshmallow that burned that was conducted through the tin can, into the water, and then into the thermometer. So, you want to maximize the fraction of the marshmallow that is burned and minimize the amount of heat lost to the surroundings.

Let me know if I need to clarify something or if I can help with anything else.

Terik,

Thank you for your email. I've done the experiment a few more times and still doesn't seem to make sense. I'm going to tell you everything that I have done and let me know what you think. I took 100 grams of water and put it in an empty can. Then I took a wine cork, stuck a needle to it and put a standard piece of marshmallow to it. I measured the temperature of the water prior to lighting the marshmallow, which was 23.5 C. Then, I lighted the marshmellow, put the can of water above it while the marshmallow burned (15-20 seconds). Then, once the flame was out, I took the can and measured the temperature of the water, which was 25.8C. Then, I applied the formula:

Qwater = 100 g x 1 cal x (25.8 - 23.5)

= 100 g x 1 cal x 2.3

= 230 cal

Does this mean that the piece of marshmallow contains 230 cal? Is it nutritional cal or Kilocalorie? Thank you for the help so far.

Bella14

I am having a hard time coming up with a hypothesis on this one. Could this be one where a hypothesis would not work?
thanks,
Bella

Hi Bella,

Eek! I am sorry I did not see this sooner! Usually I get an email when someone replies, but I didn't this time. I am SO SORRY you've been waiting so long. I hope this isn't too late!

The calculation using the numbers you gave (increasing 100 g of water by 2.3 degrees C) gives a result of 230 calories (with a little c). I agree that is a small number. Since (at least based on what I know), your results are consistent, you've followed the scientific method, and your project is not a failure--no need to worry about that.

Now is the time for thinking about why the results you obtained are different from what you expected (in other words, why your results are different from your hypothesis). To help you do that, I will need to know the answers to the questions I asked in my reply from Jan. 13th. My gut-level guess is that the marshmallow is not burning completely. Is that true? And, the calorimeter you described is not very efficient at capturing heat (a limitation inherent in all simple calorimeters), so you will underestimate the amount of energy released by combustion. But, the numbers you are getting are way less than expected. Not to be cheeky, but are you by chance using miniature marshmallows?

Also - about your hypothesis. Your hypothesis is an educated guess about what you expect to happen. In your case, a hypothesis might be "I hypothesize that if I burn then marshmallow, then the temperature of the water will increase by XX degrees." Even if you didn't write a formal "hypothesis", it sounds like you had one in your head. Otherwise, you wouldn't wonder about why things didn't turn out the way you expected (in other words, the way you hypothesized). If you need more help with your hypothesis, take a look at this really nice article about writing a hypothesis: https://www.sciencebuddies.org/science- ... esis.shtml.