Burning Calories: Question

[Emersonz98]

Hi, I am doing the project Burning Calories: How much energy is stored in different types of food? And I have a question concerning it. A couple days ago, I started doing the experiment and for a marshmallow the water only went up .2 degrees Celsius. The marshmallow, along with my other food items that Science Buddies sent me(I bought the kit), only will burn for a very short period of time and then the water barely increases in temperature. Any suggestions? Am I doing something wrong? Can you please get back to me as soon as possible!?
Thank you!

[vysarge]

Hello, Emersonz!

I've actually done a simplified version of this experiment before, so I think I have an idea of what the problem may be. There are a couple of things that you could try.

First, water doesn't change temperature very easily; its specific heat is very high. To increase the temperature change, you could try reducing the amount of water. This wouldn't mean that you would have to do all of your trials over again- the amount of water would end up canceling out the temperature change in your calculations- but might help your accuracy and make it easier to measure.

Second, looking at the design of the experiment, it seems to me like there are a lot of places that heat could escape without going into the water. You could try putting the can closer to the flame, if possible, or you could try to cover up the open top of the larger can with something nonflammable.

As for catching the food on fire more effectively- a lot of foods, including marshmallows, are a little moist, and may stop burning before all of the calories have been released as heat. To help increase the number of released calories, you might try to light the entire outer surface of the marshmallow on fire, instead of just starting a fire at one place. If that doesn't help (or you're already doing it), maybe you could try to find some larger marshmallows? Those would release more heat, as they contain more energy.

If you have any more questions, or something was unclear, don't hesitate to ask!

Good luck, and I hope this helped!

[Emersonz98]

Thank you very much now for the calculating the calories part, I measured the water in mL but the conversion is in grams. How do I convert mL to grams? Thanks!!

[vysarge]

You're welcome!

The density of water at room temperature is very close to 1 g / mL. So, 20 mL of water has a mass of 20 g.

Hope this helped!

[Emersonz98]

This project was very fun and I enjoyed doing it! I do have another question though! For calculating the calories/heat part, some of my food items such as the almond and cracker weighed 0.0 grams after being burned. Is that okay to calculate the heat capacity? Thanks!

[vysarge]

I think that should be fine; ideally, in this experiment, the final weights would be 0. So you've done a great job!

As long as the change between initial and final masses is a number that makes sense and not 0, your analysis will work and be accurate.

Hope this helped!

[Emersonz98]

Ok thank you! But what do you mean by initial and final masses?

[theborg]

Emersonz98,

When performing the experiment you should have weighed each test sample before and after burning them to determine how much was converted to heat energy.

[Emersonz98]

Yes, I did that. Then, I have to use the equation to find the heat released, correct?

[theborg]

Right, you use the equation to find the calories released during the experiment. But you'll notice weight isn't in the equation. What you do is take the total calories output from the equation and divide by the difference in weight to find the calories per unit weight. For example you test something that initially weighed 50g and 10g after the test for a change of 40g. From the equation you find the calories released is 80cal. Therefor 80cal/40g = 2 calories per gram of test material.

[Emersonz98]

Ok thank you! How did you get 80 calories? Also, on the science buddies website there is an equation and it says: Q water= mc(shape of triangle)T. What does that mean and what does the equation used for and find? I'm sorry I'm just confused on how to calculate the energy released per good item and the energy per unit weight of each food item. Sorry and thanks!

[theborg]

Emersonz98,

The numbers I used in my last post were only to illustrate the point and were totally made up to give you an example of how to use the numbers you get in your experiment.

The equation: Qwater = mcΔT

Is what you use to determine how much energy in calories were released when you burnt your test samples.

where:

Qwater = the heat captured, in calories (cal);

m = the mass of the water, in grams (g);

c = the specific heat capacity of water, which is 1 cal/g°C (1 calorie per gram per degree Celsius); and

ΔT = the change in temperature you measure of the water (the final temperature of the water minus the initial temperature of the water), in degrees Celsius (°C).

The symbol Δ is the Greek letter called delta and usually means change in whatever it is next to. In the equation above T = temperature so ΔT = change in temperature.

Once you have calculated Qwater, then you can use the change in mass of the test item to get calories per gram as I outline in my previous post.

[Emersonz98]

Ok thank you so much! So I got that! Now, for my experiment aren't I supposed to be finding the amount of energy stored JB food?

[Emersonz98]

In not JB

[theborg]

Emersonz98,

I can see where this would be confusing, but when you solve for Qwater, you are measuring the temperature change in the water and calculating the energy, in Cals, it took to raise the temp of the water.

However, the energy to raise the temp of the water comes from the energy released by burning the test sample food item. If you set up the experiment such that as much of the heat energy released by burning the food is transfered to the water as possible, then the calories released by the food item during burning closely equals the calories absorbed by the water (Qfood = Qwater) and is measured as a change in temperature in the water.