This experiment is for advanced students. Did you know that eating a single peanut will power your brain for 30 minutes? The energy in a peanut also produces a large amount of energy when burned in a flame, which can be used to boil water and measure energy.

Peanuts are part of the bean family, and actually grows underground (not from trees like almonds or walnuts).  In addition to your lunchtime sandwich, peanuts are also used in woman’s cosmetics, certain plastics, paint dyes, and also when making nitroglycerin.

What makes up a peanut?  Inside you’ll find a lot of fats (most of them unsaturated) and  antioxidants (as much as found in berries).  And more than half of all the peanuts Americans eat are produced in Alabama. We’re going to learn how to release the energy inside a peanut and how to measure it.

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Materials:

• raw peanuts
• chemistry stand with glass test tube and holder (watch video)
• flameproof surface (large ceramic tile or cookie sheet)
• paper clip
• alcohol burner or candle with adult help
• fire extinguisher

Download Student Worksheet & Exercises

What’s Going On? There’s chemical energy stored inside a peanut, which gets transformed into heat energy when you ignite it. This heat flows to raise the water temperature, which you can measure with a thermometer.  You should find that your peanut contains 1500-2100 calories of energy!  Now don’t panic…  this isn’t the same as the number of calories you’re allowed to eat in a day.  The average person aims to eat around 2,000 Calories (with a capital “C”).  1 Calorie = 1,000 calories.  So each peanut contains 1.5-2.1 Calories of energy (the kind you eat in a day). Do you see the difference?

But wait… did all the energy from the peanut go straight to the water, or did it leak somewhere else, too?  The heat actually warmed up the nearby air, too, but we weren’t able to measure that. If you were a food scientist, you’d use a nifty little device known as a bomb calorimeter to measure calorie content.  It’s basically a well-insulated, well-sealed device that catches nearly all the energy and flows it to the water, so you get a much more accurate temperature reading. (Using a bomb calorimeter, you’d get 6.1-6.8 Calories of energy from one peanut!)

How do you calculate the calories from a peanut?

Let’s take an example measurement.  Suppose you measured a temperature increase from 20 °C to 100 °C for 10 grams of water, and boiled off 2 grams.  We need to break this problem down into two parts – the first part deals with the temperature increase, and the second deals with the water escaping as vapor.

The first basic heat equation is this:

Q = m c T

Q is the heat flow (in calories)
m is the mass of the water (in grams)
c is the specific heat of water (which is 1 degree per calorie per gram)
and T is the temperature change (in degrees)

So our equation becomes: Q = 10 * 1 * 80 = 800 calories.

If you measured that we boiled off 2 grams of water, your equation would look like this for heat energy:

Q = L m

L is the latent heat of vaporization of water (L= 540 calories per gram)
m is the mass of the water (in grams)

So our equation becomes: Q = 540 * 2 = 1080 calories.

The total energy needed is the sum of these two:

Q = 800 calories + 1080 calories = 1880 calories.

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