Activity 2: Analyzing the ‘Pop” in Popcorn

Introduction

Corn is a common foodstuff, native to the Americas, which appears in many forms: corn on the cob, corn off the cob, creamed corn, and popcorn. Field corn, which is also fed to livestock, is hung on our doors at Halloween. Each variety of corn contains different amounts of water, sugars, and starches. Popcorn is a favorite as an evening snack. Popping popcorn involves heating the corn until the pressure inside the kernel is great enough to cause it to burst, turning the kernel inside out and releasing the trapped moisture.

Purpose

To determine the percent water in popcorn, to use the ideal gas law to determine the pressure inside the kernel when it pops, and to solve gas law problems.

Safety

1. Wear protective goggles throughout the laboratory activity.

2. Heat the flask evenly to prevent spattering the oil.

3. Heat with care. Oil is flammable.

4. Do not eat the popcorn.
 

Procedure

1. Record the mass of a weighing cup.

2. Add 16 kernels of one type of popcorn and reweigh.

3. Calculate the mass of the popcorn kernels.

4. Using the water displacement method, find the volume of the 16 kernels using a 10-mL graduated cylinder. Dry the kernels.

5. Add two medicine droppers of cooking oil and the 16 kernels to an empty, dry 125-mL Erlenmeyer flask.

6. Determine the mass of the flask, oil, and popcorn.

7. Assemble a ringstand with ring, placing a wire gauze on top of the ring.

8. Using a utility clamp, fasten the flask to the stand so it sits on the wire gauze.  Cover the flask with a piece of copper gauze. Bend the sides of the copper gauze around the flask’s mouth.

9. Light the burner.

10. Hold the burner, moving it back and forth slowly until the popcorn just begins to pop. Remove the heat when most of the kernels have popped. Do not burn the popcorn. If you do, you’ll need to start over.

11. If you see water condensed on the upper part of the flask, heat that part very gently with the burner until it disappears.

12. Let the flask cool, then remove and determine the mass of the flask and contents. (Remove the copper gauze before weighing.)

13. Thoroughly wash your hands before leaving the laboratory.

Help with Your Calculations

Please show all work here:
*Percent of water in “popcorn” =  (Mass of water lost x 100)/(Mass of 16 kernels)

 
 
 
 
 

** To estimate the pressure of the water vapor at the time of “pop,” use PV = nRT.
Here are some hints regarding this calculation:

Step 1. Rearrange the equation to solve for P.

Step 2. Use the “moles of water lost” from the data table.

Step 3. R = 0.0821 L atm/(mol K)

Step 4. Assume that the popcorn pops at the boiling point of the cooking oil (225 °C) and convert this temperature to Kelvins.

Step 5. Pressure will be in atm.
 

Implications and Applications

1. What brand of popcorn did you use?

2. What was the percent water in your popcorn?

3. Name one way in which popcorn, corn on the cob, and field corn are different.

4. What is the pressure of the water (as a gas) inside the popcorn just as it pops?

5. What was the atmospheric pressure in the laboratory during this activity?

6. Compare the pressure required to “pop” corn (Question 4) with atmospheric pressure (Question 5).

7. What assumptions have you made that might cause errors in your calculation of pressure?
 

Activity 2: Investigating the ‘Pop’ in Popcorn

Major Chemical Concepts

This activity involves using the gas laws to determine the pressure inside popcorn kernels when they pop. Students will apply general gas law equations and discuss differences among various types of corn.

Level

This laboratory activity is written for a general or honors level student. It involves solving the gas law equation and requires algebra skills.

Expected Student Background

Prior to conducting this laboratory activity, students should have extensive practice with Boyle’s law, Charles’ law, combined gas law, ideal gas equation, the kelvin temperature scale, converting mass of water to moles of water, percent composition by mass, and using the water displacement method to determine the volume of rregular solids.

Time

A 55-min laboratory period is ample time to complete the activity. It will help to place needed hardware at laboratory stations and the oil and popcorn at a central location.

Safety

1. Vegetable oil is flammable. Students should heat the flask with care.

2. Students must fasten the ring onto the ring stand securely. The flask must also be securely fastened to the ringstand with a utility clamp.

3. Warn students not to eat the popcorn. Prepare other popcorn for them to enjoy outside the laboratory area.
 

Materials (For 24 students working in pairs)

• 12 Erlenmeyer flasks, 125-mL
• 12 Graduated cylinders, 10-mL
• 12 Copper gauze, or aluminum window screens, 1 piece, 5-cm x 5-cm
• Pan balance
• 12 Ringstands
• 12 Iron rings
• 12 Wire gauze, 5" x 5"
• 12 Burners
• 12 Utility clamps
• 12 Packs of matches or flint lighter
• 12 Weighing cups, plastic

At a Central Location :

• Cooking oil, 3-4 containers
• 3-4 Medicine droppers
• Popcorn, 3-4 brands
• Barometer (or call radio/television station for barometric pressure)

Pre-Laboratory Discussion

1. Provide samples of corn on the cob, corn off the cob, field corn, and popcorn for students to inspect. What are some differences among these types of corn?

2. Discuss the process of popping popcorn.
    a. When you heat popcorn, what happens to the water inside the popcorn  kernels?
    b. Why don’t some popcorn kernels pop?

3. What is the pressure of 0.010 mol of water vapor at 200 °C if it occupies a volume of 0.0020 L?

4. Calculate the pressure of the 0.010 mol of water vapor at 150 °C if its volume is held constant at 0.0020 L.

5. What is the relationship between pressure and temperature?
 

Teacher-Student Interaction

While circulating through the room, ask students questions such as:

• Do you expect the mass of popcorn to increase or decrease after it has popped? Why? [The mass of the popcorn should decrease after it is popped because popping releases water vapor into the atmosphere.]
• Do you expect the pressure inside popcorn kernels just before it pops to be greater than or less than atmospheric pressure? [The pressure inside the kernels must be greater than atmospheric pressure to rupture the kernel “shell.”]
• What is the purpose of the cooking oil? [The cooking oil surrounds and softens the kernel shell and provides a medium through which heat can be transferred from the glass to the popcorn.]

Advance Preparation

1. Place all hardware at each laboratory station.

2. Place cooking oil, medicine droppers, and popcorn at a central location.

3. Decide whether all students should pop the same brand of popcorn or compare different brands of popcorn.

4. Prepare popcorn using a hot-air popper while students are completing the activity. Invite them to eat the popcorn you prepared, in a separate area, after they have cleaned all materials, washed their hands, and are working on the calculations.
 

Calculations:

n = 0.2 g x (1 mol H₂O/18g) = 0.01 mol H₂O

P = (nRT)/V = [(0.01 mol) (0.0821 L · atm · mol-1 · K-1) (273 + 225 K)] / 0.0014 L

P = 292 atm, or 3 x 10² atm

Answers to Questions
Pre-Laboratory Discussion Questions

1. a. Different types of corn contain varying amounts of water. Corn on the cob and corn fresh off the cob contain the largest percent water. Popcorn and field corn contain less water. Fresh corn on cob will not work as popcorn because there is no hard kernel to allow pressure buildup.

2. a. When the popcorn is heated, water in the kernels becomes hotter and vaporizes.
b. Unpopped kernels possibly were too dry (insufficient moisture) and failed to build up sufficient pressure to pop, or had porous shells allowing gas to escape.

3.

4.

 

Answers to Implications and Applications

1. Answers will vary depending on the brand used.

2. Answers will range from 2% to 10% water.

3. Corn on the cob contains a much larger percentage of water. Field corn and popcorn contain much less water.

4. The pressure inside the popcorn will vary greatly. Answers should, however, range from 100 to 350 atm.

5. Read barometer and convert to atmospheres.

6. It is much larger.

7. Throughout the laboratory activity, the volume of popcorn kernels was assumed equal to the volume of displaced water. The pulp of the popcorn occupies much of the space inside the kernel; the volume recorded is larger than the actual volume occupied by the water. The kernel temperature at the time of popping was not directly measured. The assumption of 225 °C used in calculations is probably not highly accurate. Thus, for all these reasons, calculated results contain some degree of error.