Popcorn is a great real-world example and science phenomenon to use when discussing the kinetic molecular theory of gases, the phase change of water from a liquid to a gas, Gay-Lussac’s gas law (pressure directly related to temperature), Charles’ law (volume and temperature directly related), or the ideal gas law (PV = nRT). After covering the KMT and gas laws in class, complement the lesson with a student-designed inquiry activity using several brands of popcorn, both bag and microwave varieties to examine and explain the phenomenon of popping corn.
When a popcorn kernel is heated, the trapped water in the endosperm turns into steam, building up pressure inside the pericarp to more than 9.0 atm. This pressurized, super-heated steam transforms the soft starch in the endosperm into a gelatinous material. The pericarp ruptures, releasing the steam and gelatinous starch that solidifies upon cooling. The resulting popped kernel is 40 to 50 times its original size. The optimum moisture level for popcorn is 14%, below that value, the size of the popped kernels is smaller and the number of kernels that pop decreases.
Students observe popping corn.
HS-PS3-2. Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles and energy associated with the relative positions of particles.
Students should wear goggles and aprons or lab coats during the activity and exercise due caution around Bunsen burners or hot plates. Inform students that cooking oil boils at a higher temperature than water (225° C), and have them cover their beakers with aluminum foil to contain the popping corn and boiling oil.
Note: Remind students not to eat any of the popcorn produced in the lab.
To save time, you may wish to pre-count popcorn kernels and have them in a labeled cup ready to hand out to students. Dispose of the popped corn in the trash. All glassware will need to be washed with detergent and hot water to remove the oil.
What is the percent by mass of water in your sample of popcorn? Show your work and include all units.
% of water in kernels = water lost ÷ mass of kernels × 100
0.20 g ÷ 2.10 g × 100 = 9.5%
Compile the class data and calculate the mean water percentage for bag popcorn and microwave popcorn.
Student answers will vary depending on the values submitted by groups. Students may need to be reminded of how to calculate mean or average.
Use the class data to explain any differences between bag and microwave popcorn.
Check the averages. There will likely not be significant differences in percent mass when comparing the types of kernels. The corn kernels are of the same variety but packaged differently.
Use the kinetic molecular theory, gas laws, and class data to construct a model explaining why popcorn pops.
Popcorn kernel is heated → KMT: heat added, temperature increases, KE increases, particle motion increases
Trapped water in the endosperm turns into steam → KMT: heat added, boiling point of water is reached, phase change from liquid water to steam, heat of vaporization is absorbed so liquid water turns to steam, gas laws apply
Pressure inside the pericarp increases → Gay-Lussac’s law: as temperature increases, pressure increases; Charles’ law: as temperature increases, volume increases (combined gas law can also be stated)
Pressurized, super-heated steam changes the soft starch into a gelatinous material → KMT: heat is added, particle motion is increased, allowing the steam to mix with the starch, creating a gelatinous material
Pericarp ruptures → KMT: structural failure of the pericarp resulting in the loss of steam/water molecules
Steam is released Ideal gas law: amount of water molecules decreases, moles of water decrease, and the quantity of (PV/T) decreases
Heat source is removed → KMT: temperature decreases, KE decreases, particle motion decreases
Gelatinous starch cools and solidifies → 2nd Law of thermodynamics: thermal equilibrium is reached, popcorn is a room temperature
Written descriptions, particle diagrams, flow charts, or procedural sketches may all serve as models.
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