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Why Some Kernels Never Pop: Particles in a Popcorn Popper

Use data and a particle model to explain how thermal energy pops a kernel.

Answer Key

Three kinds of kernels

Read the passage and study Table 1. Then answer the questions.

The Kernels That Stayed Hard

1The teen maker club at the Eastbrook Community Center builds one simple device each month. In March, the club put together a hot-air popcorn popper from a kit. A small fan blows air across a hot metal coil and up through a chamber, where the kernels tumble in the hot air until they pop. The club adviser, Mr. Faisal Mirza, plugs in and runs the popper, while members Zuzana Horvath and Bodhi Ramaswamy record data from a safe distance. After the first batch, Bodhi counted many kernels that never popped at all.

2A popcorn kernel has a hard, tight outer hull. Inside is soft starch that holds a small amount of water. As the popper's hot air adds thermal energy, the temperature of the kernel rises. Water boils at 100 °C at normal air pressure, but the hull traps the water inside, and the pressure inside the kernel builds as it gets hotter. When the hull finally bursts, the water rushes out as steam, and the soft starch puffs into the white foam that people eat.

3Zuzana wondered whether the water in the kernels was the key. She split one bag of kernels into three 100 g samples. Sample A came straight from a sealed jar. Sample B sat in an open bowl in the warm, dry clubroom for 3 weeks. Sample C was dried by Mr. Mirza in an oven set to 60 °C, too cool to pop corn, for 6 hours. The club borrowed a grain moisture meter to measure how much of each sample's mass was water. Then each sample was popped for 3 minutes, and the popped and unpopped pieces were counted and weighed together.

4During each run, a cloud of white mist rose from the chute of the popper, and small drops of water formed on the cool glass of the window above the counter. After the run, Bodhi dropped a pat of cold butter onto the warm popcorn and watched it soften into a yellow puddle.

Table 1. Three 100 g Samples, Popped for 3 Minutes
SampleStoredWater (% of mass)Kernels popped (%)Mass after popping (g)
ASealed jar149688
BOpen bowl, 3 weeks107193
COven-dried at 60 °C61898
1.
Use Table 1. Each sample had a mass of 100 g before popping.

Sample A held 14 g of water, and Sample C held 6 g of water.

2.
Use Table 1.

During popping, Sample A lost 12 g of mass, and Sample C lost 2 g of mass.

3.
Which explanation of the mass lost during popping fits the passage and Table 1 best?
  • A. Heat, which has mass, flowed out of the kernels once they had popped.
  • B. Starch particles were destroyed by the heat and turned into thermal energy.
  • C. Water particles left the kernels as a gas and mixed into the air.
  • D. Air particles trapped in the hull were squeezed out as the kernels cooled.
4.
Which pattern does Table 1 show?
  • A. The samples with more water popped fewer kernels and lost less mass.
  • B. The samples with more water popped more kernels and lost more mass.
  • C. The oven-dried sample lost the most mass because it was the driest.
  • D. Samples A and C popped about the same share of their kernels.
5.
All three samples were heated in the same popper for the same time. Which particle explanation best fits why so few Sample C kernels popped?
  • A. At the same temperature, the water particles in dry kernels moved much more slowly than those in fresh ones.
  • B. Fewer water particles turned into gas, so the pressure in most hulls stayed too low to burst them.
  • C. The oven gave the starch particles extra cold, which kept the dry kernels from warming up in the popper.
  • D. Drying made the starch particles grow larger, so they filled the hull and left no room for the steam.

Inside one kernel

Use Table 2, the passage and a particle model to answer the questions.

Table 2. Temperature Inside One Fresh Kernel (from the kit's manual)
Time in popper (s)Kernel temperature (°C)What happened
020Hard kernel
3075Hard kernel
60115Hard kernel
90150Hard kernel
120175Hard kernel
135180Hull bursts; kernel pops
6.
From 0 s to 120 s, the kernel warmed from 20 °C to 175 °C. What happened to the water particles inside it?
  • A. They moved faster on average, because they gained kinetic energy.
  • B. They slowed down, because the hull kept them from moving around.
  • C. They grew larger, so each one took up more room inside the hull.
  • D. They stayed still until the hull burst, and then began to move.
7.
Use the particle model. Why did the pressure inside the hull keep rising as the kernel got hotter?
  • A. Faster gas particles hit the inside of the hull more often and harder.
  • B. Thermal energy is a kind of gas, and it filled up the space in the hull.
  • C. The water particles joined together into fewer, much heavier particles.
  • D. The cold inside the kernel escaped, and the hull shrank around the starch.
8.
Use Table 2 and paragraph 2.

When it popped, the kernel was 80 °C hotter than the temperature at which water boils at normal air pressure.

9.
Use paragraphs 2 and 4. Sort each event by the change of state of the substance named, if there is one.
Butter softening into a puddle on the warm popcornWater vapor forming drops on the cool window glassWater in a kernel turning to steam as the kernel heatsA popped piece of popcorn cooling in the bowlA hard kernel warming from 20 °C to 75 °C
Solid to liquidLiquid to gasGas to liquidNo change of state
Butter softening into a puddle on the warm popcorn
Water in a kernel turning to steam as the kernel heats
Water vapor forming drops on the cool window glass
A popped piece of popcorn cooling in the bowl
A hard kernel warming from 20 °C to 75 °C
10.
Zuzana finds a fresh kernel with a small crack in its hull. Predict what will most likely happen when it is heated in the popper.
  • A. It will pop at a lower temperature, because its water particles move faster.
  • B. It will pop like the others, because its water still turns to steam inside.
  • C. It will pop sooner, because the crack lets extra hot air rush into the kernel.
  • D. It will likely stay unpopped, because steam leaks out and the pressure stays low.
11.
Bodhi says, "Popcorn pops because heat gets trapped inside the hull and piles up until the kernel explodes." Do you agree? Write a claim, use evidence from Table 1, and use the particle model to explain what really builds the pressure.

I disagree. Heat is not a substance that piles up; the pressure comes from water inside the kernel. Evidence: all three samples got the same heating, but Sample A, with 14% water, popped 96% of its kernels, while Sample C, with only 6% water, popped just 18%. Sample A also lost 12 g during popping, which is most of its 14 g of water leaving as steam. Reasoning: thermal energy from the hot air makes the water particles move faster. Some of the water turns into gas, and the fast gas particles hit the inside of the hull harder and more often, so the pressure rises. With enough water, the pressure bursts the hull. With too little water, there are too few gas particles, and the hull does not burst.