Learning Objective: To determine the volume and water displacement of popcorn.
Key Terms: Volume, Water Displacement, Popcorn, Scientific Method, Food Science
Grade Level: 3-6
- 2 same size glasses (or if accessible, 2 graduated cylinders or beakers to measure water displacement)
- Popped popcorn
- Caramel popcorn
Estimated Time: Approximately 15-20 min
Cereal is a breakfast favorite in the U.S., with Americans consuming around 2.7 billion boxes of cereal each year. That is about 14 pounds of cereal the average person consumes every year.
We shovel a variety of sugar-coated corn flakes and rice puffs into our mouths each morning, so why not popcorn? Similar to rice and wheat, corn is considered a starchy, whole grain, but once in its popped form, we don’t consume popcorn with milk.
This was not always the case. History suggests that popcorn and milk, sprinkled with a hint of sugar, was one of the first examples of cold breakfast cereal. The most notable is the specific passage read from Laura Ingalls Wilder’s Farmer Boy that described filling a single glass with popcorn and another with milk, then placing all of the popcorn pieces into the milk without it ever overflowing.
“You can fill a glass full to the brim with milk, and fill another glass of the same size brim-full of popcorn, and then you can put all the popcorn kernel by kernel into the milk, and the milk will not run over. You cannot do this with bread. Popcorn and milk are the only two things that will go into the same place. Then, too, they are good to eat.” (Laura Ingalls Wilder’s Farmer Boy)
Now, this passage has turned into inspiration for many grade-level science experiments.
A photo of Almanzo Wilder and the book “Farmer Boy” by Laura Ingalls Wilder
Popcorn Anatomy – Kernel to Popcorn
A popcorn kernel is comprised of three parts:
- Outer shell (known as the bran, hull, or pericarp) – this outer shell surrounds the popcorn kernel and protects the kernel from environmental factors.
- Endosperm – the middle layer that stores starch and water, which serves as food for the germ.
- Germ – is the seed embryo inside the kernel. It contains genetic information (used to grow into a new popcorn plant), enzymes, vitamins, and minerals to help the kernel grow into a corn plant.
The popcorn kernel is unique because when it is heated to a temperature of about 400-460°F (204-238°C), it will explode. The moisture contained in the endosperm layer begins to boil and turns into steam.
The increasing amount of steam increases the pressure inside of the kernel because the outer shell prevents the steam from escaping. As the kernel continues to build pressure, the starch also begins to gelatinize.
Once the pressure reaches about 135 pounds per square inch, the outer shell breaks away and the kernel bursts open. The starch solidifies into a white, puffy popcorn, essentially turning the popcorn kernel inside out. Get a close-up of the popcorn popping process, here is a video of popping popcorn in ultra slow motion.
Now that the kernel has been popped, we can absorb and digest the amazing nutritional benefits of popcorn! One serving of popcorn, roughly 4 to 5 cups, contains about 70% of the recommended daily intake of whole grains. Additionally, popcorn contains fiber, iron, and other essential nutrients. Without any added butter, per cup of popcorn contains ~30 calories making it an optimally healthy, whole grain snack option.
Before beginning the popcorn and milk experiment, introduce the concept of volume and measuring volume using water displacement to the kids. This will allow them to begin to explore what is happening when popcorn is placed in milk.
Volume is known as the amount of space an object takes up. It is one of the many ways that an item can be described, known as physical properties. Similar to height and weight, it is a way to describe an object’s size. Instead, volume can only measure objects that are 3-dimensional, meaning they contain three parts:
This allows us to measure volume in cubic units.
For example, if you wanted to find out how much a rectangular container holds, you need to measure the container’s volume. The formula to find the volume of a rectangular container (prism) is:
Volume = length x width x height
To find the volume, you first need to measure the length, width, and height of the container.
V = l x w x h
V = 14 in x 5 in x 10 in
V = 700 cubic inches
The rectangular container can hold a volume of 700 cubic inches.
While a rectangular container has three distinct sides to measure length, width, and height, this is not the case for a majority of objects. Instead, we can use a technique called water displacement to determine the volume of abnormal objects.
Water displacement is when the object is placed in water and in order to ‘fit’ in the water, it pushes the water out of the way to make room for itself. However, the water has to go somewhere. The object will push out the volume of water that is equal to its own volume.
A great example of this is when we take a bath. When we get into a bathtub, the water level rises. This is also a good reason not to fill the bathtub all the way before getting in!
The concept of water displacement was discovered by Archimedes when he needed to determine if a crown was made of true gold or not.
Finding Volume With Water Displacement
In order to find the volume of an irregularly shaped object, you will need to use the water displacement formula:
The final volume of water with the object – starting volume of the water before the object was added = volume of the object
In order to use the water displacement formula, you would need a container of water with a known starting volume. It is best to use a graduated cylinder or beaker to accurately measure the volume before and after the object has been added. However, if you do not have a graduated cylinder, a measuring cup can suffice but will be less accurate.
Next, add the irregularly shaped object to the container of water until completely submerged. You should be able to see the rise in water volume once the object has been added. This is the final volume that needs to be recorded. Now subtract the final volume of water from the starting volume of water to get the volume of the object.
Further, the density of the object can be calculated (if the mass is known).
Note: Initiate a discussion on whether water is the required liquid or could other liquids be used to calculate water displacement.
Water Displacement Example
A graduated cylinder is measured to have a known volume of water of 12.5 mL of water. A ball is then added to the graduated cylinder causing the water level to rise up to 17.5 mL. By using the final volume and subtracting the starting volume, the volume of the ball can be found.
Volume of the ball = final volume of water – starting volume of water
Volume of the ball = 17.5 mL – 12.5 mL
Volume of the ball = 5 mL
Given that the ball is solid, the volume will be recorded in cubic centimeters as the unit for volume rather than mL, since 1 mL = 1 cubic centimeter.
Popcorn and Volume
Popcorn is an irregular object, so would using water displacement be the best method to measure the volume of popcorn?
Unlike most objects, the starch of the popped popcorn, and most other starches, are filled with air instead of solid material. This is why popcorn appears to melt in your mouth, Water (saliva) fills in and collapses the pockets of air found within the popcorn.
Is there a way to accurately measure the volume of popped popcorn? Is there a way to prevent the water from filling the air pockets of the starch portion of the popcorn?
Below are directions on how to conduct the milk and popcorn experiment. “Popcorn and milk are the only two things that will go into the same place.” (Laura Ingall Wilder’s Farmer Boy)
- Fill one glass about ½ a centimeter from the top with milk.
- Fill the second glass to the same spot with air-popped popcorn.
- Slowly place the popcorn into the glass of milk.
- Continue until all of the popcorn from the glass is in the glass of milk.
- Make observations. Create a discussion with the class as to why the same volume of popcorn fits into the glass of milk without causing it to overflow. Did the popcorn change size when wet? Did it make a sound?
- Repeat steps 1-5 with caramel popcorn.
- Discuss the differences between the plain popcorn and caramel popcorn. What were the differences? Did the same thing happen? If not, why?
For fun: allow the kids to test taste what regular and caramel popcorn taste like when consumed with milk, just like the boy in the book, ‘Farmer Boy’, did.
Here are a couple of ideas of how you can expand on the experiment, enable curiosity, and encourage scientific thinking.
- Test the popped popcorn in a variety of different liquids, other than milk. Milk is a unique liquid that contains a variety of proteins, fats, and other molecules that could potentially be causing the phenomenon.
- Test the experiment with other products that are similar to popcorn, maybe they contain the airy, starchy texture of popcorn (i.e. puffed rice cereal, marshmallows, bread).
- Compared popped popcorn to popcorn kernels either of the same quantity of popcorn pieces, the same volume of popped popcorn, or the same weight of popped popcorn. This could expand on additional concepts such as density, matter, and weight.
- Change a variable in the experiment such as time. Maybe placing the popcorn in the milk slowly allows for the milk to dissolve the popcorn.
- If there is extra popcorn available, see how much popcorn can fit into the glass of milk before it overflows.