Popcorn is a modern day snack but it has been around for a long time. Ears of popping corn dating back 5,600 years were found in New Mexico in 1948-1950. A 1,000 year old popped kernel of popcorn was found in a dry tomb on the east coast of Peru. More history can be reviewed using this source.

Popcorn kernels are corn seeds consisting mostly of the carbohydrate starch; they also contain small amounts of other substances, and a variable amount water. In the popcorn kernel, the water molecules are found in between the granules of the starch. All substances are enclosed in a hard shell forming what we call the kernel of popcorn. This link provides background and history about popping corn.

To understand what happens when we pop our popcorn, a bit of background about liquids and gases (vapors) is useful. This is studied in detail in when dealing with states of matter, but for now a short explanation is sufficient. A liquid substance has a layer of vapor above it this is why we can smell liquids; the vapor contains the same molecules found in the liquid, just in the form of the gas state. (Review Chapter 1 if necessary.) These gas molecules bump into and push against the walls of the container, exerting what we call "pressure". Since these are molecules of vapor above the liquid, this pressure is called the vapor pressure of the liquid. Higher temperatures produce more gas molecules, this means more molecule impacts on the kernel wall. The greater the temperature, the faster the molecules move; this means they hit the walls more often and with more energy.

When a kernel of popcorn is heated, the temperature of the water inside rises. Normally, at 100^oC water boils and turns to steam. However, the sealed kernel acts like a pressure cooker and the trapped water is superheated to more than 100^oC inside the kernel. The vapor pressure of the superheated water rises until the pressure ruptures the kernel. The pressure "pushes" against the hard shell of the kernel until the kernel explodes or "pops". The superheated water converts totally to steam once the kernel is forced open. The volume of the steam will be greater than the volume of the liquid water it comes from at the same temperature. This expanding steam "fluffs" the carbohydrates and starch into the fluffy popcorn we enjoy eating. This is somewhat like blowing up a balloon until it breaks. You also know that if you let air escape slowly from the balloon, it will deflate but will not pop. Similarly, a little crack in the hard shell of the corn kernel will keep the water inside from superheating and will let the steam leak out or escape without expanding the kernel to its fluffy best.

You will weigh your popcorn before you heat it. (Chemists still say "weigh" the popcorn, even though we mean "measure the mass of" the popcorn.) Then you will pop the corn, allowing its water to turn to steam and expand the kernels. Once the kernel has exploded, its water (steam) will escape to the air. When you weigh the popped corn after it cools, its mass will be less by the number of grams of water that have escaped.

In this activity you will build a simple balance and then measure the mass of your popcorn sample both before and after popping. This will allow you to calculate the mass of water lost by your popcorn during the popping process. This enables you to calculate the % of the popcorn mass that is water content. Popcorn and other materials typically have definite composition. In the case of popcorn the percent water is very consistent. The percent water influence's the "popping" potential for the corn. If the kernels are too "dry" they will not pop. The water is essential to the popping process. We usually heat or 'microwave' popping corn to vaporize and expand the steam in the kernels. Check this video showing how cell phones can pop corn. cell phone corn poppers

Percent composition is especially significant for pure substances. The percent by mass is related to the formula of the substance. Materials like water, H₂O, and hydrogen peroxide, H₂O₂ have different percent by mass of oxygen. Water is always 89% O by mass while hydrogen peroxide is always 94% O by mass. These differences in percent by mass composition can be used with other data to identify pure materials.

The % water in the unpopped corn is found by dividing the grams of water by the mass of the unpopped corn and then multiplying by 100.

mass of unpopped corn  -  mass of popped corn   =  mass of water

Example

A sample of unpopped corn weighed 58.2 grams. After popping the popped kernels weighed 51.1 grams.

The mass of water in the unpopped kernels is   58.2 g - 51.1 g = 8.1 g

The percent water is

List of materials and supplies

Package of plain microwaveable popcorn

10 pennies, undamaged and relatively unworn

25 nickels, undamaged and relatively unworn

 

Sturdy cardboard box approximately 12" wide, 15-18" tall, and 3" deep*

Metal or wood skewer that is at least 6 inches longer than the depth of your box (a bamboo or metal skewer, a long pencil or a nail 9 in. or more in length will work)

 

Rubber band able to stretch and snap back to original size several times (1/16 inch thick 3 to 31/2 inches long before cutting open) The rubber band must stretch at least 10 cm when loaded. ( THE RUBBER BAND NEEDS TO BE thin enough to STRETCH A LENGTH OF ABOUT 5-10 cm ) This is needed so the distances measured show SIGNIFICANT differences when recording the weight loss.

4 pieces of Al foil, each 10 cm x 11 cm

2 resealable plastic bags like Ziploc sandwich bags

2 large paper clips

Tape

Scissors

12 " ruler marked in centimeters

One-hole punch if available

*The type of box used for FedEx or Priority Mail packages works well; the "gift box" from most department stores is not strong enough.

Since you probably do not have a lab balance at home, you are going to make a substitute. It will not work in exactly the same way as a lab balance, but it will do what is needed for this experiment. This device utilizes the stretching characteristics of your rubber band. The more mass you hang from the rubber band, the more it stretches. The rubber band needs to stretch ablest 10 centimeters when the package of popping corn is suspended or else the changes in length will be too small to give meaning ful changes. We will use the nickels, pennies and Al foil pieces to find the amounts of mass to associate with particular stretch distances.

• Stand your box on one of its 3"x12" sides; this 3"x12" side will be the base of your device during the experiment.
• Push your skewer into the center front of the box, about 2 inches down from the top, and then on out the back. Leave 6-7 in. of skewer sticking out of the front of the box.
• Stand your box on a counter or in the front of a drawer that has been pulled out just enough for the box to fit inside. Tape the box in place so it won't tip over.
• Unbend the outer part of a paper clip to form a hook, leaving the rest in its original shape to form the top of your hook.
• Cut your rubber band and tie one end to the top of your hook. The rubber band should be thin enough so it stretches at least 10 cm.
• Then tie the other end of the rubber band to the skewer.
• Tape your ruler to the box so the ruler extends downward from about the top of the hook. (Have the "1" on the ruler scale to the top.) You want to be able to look at the bottom of the hook and read the scale on the ruler.
• Be sure the hook swings freely and does not rest on the counter below your box.

• Remove plastic overwrap from the package of popcorn. Unfold the package and hold it so you can read the "This Side Up" lettering. Punch a hole in the paper package where it is glued together at the top. A good position for the hole is 1/2 inch below the top and 1/2 inch in from the left side.
• Hang the package by inserting the hook through this hole.
• Read the position of the bottom of the hook relative to the ruler by looking through the hole and lining up the hook with a spot on the ruler scale. Record this reading as "Unpopped popcorn hook position".

• Punch a hole in the top center of your Ziploc bag so you can hang it.
• Remove the popcorn package from the hook and replace it with the plastic bag.
• Carefully add nickels to the bag until the hook position is about 1 cm above the "Unpopped popcorn hook position". Then add pennies until you get the hook position close to that of the unpopped popcorn. Finally, add pieces of Al foil to get the hook position to match the "Unpopped popcorn hook position".
• Remove the Ziploc bag from the hook and count the pennies, nickels, and Al foil pieces. Record the number of each.