Percent Water in Popcorn

Percent Water In Popcorn, Definite composition and Counting By Weighing

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discussion	percent water calculation	popped corn mass	building a scale
	materials list	counting by weighing a model of the mole

Percent Water In Popcorn

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. From this you can calculate the % of the popcorn mass that is due to the 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.

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.

Discussion

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.

To understand what happens when we pop our popcorn, we need a bit of background about liquids and gases (vapors). This is studied in detail in Chapter 8, but for now a short explanation is sufficient. A liquid substance has a layer of vapor above it; the vapor contains the same molecules found in the liquid, just in 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. The more gas molecules, the more the wall gets bumped. 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 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.

grams of unpopped corn - grams of popped corn = grams of water

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.

Procedure

List of 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 is ideal; the "gift box" from most department stores is not strong enough.

Construct a Device to Weigh Your Popcorn

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.

Experimental Procedure

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.

Pop your popcorn in the microwave, following the directions on the package.

Carefully remove it from the microwave and cut a small opening at the center top of the package to allow the steam to escape. The steam will be very hot so use caution when cutting into the package. You should be able to see the ripples the steam makes in the neighboring air as it escapes.

Do not allow any popcorn to leave the paper bag. Allow the package to cool.Then hang the package from your hook as you did before. Locate the position of the bottom of the hook relative to the ruler scale. Record this as "Popped popcorn hook position".

Remove package of popcorn from the hook and replace it with your Ziploc bag.

As you did before, add nickels, pennies, and pieces of Al foil to the plastic bag until the hook position matches the "Popped popcorn hook position".

Remove the Ziploc bag from the hook, count the nickels, pennies, and Al foil pieces and record these numbers to figure the mass of popped corn.

Calculate the mass of the package of unpopped popcorn by finding the total mass of nickels, pennies, and Al foil needed to give the same hook position. A nickel has a mass of 5.0 g. A penny has a mass of 2.5 g. Each piece of Al foil with dimensions 10 cm x 11 cm has a mass of 0.5 g.

Calculate the mass of the package containing the popped popcorn in the same way.

Calculate the mass of water lost as a result of the popping process.

Calculate the % Water in Popcorn.

Answer other questions, if any, on the report sheet.

Counting By Weighing

When we want to "measure out" a quantity of something that exists as small particles, we often use its weight or mass instead of counting out individual pieces. We do this when we buy nails in bulk, when we use chocolate chips in a cookie recipe, when we cook rice or beans, and when we perform numerous other tasks. Chemists do this when we "measure out" quantities of atoms or molecules. In this activity, you will examine the concept of counting by weighing, using grains of rice to simulate atoms and molecules.

Report sheet link

Procedure counting by weighing

Obtain about 1/2 Cup of uncooked grains of regular (not instant) rice.

You can also use dried beans, dried peas, whole grains of wheat, marbles, screws, nails, pins, or anything in place of the rice grains.

Use your weighing device from the popcorn experiment.

Place 10 nickels in your Ziploc bag and hang the bag from the hook.Read the position of the bottom of the hook. Record this as "Hook position for 10 nickels".

Remove the Ziploc bag from the hook, and empty it. Hang the empty Ziploc bag back on the hook. Add the rice grains (objects) until the hook position matches the "Hook position for 10 nickels".

Remove the plastic bag from the hook and count the grains of rice (objects) in the bag. Record this as "Number of objects = 10 nickels".

1.Calculate the mass of 10 nickels. A nickel has a mass of 5.0 g.

2.Calculate the mass of the rice (objects)that you had in your plastic bag.

3. Calculate the mass in grams of 1 grain of rice (object).

4. Calculate the number of grains of rice(objects) in one pound or 454g of rice(objects).

5. Predict how many objects would be in 100 pounds.