Percent Water In Popcorn, Definite composition and Counting By Weighing
Dr. Walt Volland, All rights reserved copyright Revised August 8, 2010
Report for popcorn counting by weighing as assigned in class Report sheet link
Discussion why popcorn pops
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 100oC water boils and turns to steam. However, the sealed kernel acts like a pressure cooker and the trapped water is superheated to more than 100oC 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.
Water In Popcorn-definite composition
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.
mass of unpopped corn - mass of popped corn = mass of water
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
Materials and Supplies
*The type of box used for FedEx or Priority Mail packages works well; the "gift box" from most department stores is not strong enough.
Constructing a Scale 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.
Calibrating your scale
Popping corn and weighing to figure mass of water
- 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.
Figure the mass of the package of unpopped popcorn by finding the total mass of nickels, pennies, and Al foil needed to stretch the rubber band to a matching hook position.
A nickel has a mass of 5.0 g. A penny has a mass of 2.5 g. Click here for US mint coin masses. 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 and use these to complete the Angel course equiz.
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 nuts 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.
Procedure for 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.
1. Calculate the mass of 10 nickels. A nickel has a mass of 5.0 g
2. Calculate the mass of the rice (whatever objects) 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 454 g of rice (objects).