Group Members:
Tete Dickson and Dana Anzalone

Popping Fun

Popcorn from Dana Anzalone on Vimeo.

Chemistry Background Information

All of us enjoy a bag of popcorn every once and a while, but how does it work? When you heat the popcorn kernels in the microwave, it causes its contents to expand, eventually causing the kernel to pop. Each kernel is composed of 13% to 14% water with the rest as starch. With the addition of heat and energy, the water inside the kernel changes phase form a liquid to a gas, causing it to expand inside the kernel, thus increasing the pressure. Eventually the pressure gets high enough that it causes the shell to break, and turn inside out, exposing the tasty, fluffy starch.In popcorn the natural moisture inside the kernels build up as the pressure also grows in strength causing them to explode. A kernel will explode at 400 degrees Fahrenheit or 204.4 degrees Celsius. Those kernels that are left in the bottom of the bag either didn’t have the correct percentage of water (either too much or too little), or had a hole in their shell. Both of these factors affect the pressure within the kernel, and when the pressure isn’t just right, the kernel wont pop.

When temperature is increased and energy is added to a substance, in this case water, it causes it to change phase form either a solid to a liquid or a liquid to a gas. The energy is transferred to the molecules of the substance, allowing them to break their intermolecular bonds which cause the substances to expand. The opposite happens when energy is removed.

Water comes in many phases. At low temperatures what becomes a solid where the individual molecules create a tightly packed structure. The increase of temperature also increases the kinetic energy of the molecules which then increase the rate of movement of the molecules. Water moves in three ways vibration, rotation and translation. Molecules vibrate when the bonds are stretched or bent. “Rotation involves the motion of a molecule around its center of gravity”. And translation means the motion of molecules through space.

Covalent bonds between hydrogen and oxygen atoms are intramolecular bonds. While the bonds between the molecules as a whole are intermolecular bonds. Intramolecular bonds are stronger than the intermolecular bonds. As a “system” gets warmer, the energy of the molecules becomes to big to permit the solid structure. The solid them melts and becomes a liquid. Again the energy becomes too big for the molecules, that they move rapidly forming intermolecular bonds. The liquid then turns into gas. The difference between the different phases is based on the “competition between the strength of intermolecular bonds and the thermal energy system”. Substances that have strong intermolecular bonds are solids. The loss of energy causes the molecules to come closer together, which increases the intermolecular forces, in turn limiting the movement of the molecules. As substances get hotter, the molecules tend to move faster causing the thermal energy to increase. The molecules are vibrating in a solid, while in a liquid they hold so much energy that they are released as steam in the air. As the steam increases in heat, the gases float in different direction, causing the gas to expand in volume. When enclosed in a single space, molecules bounce around hitting the limits faster as the temperature also rises.

When a phase change occurs, the physical structures of the molecules is being changed. Because of the charges, when molecules join together they arrange themselves in a pattern in which the positive charge is located close to the negative and vice versa. In a solid structure the molecules align themselves in a fixed crystal like formation. A solid has a exact shape and volume. When the solid becomes a liquid, the molecules disjoin and “flow over one another, no longer keeping fixed neighbors.” A liquid has a volume, but takes the form of the container in which it is held. When the liquid becomes a gas the molecules are in a spontaneous motion. The gas has a volume, but no shape. In each transition a type of energy is required. Latent energy is the amount of energy need for a phase change to occur. The transition from a solid to a liquid is known as latent energy of fusion. The transition from liquid to gas is known as latent energy of vaporization. Due to the strong hydrogen bonds in water, the numerical latent energy of vaporization is quite high.


Stoichiometric Component

O.15 grams = Kernel of Popcorn
0.15 x 0.14 (percentage of water in kernel) = .021grams of water in a single kernel

Heat Energy

q(heat) =m(mass of water) x Lvapor (Latent heat of vaporization energy)

Conversion from grams to kilograms = .021 grams x (1/1000kg) = 2.1 x 10^ -5 kg of water

2.1 x 10^ -5 x 2260kJ/Kg = .047kJ = heat energy

Corresponding pressure

PV = nRT

P = Pressure
V = Volume = 0.15 L
n = moles of water
R = Gas constant ( 0.0821 L atm mol-1 K-1
T = Temperature

conversion from grams to moles = .021g/ 18.016 ( molar mass of water) = .0012 moles

400 degrees Fahrenheit = 477.59 Kelvins

P(0.15) = (0.0012)(0.0821)(477.59 K)

P = 0.31 atm

Additional Resources:
short quiz -
References/Useful Links: