Popping

A chemical or physical change?

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A Middle Grades

Activity

 

Popping Popcorn: A Chemical or Physical Change? Teacher Instructions

Lesson Summary and Teacher Background Information: In this problem-based learning activity, students will make careful observations regarding the following in order to distinguish between chemical and physical changes:

A) Popcorn popping in slow motion B) An un-popped kernel of popcorn C) A popped kernel of popcorn

The teacher will serve as a facilitator to student discovery. Students will discover that, while a popped piece of popcorn appears to have undergone a significant change, no new substance is created in the process and popping popcorn is, thus, a physical change. All of the materials that are in an un-popped kernel are the same as what makes up a popped piece of popcorn. The moisture that is inside the popcorn kernel changes states (a key sign of physical change) to a gas when heat is applied. When the starch inside the kernel meets with heat, moisture, and air, it becomes the fluffy outside of the popped piece of popcorn that we all know and love. The kernel is essentially turning inside out, due to the application of heat creating the state change inside the kernel. This is a difficult problem because there are signs of a chemical change (heat involved, appearance of a new substance, new odor, appearance of a new color), but there are no atomic bonds broken and/or created, so there is no new substance and, thus, it is a physical change.

Lesson Objectives:

- Students will recognize the differences between chemical and physical properties and changes. - Through a problem based learning activity, students will discover that it is not always simple to distinguish between chemical and physical changes. - Students will understand that to have a chemical change, atomic bonds must be broken and/or formed (a new substance is present).

Learning Target:

“I can distinguish between chemical and physical properties and changes.”

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Materials:

- Copies of student handouts included in this product - Enough un-popped popcorn kernels to give each group of four in each class 10 – 20 kernels for the purpose of student examination - Enough popcorn to make a couple of cups of popped corn per group of four. Teacher tip: Use microwave bags of popcorn; you will get a faster, more plentiful, and easier result from this than you will trying to pop the corn on a stove top. I used two bags per class of 30 – 33 students and brought my portable microwave into class. If you do not have easy access to a microwave near your classroom, pop ahead of time and bring in, or buy bags of popped corn. Popping it in class is ideal, as it adds to the impact of the lesson. This way, students also might discuss the change in odor (a sign of a chemical change), and whether that odor is actually from the popcorn itself or not (it’s primarily from the butter/oil/flavor mixture). - Paper Towels - Magnifying glasses for groups (optional) - One or two wooden mallets per group (optional) - Internet access and an overhead projector where you can show a video from the Internet to the whole class - Chart paper or marker/smart board and markers Procedure:

1. Prior to this lesson, students should have basic knowledge of what physical and chemical properties and changes are. Included in this product is a notebook foldable that you may have students complete to help them learn these concepts. There will most likely not be time to complete the foldable and this lesson in one class period, so it should be done prior to this lesson. Main information that students should write in the properties foldable includes:

Physical: Can be observed without changing the substance - Shape - Color - Size - Luster - Weight - Malleability -Volume - Conductivity - Density - State of matter - Length - Width

Chemical: Describes the substance’s ability to change or react - Acidity - Flammability - Toxicity - Oxidation state

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Main information students should write in the changes foldable includes:

Physical: A physical property is changed, but the substance keeps its identity - Changes of state (melting, freezing, evaporating, condensation) - Dissolving - Tearing -Breaking

Chemical: Atomic bonds are broken and/or formed, making a new substance - Rusting -Signs include: change - Tarnishing in odor or color, - Digestion bubbling, change in - Decay temperature - Burning

2. Pose this question to the class: “Is popping popcorn a chemical or a physical change?” You might do this by writing the question on the board or chart paper and having students respond on Post-it notes.

3. Formative Assessment Strategy (Beginning): After students have

had time to contemplate their answers on their own, have students who believe it is a chemical change stand on one side of the room, while students who believe it is a physical change stand on the other side. Allow students to talk with their own cohort for one minute about why they agree with each other. Then, instruct students to find people on the opposite side of the room with whom to have a friendly debate. Allow a few minutes for students to mingle and discuss, then ask them again to go to either the chemical or physical side of the room. Be sure to emphasize that they may have changed their minds based on their conversations and can change sides of the room accordingly. Record the number of students whom are standing on each side of the room and display those numbers somewhere in the room so that students may remember their original thinking as they progress and make discoveries during this activity.

4. Provide students with the video observation handout chart. Have students record information in the chart while you show them the following video clip from YouTube (Slo Mo Guys, “Popping Popcorn in Super Slow Motion”): http://www.youtube.com/watch?v=p_7Qi3mprKQ

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5.  Allow students a few minutes to record all of their observations in the chart. Then, ask for them to share what they observed. Likely student observations are that it appears that the kernel is turning inside out, or “blooming.” It is helpful here to turn the video back to both the 2:12 and the 2:50 mark and ask students to observe even more closely. At both of these points in the video, you can see that as the kernel pops, a burst of steam comes out from the kernel. This indicates a change of state (liquid moisture inside the kernel turning to a gas), which is a physical change. Do not blatantly state this to students, but allow them to make further observations based on viewing those two segments more closely.

6. Have students share their inferences from their charts with each other in table groups or shoulder/elbow partners.

7. Record the questions that they have as a class on the marker/smart

board or on chart paper so that their thinking is displayed and can be referred to again during the lesson.

8. Formative Assessment Strategy (Middle): Have students repeat the

opening formative assessment activity with the same question, “Is popping popcorn a chemical or physical change?” This time just have them take a side of the room, since discussion was just had. Stress that it is okay to fluctuate and change minds throughout the activity. Record their thinking again so that it is visible to the class.

9. Give each table group or group of four 10 – 20 un-popped kernels

and the “Physical Properties of Popcorn Before and After” student handout. (I pour the kernels in a plastic cup ahead of time and set at each table.) Let students examine the kernels and write down their physical properties. Optional but highly recommended: Obtain a wooden mallet (or other utensils) for each group and magnifying glasses and have students carefully crack open the kernels to examine the insides. Make sure they record all observations.

 

10. Pop the popcorn. Place a paper towel in the middle of each table and pour about two cups per group of four. Instruct students that they may examine the popped pieces and record their observations on their handout. When they are finished, they may eat the popcorn (barring any allergies).

11. After several minutes, pick student helpers to clean up.

12. Students should reflect by completing the “signs of change” handout. Discuss responses. During this discussion, be sure to emphasize that the product of a chemical change is a new substance. Let students arrive at the conclusion that the popped pieces of popcorn have the same substances as the kernels.

13. Formative Assessment Strategy (End): For the final time, have

students choose a side of the room – physical change or chemical change – to stand on. Record the number of students on each side in the same area of the room where you have displayed the numbers for the beginning and middle formative assessments. If any students are left on the “chemical” side, allow students to discuss and debate. Students should be arriving at the conclusion that this is a physical change, as the popped pieces contain the same substances as the kernels and no new substance is created. Again, the idea here is that the moisture and air inside of the kernel has become heated and the particles began to move so quickly that they escaped the kernel and forced the enamel of the shell open. The moisture inside the kernel becomes so heated that it turns to a gas, and the starch inside the kernel expands and becomes fluffy in this process. This is a state change, which is a physical change.

14. As part of a closing discussion, ask students why this was a hard

problem to solve. Their responses should indicate that, although this was a physical change, there appeared to be some signs of a chemical change that made it confusing. Those signs include the need for heat in this change, the change of temperature of the kernels, the apparent change of color from the kernel to the popped piece, and the smell. Stress that although some of these signs were present, no new substance was formed, and no bonds were broken or created in the process.

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15. Ask students: “How is a burnt piece of popcorn different than a regular piece?” Students should arrive at the conclusion that a burnt piece has a new substance – ash – while a regular piece of popcorn does not. The burnt piece has undergone a chemical change while the regular piece has not.

Student Handouts on Following Pages Teacher Tip: Students get all papers and secure them in their notebooks

as a warm up for class.

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Rebecca’s Resources on Teachers Pay Teachers

Please cut out the two foldables along the bolded solid lines. Then, cut between the flaps on the dotted lines. Glue the back of the top title strips in your notebook,

leaving the flaps unglued. You will take notes under the flaps on your notebook paper.

Chemical And Physical Properties

Chemical Physical

Chemical And Physical Changes

Chemical Physical

 

Popcorn: Chemical or Physical Change?

As you watch the video clip, complete the following chart:

What I observe (be detailed!):

What I infer (my thoughts about what I observe):

What questions I have:

 

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Chemical or

Physical?

 

 

 

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Physical Properties Of Popcorn Before And After Being Popped!

List all of the physical properties you can of a popcorn kernel and a popped piece of popcorn

in the chart below:

Kernel Popped Piece

Chemical or

Physical?

 

Signs of Change

1. What are some signs of a physical change? List them below: ____________________________________________________________________________________________________________________________________________________________________________________

2. Did you see any of these physical change signs with the popping of the popcorn? If so, what? ____________________________________________________________________________________________________________________________________________________________________________________

3. What are some signs of a chemical change? List them below: ____________________________________________________________________________________________________________________________________________________________________________________

4. Did you see any of these chemical change signs with the popping of the popcorn? If so, what? ____________________________________________________________________________________________________________________________________________________________________________________

 

 

 

 

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