Lesson Plan: Alien Invasion?

Summary

In this lesson, students will explore both physical and chemical changes as they try to help the Department of Homeland Security analyze and identify materials found in a mysterious capsule. Students will examine and observe an unknown object (a frozen water balloon) as it changes (melts). They then mix combinations of everyday substances to observe chemical changes.

Resource Type

Lesson plan

Grade Level

Elementary school

Objectives

By the end of this lesson students should be able to:

· Describe the difference between a physical change and a chemical change.

· Identify examples of physical changes and chemical changes.

Chemistry Topics

This lesson supports students’ understanding of the following topics in chemistry:

· States of matter

· Physical change

· Chemical change

· Solutions

· Observations

· Inferences

Time

Teacher Preparation: 15 minutes for each part of the lesson

Lesson: 2 or 3 classes, 45 minutes each

Materials

For each group:

Part A

· A balloon full of water, frozen

· Bowl

· Hand lens

· Ruler

· Thermometer

· Science journal

· Safety goggles

· Scale or balance

Part B

· Water

· Vinegar

· Seltzer tablets

· Salt

· Sugar

· Baking soda

· Food coloring

· Thermometer

· Test tubes, vials, or mixing containers

· Measuring spoons

· Stirring rods or spoons

· Safety goggles

· Science journal

Safety

· Students should wear goggles throughout the activity.

· Remind students that although they are mixing unknown substances in this activity, they should never on their own mix substances that are unknown.

· Clean up should be done carefully so that materials are not mixed together.

· Remind students not to put anything in their mouths.

Vocabulary Terms

· Chemical change

· Inference

· Observation

· State change

· Physical change

Keywords

chemical change, physical change, state change, solvent, solute

Teacher Notes:

· This activity can be done with students who already have some background with physical vs. chemical changes, or it can be modified to be an introduction to the topic.

· To engage students quickly in the scenario, you may wish to be dramatic. For example, students from another class could rush in to bring the news or the principal could cut into the class on the intercom to give students their instructions. You may wish to create a capsule for students to find the materials in.

· It is important not to tell students what any of the materials or substances are (for example, don’t refer to the frozen water balloon as a frozen water balloon).

Teacher preparation

Prepare the “alien” capsule for students to explore or for you to present.

· Freeze water in balloons (have enough for one balloon per group)

· Fill vials or small containers, each with one of the following: sugar, salt, baking soda, vinegar, water, over-the-counter antacid tablets (sodium bicarbonate), and any other safe substances from the kitchen. Label each vial of the same substance with the same symbol, letter, or number so students can identify which substance is which. There should be enough so that each group gets one vial of each substance.

Lesson

Engage

Begin by explaining to students that an object thought to be a space capsule has been found in the schoolyard. It contained a refrigerated compartment with large, round objects in it and several smaller unrefrigerated compartments with different vials of fluids. The Department of Homeland Security (DHS) wants to know what the contents are so they can figure out where the capsule came from and what it means. DHS has asked your class to help analyze the materials. They are looking for younger students that can think creatively and help to resolve the issue.

As a class, you need to answer the following questions:

Objects in the refrigerated compartment

a) What are the physical properties of the object in the refrigerated compartment?

b) What are the objects? What are they made of?

c) Are the objects in the refrigerated compartment changing? If so, are they undergoing physical changes or chemical changes?

Substances in the vials

a) What are the physical and chemical properties of the substances in the vials?

b) What happens when the substances in the vials are mixed with other substances?

c) If there is a change, is the change physical or chemical?

Challenge the class to come up with ways to answer the questions. Explain that DHS will require explanations for all of the answers. They will need to know what observations were made and what analyses were conducted, so it is important to take careful notes in the science journals.

Part A: Alien Frozen Material (Physical change investigation)

Explore

1. Put the frozen balloon in a bowl in the middle of each group of students and instruct them to examine it, describe it, and infer what it is. Explain to students that they MUST NOT destroy the object in any way. They should record their observations (something directly observable) and inferences (understanding that is not directly observed but is based on direct observations) so that they can compare with the other groups. As they are working, challenge students to think critically about their observations and inferences:

· What is this made of? What is on the outside? What is on the inside?

· What can you actually observe with your eyes, hands, ears, and nose?

· What are you inferring: what do you think you know based on your observations?

· Can you be 100% sure?

· What could you do to be 100% sure?

2. Have students continue to observe the object and make observations over time. You’re going to see if the object changes over time or stays the same. Guide students to observe using their eyes, ears, nose, and hands. Students should think about changes in temperature, flexibility, color, size, solidity, etc. Students may first observe condensation on the balloon. After a while, they may infer that the inside is melting.

· How is the object changing? [the inside is melting; the outside is warming up; the skin of the object is becoming more flexible]

· Where is the liquid on the surface coming from? [condensation from water vapor in the air]

3. Once students have made their observations, ask:

· Are these changes permanent or can they be reversed? What makes you think so? How can you prove it? Students may note that if the object is really melting inside, they should be able to put it in a freezer to make it solid again. They may not be sure how to reverse some other physical changes.

· As the object changes, is it changing into new substances? Or is it just changing form? Why do you think so? Students will likely infer that the object is just changing form.

Explain

Come back together as a class to discuss observations and inferences.

1. Create a chart with two columns: observation and inference, and challenge students to fill in the chart accurately. Students may at first have difficulty separating observations from inferences.

For example,

Observations / Inferences
Object is round.
Object is red.
Object is cold.
Outside is rubbery.
Inside feels hard.
Over time, outside is getting wet.
Over time, the object is getting warmer.
The inside of the object seems to be melting.
Over time, the outside of the object became more flexible and not as hard. / Object is a balloon.
Inside the balloon is ice.
The ice is melting.
The water on the outside is leaking out of the balloon.
The water on the outside is condensing from the air.

2. Discuss the properties that the students observed. Have them identify each as either a physical property (P) or chemical property (C). (All listed above are generally described as physical properties.)

3. Discuss the changes that the students observed. Were these chemical changes or physical changes? How do you know? (Were the changes reversible? Did the substances change into new substances?) Students may or may not know that physical changes are generally reversible and do not change the actual composition of the substance. Chemical changes are generally irreversible and do change the composition of the substance.

4. Move on to introduce or review the differences between physical and chemical changes. Have students think-pair-share to complete the table below. This will prepare them for Part B.

Physical Change Examples / Signs of a Physical Change / Chemical Change Examples / Signs of a Chemical Change
Melting
Freezing
Evaporating
Tearing
Mixing / Change can be reversed
The substance does not turn into a new substance / Burning
Rusting
Cooking
Rotting / A release of gas
Change in color
Change in temperature
Production of a new material

Part B: Mysterious Substances (Chemical Change investigation)

Explore

1. Explain to students that they will now investigate the substances found in the vials in the unrefrigerated compartment. DHS wants to know what happens when different combinations of the substances are mixed together. They want to know if chemical or physical changes occur.

2. Review what can happen when two substances are mixed. What are some signs that a chemical change has occurred? NOTE: Remind students that you are doing this activity because you know that mixing the substances will not result in a dangerous chemical reaction such as one that releases a lot of heat or causes an explosion. They should never mix unknown substances on their own.

3. As a group, decide how to conduct the investigation and how to record observations. Emphasis should be put on combining only two materials at a time. Quantities of each material should be limited. The number of materials tested will be limited by the length of the lesson. Instruct students to measure the amounts in each experiment so that it could be communicated and replicated.

4. You may want students to make tables like the ones below:

Substance Label / Description / What it might be
A / White grains / Sugar or salt?
B / Clear liquid / Water? Vinegar?
Substance #1 / Substance #2 / Observations: What happens when they are mixed / Did a chemical change occur?
Identity / Amount / Identity / Amount
A / ½ teaspoon / B / 1 Tablespoon / The white grains disappear / I am not sure; the grains could have just dissolved

5. Have students perform the investigations, carefully mixing one substance with another and making observations. As students are conducting the investigation, ask guiding questions such as:

· What happened?

· Why?

· Did you record your observations?

· If a temperature change is involved, did you take the temperature before and after the change?

· Could this have been only a physical change? Does it fit your definition of a chemical change?

· Can you reproduce your results?

· Does mixing different amounts create different results?

Explain

1. Have groups share their results, comparing results of similar mixtures. If the results vary drastically (e.g., a combination was found by almost every group to be a chemical change but one group had the opposite result) these differences should be examined. Were there flaws in the observations or procedure?

2. Have students further discuss the difference between physical and chemical changes. Which combinations cause chemical changes? How do you know? What other chemical changes can you reproduce in the lab? What other chemical changes exist that you have not talked about? Is wood burning a physical or chemical change? How about water boiling?

3. As a group, formulate a definition of chemical change and a definition of physical change, based on Parts A and B of the investigation and prior knowledge.

Elaborate

1. Have students prepare a presentation of their findings for DHS. The presentation can be in the form of a written report, an oral presentation, a video, or a multimedia presentation. It should include the following:

· Answers to the questions posed in Engage

· Evidence to support the answers to the questions

2. Have students research other examples of chemical changes or physical changes and present their findings to the class. Guide students to investigate something of interest to them (e.g., rockets, cooking, sports, plants, animals).

Evaluate

There are numerous opportunities for formative assessment within the activities, and summative assessment in Elaborate. In addition, you may want to use the following:

Multiple Choice Items

Which is the best evidence of a chemical change taking place in a substance?
The substance breaks into pieces.
A change to the substance can be reversed.
The substance changes shape.
A gas is released from the substance.*

Which is the best evidence of a physical change taking place in a substance?

The substance burns.
A different substance forms.
The substance changes state.*
A change to the substance cannot be reversed.

Open-Ended Questions

1. Why is heating water to form water vapor not considered to be a chemical change?
[Heating water to form water vapor involves a change of state, not a change to the substance. The water is still water, but in a different form It’s a reversible process—the water vapor can be collected and condensed back into liquid water.]

2. Why is it important to keep accurate and organized notes during a lab?
[It is important to be able to communicate clearly with other scientists and with other people who are interested in the results; it is important to be able to replicate the lab (do it exactly the same way again); if data are not recorded accurately, the scientist may draw the wrong inferences or conclusions]

Cross-Disciplinary Extensions

Connect to Math

· Have students weigh the object in the refrigerated compartment (the frozen balloon) at the beginning of the investigation and again at the end after it has melted partially or completely. Has the weight changed? Why or why not?

· Have students weigh the mixture components before and after mixing. Does the combined weight change? Why or why not?

· Have students convert weights or masses into other units of measurement (e.g., ounces into pounds; grams into kilograms).