Talk About Burning Your Money! (Phase Changes, Endothermic and Exothermic Reactions)

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Objective

Students will be able to describe the changes in the motion of the particles, temperature, and state of matter of a substance when thermal energy is added or removed.

Big Idea

Students investigate endothermic and exothermic reactions with dry ice!

Introduction and Connection to the NGSS and Common Core

In this lesson, students are introduced to endothermic and exothermic reactions and phase changes. Students note patterns in thermal energy changes by watching a dollar bill be "burned" and dry ice demonstrations!

This lesson is designed to address the following NGSS and Common Core standards:

MS-PS1-2  Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.

MS-PS1-4  Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.  

CCSS.ELA-LITERACY.RST.6-8.9  Compare and contrast the information gained from experiments, simulations, video, or multimedia sources with that gained from reading a text on the same topic.

Science and Engineering Practices:

In reading and completing the "Let's Get Physical With Phase Changes" document, students use strategies to obtain scientific information and evidence from text (SP7).  In addition, during class discussion, students back up their explanations in the lab document with evidence from their qualitative observations (SP8).

Crosscutting Concepts:

When students discuss what they saw in the lab in comparison to what they read in the text, students can begin to see patterns in the way that the state of matter and the molecular arrangement can change when thermal energy is added or removed. Thus, students can begin to see that macroscopic patterns are related to the nature of microscopic and atomic-level structure (Patterns).  

In creating patterns of energy transfer that occur in both phase changes and endothermic and exothermic reactions, students track how energy moves during various phase changes (Energy and Matter).

Connecting to the Essential Question: What are you going to learn today?

5 minutes

Ask students, "What are you going to learn today?".  Students should respond by saying that they will be answering the Essential Question, "How do particles combine into new stubstances? And, what evidence can show how the physical and chemical properties of the substances change?"  This EQ is posted on my board and on the student's Chemistry Unit Plan.  

Explain that the focus of this lesson is to relate to Skill 4, "I can identify the physical and chemical properties of the reactants and products in a reaction."  (It is important to note that my students have already been introduced to physical and chemical properties before this lab.  The notes sheet along with some true/false statements relating these notes along with the answer key is included in the resources.)  Let students know that this lesson specifically is targeting how adding and removing thermal energy can change the physical properties of substances such as state of matter, volume, density, and temperature.

As students went through a series of lab stations in the previous lesson, ask students to change their mastery score on their Chemistry Unit Plan if they feel it is necessary.  Notice in the picture below that the student has self-assessed on all skills and adjusted her mastery score (on a scale of 1 - 4, with 4 being mastery).   For a look into what students completed in the previous set of lab stations in which they were introduced to physical and chemical properties, check out this Crack That Marble! Lesson.

Mini Lesson: Thermal Energy and Phase Changes

25 minutes

Ask students to read Let's Get Physical (With Phase Changes) and complete the sheets that follow. In doing this, students can begin to see patterns between the change in molecular motion and whether a reaction is endothermic or exothermic.  They begin to see that if a phase change causes molecules to move faster after the change, that it is endothermic and that if the molecules slow down after the phase change, it is exothermic.  

I like starting discussions of endothermic and exothermic changes with phase changes so students realize,  in later lessons, that these reactions can be chemical or physical changes.  By completing this reading, students notice patterns in how adding and removing thermal energy affects phase changes and other physical properties.  

One key to discussing this with your students is to have students reflect on how the reactions will "feel".  For example, if a reaction is endothermic, it must take energy "in", thus making the surroundings feel colder.  If a reaction is exothermic, it must have energy "exit", thus making the surroundings feel warmer.  Furthermore, it is important to note that "warmer" and "colder" are words used to compare two temperatures.  They do not mean "cold" and "hot".  It simply means that the temperature changed and is now higher or lower than the original temperature.

Exothermic Reactions: Talk About Burning Your Money!

15 minutes

You and any student involved in this demonstration must wear goggles.  I start this demonstration by saying, "When I was your age, my parents were always advising me on how to spend my money.  They said, don't burn your money all in one spot.  So, I thought what would happen if I did burn my money?  I will dip this bill in a beaker of rubbing alcohol and light it on fire."  After lighting the bill, the bill is left untouched and it actually feels cold!  I actually let the students touch the bill to feel that it has decreased in temperature.

It is only at this point that I let students know that I had mixed equal parts of water and rubbing alcohol.  I explain that this is not something to ever repeat at home.  The mixture of rubbing alcohol and water has to be "perfect" in order to remain safe.  Without the right amount of water, the dollar bill would burn.  Knowing that, have students think about why adding water to the mixture keeps the dollar bill intact.

Students should eventually determine that the water evaporates as the rubbing alcohol burns. When the water evaporates it takes energy in from the dollar bill in order to burn, thus lowering the temperature of the dollar bill.  Evaporation is an endothermic reaction that makes the surroundings (the dollar bill) feel colder.  Also, we discuss that the fire itself was exothermic as it was giving off thermal energy.

Endothermic Reactions: Dry Ice Demonstrations

30 minutes

Dry ice is easily purchased at stores such as Meijer and is a great opportunity to discuss phases changes and endothermic and exothermic reactions.  One way to find a store near you that supplies dry ice is to Google “buy dry ice”.  You will notice that different brands of dry ice have their own website that can help you find a store that sells it.  For example, Penguin Brand Dry Ice has a website that locates the dry ice supplier closest to you. 

Safety:

Do not touch dry ice with bare skin.  Let students know if any dry ice comes near them, to resist the urge to reach out and touch it.  Ask students to keep their distance and explain that if there is an opportunity to touch anything safely, you will specifically tell them to do so.

After each of the following demonstrations, I ask students the same line of questioning:

  • What properties changed?
  • What phase change(s) are occurring?
  • Are these phase changes endo- or exo- thermic?
  • What evidence do you have to support this?

Demonstration 1:  Dry Ice In a Balloon

To start, place some dry ice in a balloon and tie it off.  Throughout the rest of the demonstrations, students can observe the balloon expansion.  Then, at the end of the demonstrations, drop the dry ice balloon at the same time you drop a balloon full of air to determine if dry ice is more or less dense than air.

Demonstration 2:  Bubbles in a Bucket

Make a mixture of dish soap, water, and a little corn syrup.  Cut up an old t-shirt to make a string that is longer than the width of the bucket you are using.  The bucket needs to be clean!  Dip the shirt in the soap mixture, hold it with tension, and run it over the bucket to "seal" the bucket with a bubble.  This may take a few tries!  Practice before you do it with students!

 

              

Demonstration 3:  Staircase of Carbon Dioxide

Place about 2 inches of water. in an aquarium (15 gallons or larger).  Then, turn over three different sizes of beakers and place them upside down in the water.  Place a votive candle on top of each.  Light all three votive candles.  Drop large chunks of dry ice in the water.  The carbon dioxide will rise gradually, causing each candle to be "put out" in sequential order as the carbon dioxide level rises and smothers the flame.

Demonstration 4:  Blowing Bubbles

Blow bubbles so that they land on top of the carbon dioxide gas produced in the previous demonstration in the aquarium.  This may take practice, but it is worth it!  The bubbles float!

Demonstration 5:  Film Canister Rockets

Fill a film canister 3/4 full of water, place dry ice in the canister, and quickly place the lid on tightly.  The cap will shoot to the ceiling!  For fun, repeat, but turn the canister upside down....get ready for a mess!

Demonstration 6:  Carbon Dioxide Bubbles

In a large graduated cylinder, add a water and dish soap mixture.  Then, drop in a large chunk of carbon dioxide.  The bubbles are full of carbon dioxide and are really cool when popped!

Closure

5 minutes

Two of the Crosscutting Concepts that the students have been working on as they interact with text and engage in discourse are "Patterns" and "Energy and Matter".  Students have been learning that scientists are constantly searching for patterns in phenomena as well as in data.  In addition, scientists track the movement of energy through systems.  Before having students discuss the question that follows, I revisit these Crosscutting Concepts so that they are fresh in their minds before having the discussion.

After completing the "Let's Get Physical With Phase Changes" worksheet and watching the "Talk About Burning Your Money" and dry ice demonstrations, ask students to search for any patterns they notice with endothermic and exothermic reactions and the shifts in energy that take place in those reactions.  Have students discuss this in table groups and then share their conclusions with the whole group.