Is it chemical or physical?

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Objective

SWBAT identify the difference between a chemical change and a physical change.

Big Idea

Demonstrations provide context for a lesson on physical or chemical change that helps students build their ability to make evidence-based arguments.

NGSS Standard Alignment

Performance Expectation/DCI

This lesson is aligned with HS-PS1-7, the uses of mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction; and DCI-PS1.B, the fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions. An important part of this DCI is to be able to recognizing evidence of chemical reactions based on MS-PS1-2.  In this lesson students will recognize evidence of a chemical reaction and differentiate between chemical and physical changes.

Science and Engineering Practices (SP)

This lesson will cover all aspects of the 5E model of inquiry.  The Explore portion of the lesson has students observe a demonstration and create a mental model (SP 2, Develop and using models) that differentiates between chemical and physical changes.  Students will also have to provide evidence to justify claims based on observations of a chemical and physical change.

Crosscutting Concepts (XC)

During this lesson students will recognize that energy and matter is an underlying crosscutting concept involved in all chemical reactions. During the Explain portion of the lesson students will receive notes that explain that chemical reactions can be endothermic and exothermic.  This is not the main focus of this lesson, but as students continue to develop an understanding of chemical reactions throughout the unit they will build on the idea that matter and energy are part of chemical reactions.

 

Engage/Explore

15 minutes

The first part of this lesson is a demonstration that has students make observations that differentiate between chemical and physical changes.  Demonstrations are an excellent way to help students create a mental model (SP 2) that can be used to connect present thoughts to future learning.

During the demo students will explore their understanding (prior knowledge) of different types of evidence that can explain whether a chemical or physical change has occurred.  Even though many students come with background knowledge from middle school about chemical and physical changes, most still struggle with providing scientific evidence that back their claims.  This demonstration helps students build their argumentation skills by asking them to justify their claims based on observations.

Demonstration

The demo I use to illustrate the distinction between physical and chemical change is mixing water with sugar and sulfuric acid with sugar- often referred to as “The Black Snake” demo.  This demonstration works well because students make the observation that water and sulfuric acid are the same substance because they are both clear and think nothing will happen.  The following steps will explain the materials needed and how I perform the demo.  Here is a YouTube video that can be shown if a hood is not present.

Safety:

  1. Wear goggles and lab coat
  2. Perform in fume hood
  3. Handle concentrated sulfuric acid with care
  4. Do not touch finished demonstration product with bare hands; it contains sulfuric acid!
  5. Wait several minutes before placing demonstration in neutralizing solution.
  6. Do not throw carbon waste product in the garbage can; it contain sulfuric acid.

Materials:

  • 2 150-mL Beaker (tall form)
  • Sucrose to fill Beakers about 1/3 full
  • 50 ml of conc. (18 M) Sulfuric Acid
  • 50 ml of water
  • Glass Stirring Rod
  • Large glass Dish for overflow
  • Fume hood
  • Gloves and Goggles

Procedure:

Do this demonstration in a fume hood and stand back when it starts to react.  The fumes given off are Sulfuric Acid.

  1. Fill two beaker about a 1/3 full of sugar and set aside.
  2. Have two beaker: one with 50 ml of water and the other with 50 ml of sulfuric acid.  Set aside.
  3. Add water to one of the sugar beakers and mix with glass stirring rod for 1-2 minutes.
  4. Ask them to make observations about what is occurring and think about questions they have.
  5. After stirring, set aside
  6. Add conc. Sulfuric acid to other beaker until it just covers the sugar and mix with the stirring rod. You may need to add a little more sulfuric acid-up to 50 ml.
  7. When you feel the beaker heating up, stop stirring and put the beaker down in the crystallization dish
  8. The reaction takes several minutes to start to fully react.
  9. While the reaction is occurring ask them to make observations about what is occurring and think about questions they have.
  10. Let students gently touch the beaker so they can feel the exothermic reaction starting.
  11. Result should be a nice column of graphite.
  12. While the reaction is taking place have a discussion about the two reactions.
  13. After the reaction is completed let it sit 30 minutes before putting in a neutralizing solution of water and sodium bicarbonate.  Acid neutralizer works too. 

Waste Management:

The graphite column is saturated with sulfuric acid. It should be placed in a bucket of water and the whole thing neutralized with sodium bicarbonate. Then it can be dumped down the sink. 

Discussion 

While I am doing this demo I am talking about the difference between chemical and physical changes. I don’t specifically tell them evidence, I expect them to tell me their observations which leads to evidence. 

Students typically have a lot of questions which I try to get them to answer through peer discussion.  I also ask them to think about how they are asking questions.  Are they open- or close-ended questions?  This helps reinforce what we worked on in the first 3 lessons in this unit.

Students' questions might be:

  1. What causes the snake to form?  This will be explained later by showing them the chemical equation.
  2. What did you add to the beakers?  It looked like water.  Sulfuric Acid and water.

After several student questions I ask them to state some differences between the two processes they observed.  They typically say the water dissolved the sugar and it disappeared.  Most students realize this is a physical change, but for the students that still struggle with this concept (which there are always a few) I ask them what would happen if I let this evaporate.   They then realize that a chemical change requires more evidence than just something dissolving (or disappearing) in solution.

This leads to conversation about the “Black Snake Demo”.  I ask them what evidence do you have that a chemical reaction occurred when I mixed the other clear substance with the sugar?

Student reponses:  Heat, smoke (water vapor), color change and a change in properties of the sugar.

These are all good responses which leads to providing notes about evidence of chemical reactions during the next portion of the lesson.

Explain

20 minutes

As student head back to their seats I handout the Chemical_Reactions Concept_Map they will use for notes. On the board I have the chemical reaction for the “Black Snake”;  2C12H22O11(s) + 2H2SO4(aq) +  O2(g) → 22C(s) + 2CO2(g) + 24H2O(g) + 2SO2(g).  I will use this as reference during the notes to illustrate that a new substance was formed that produces gas.

As they sit down I take volunteers to provide evidence of a chemical reaction.  As they are providing answers I fill in a blank graphic organizer (GO) (Chemical_Reactions_Concept_Map answer key) under the document cam.  I also provide examples of each type of evidence as shown in the answer key.  After all evidence and examples are provided, I ask what type of reaction gives off heat.

This leads to filling in the energy portion of the graphic organizer with exothermic and endothermic.  I add to this portion of the graphic organizer by providing information about energy in and energy out.

Finally, I ask for volunteers to provide information about chemical equations.  As they say the correct information I fill in the graphic organizer.  This provides a simple, organized way for students to review chemical equations.  It also acts as a review because some of the information they have seen in a previous lesson when they took notes on chemical equations.  

Elaborate/Evaluate

15 minutes

While students are completing the graphic organizer I handout the Physical and Chemical Changes Worksheeet for this portion of the lesson.  This will also be homework because we will not have time to complete it in class. 

To start this off I have students take a couple of minutes to answer questions 1-14.  After a couple of minutes I have them share their answers with the person next to them.  This stimulates good conversation and requires students to provide evidence to justify their claim (SP 7).  This is followed by providing the answers and justifying the answers if any students are still having difficulty after sharing with a classmate.

I continue this same process with each section till the bell rings.  Just before the bell rings I ask them to complete it for homework (student work).  The point value on this assignment is 10 points based on completion.  I only look for completion because I want students to think about chemical and physical properties, but it's not the main objective of this unit.  The students need to be aware of how observations play a role in determining a if a chemical reaction has occurred.