Decomposing Sucrose

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Objective

SWBAT identify the elements that makeup sucrose (common table sugar).

Big Idea

Sugar is carbon and water?

The Need for This Lesson

This lesson is the first in a series that introduce basic chemistry to middle school students. It is set up to be a discrepant event that creates a need-to-know learning situation. 

Chemistry, especially the bonding of elements to create new substances, is an abstract concept for students still deeply seeded in concrete thinking. To imagine how carbon, hydrogen and oxygen can combine to form sugar is difficult.

Students use a flame to break chemical bonds. They will see for themselves the elements in sucrose.

This lesson was inspired by a now archived website, Armchair Chemistry*. I have formatted the lab to fit in a journal and modified the text as needed to support the learning of my students.

*Streitberger, Eric. Armchair Chemistry. 2015-02-15.  Accessed: 2015-02-15. 

Investigation Preparation & Summary

10 minutes

Students find it hard to believe that the sugar they see everyday is made-up of the elements, carbon, hydrogen and oxygen. By using heat, they will be able to see the atomic composition of sucrose. (MS-PS1-1 Develop models to describe the atomic composition of simple molecules and extended structures.)

Students will notice changes in appearance indicating that a chemical reaction has occurred. (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.)   

Questions will be asked to encourage students to analyze their results as they explain how they know sucrose is a carbohydrate. This is an aha moment for students. They use the word carbohydrate frequently without thought to what the word or word parts mean. (SP4 Analyzing and Interpreting Data). Their evidence is derived from their careful observations (SP7Engaging in Argument from Evidence).

The lesson walks students through the the chemical equation where they see that matter may be changed but mass is conserved. (MS-PS1-5 Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.)

The materials list is written for small student groups. This lesson could also be done as a demonstration.

Students in Action

35 minutes

This lesson begins with activation of student background knowledge. Students Turn/Talk/Record their answers to a series of questions designed to develop a simple definition of carbohydrates as hydrated carbons and discover sugar to be a hydrated carbon.

  • What does it mean to be dehydrated? 
  • What does it mean to be hydrated? 

 

Students should be able to answer these questions easily.  Next students are asked to apply their background knowledge to define carbohydrate.

 

  • How would you define carbohydrate? Hint – we might also say that sugar (sucrose) is a hydrated carbon.
  • Some of the sweet treats we like, cookies and cakes, contain carbohydrates.  What happens when we leave cookies or cakes in the oven too long? Describe what they look like. Have you ever roasted marshmallows? What happens to this carbohydrate when you roast it? What does it look like if you roast it too long? The goal here is for students to see that heating the marshmallow causes it to become dehydrated. The blacken substance remaining is the carbon.
  • Next students are introduced to the chemical formula - Sugar (sucrose) has the chemical formula of C12H22O11. A chemical formula is a way to write the atoms making up a molecule. The C12 is Carbon and the H22O11 can be reduced to H2O or water. Why is sucrose a carbohydrate? Students are skeptical that there is water in sugar even though they clearly see the chemical formula has Hydrogen and Oxygen C12H22O11.

 

I give students a few minutes to complete these questions with their elbow partner, then ask for student volunteers to share their answers aloud. I want to insure that students have been able to apply their background knowledge and extend their thinking to define carbohydrates and set the stage for their observations as they decompose sucrose at the start of the lesson.

 

Activating background knowledge is a great strategy to use when introducing new content.

Before students start their procedures, I casually ask what is the color of sugar. All my students answer, "white". Anticipating this answer, I have setup several microscopes with a slide of sugar and invite students to take a moment to check. They are surprised that sugar is like a crystal and it is clear. I explain that light is scattered by the imperfections of the sugar crystals making them appear white. Another discrepant event for students.

I keep a collection of homemade slides ready for student exploration. These are recycled file folders. I punch a hole in the center and then attach a piece of high quality clear tape. The sugar, salt and other dry materials stick to the tape. 

I also ask students, "What color do you associate with water and carbon?" Students answer as expected that water is clear and carbon is black. This questioning create a bit of cognitive disequilibrium with my students as they question how sugar can be made from carbon and water.

Students are reminded to put on their goggles and we review the procedures. I remind them that I will light the candle and show them how to gently extinguish the flame at the end of the procedure. We review our classroom safety procedures.

 

In this video, you will see the students using the tea candles for the decomposition of sucrose. 

What students should observe is the steam rising from the heated sucrose. Sometimes students confuse the steam with smoke so I make sure I walk around during the procedure to make sure they are seeing steam. I am always amused at the startled look on their faces when I ask them for the chemical formula for steam. Some students quickly answer H2O while others need to be prompted to think of steam as a form of water.

Since this is our first lesson on chemistry, I walk the through the rest of the questions with the students. I have no expectation that they will have experience with chemical equations or how to draw the chemical elements. The table helps students see multiple representations of their procedure designed to help them transition to thinking like a chemist.

Connecting the Learning

10 minutes

We take a few minutes at the end of the lesson for reflection. This lesson was a discrepant event for students on two levels. One that sugar is composed of carbon and water. The other that sugar is not actually white. 

What surprised you about this lesson? Students will acknowledge that they were surprised to see that you could decompose sugar into water and carbon.

Any other surprises? Yes, sugar is not really white!

What do you know now about chemistry that you did not understand before this lesson? I know what carbohydrate means. You can decompose sugar into carbon and water. When the state of matter changes its chemical formula does not change. The chemical formula for steam is the same as water. I know the chemical symbol for carbon. 

What are you wondering now? Does heat always decompose substances? Is salt really white?

For this lesson I elected to have an oral discussion. With other lessons, students will reflect in their science journal using the same prompts - what surprised you about this lesson, what do you know now that you did not know before this lesson and what are you wondering now.