Decomposing Copper Sulfate Pentahydrate

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SWBAT explain in terms of a chemist the decomposition of Copper Sulfate Pentahydrate

Big Idea

Explaining our observations when molecules decompose.

The Need for This Lesson

This lesson is the second in a series to introduce basic chemistry to middle school students.

Chemistry, especially the bonding of elements to create new substances, is an abstract concept for students still deeply seeded in concrete thinking. This decomposition lesson creates a concrete, visible separate of chemical bonds as the color changes from blue to white and the water condenses on the test tube

Students use a flame to break chemical bonds. 

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

5 minutes

Using heat, students will be able to see the atomic composition of Copper Sulfate Pentahydrate. (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 that the Copper Sulfate Pentahydrate has decomposed (SP4 Analyzing and Interpreting Data). Their evidence is derived from their careful observations (SP7 - Engaging 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.)

Here is a link to the MSDS for Copper Sulfate Pentahydrate to meet your school's safety protocols and systems.

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

Students in Action

35 minutes

This lesson begins with students activating prior knowledge. In the Turn/Talk/Record section students work with their elbow partner first to examine closely the chemical formula for the Blue Copper Sulfate crystals. Students will be learning to balance equations in a future lesson so taking the time to examine the quantity of each of the elements will help lay a foundation for future learning. I have included a drawing of the formula so students can check their element count.

Blue copper sulfate crystals have the formula: CuSO4•5H2O.

Write the number of each element in the formula

____Cu    ____S    ____O    ____H

In this decomposition lesson, students will heat the Blue Copper Sulfate Crystals to remove the water. Students are asked to recall their understanding of hydrate / dehydrate we examined in the Decomposing of Sucrose lesson.

Copper sulfate pentahydrate has water molecules as part of the chemical formula. So as the name implies, we have hydrated copper sulfate. If copper sulfate pentahydrate is hydrated then what can we say about it if we remove the water? 

What is the first sign that you are dehydrated? Students know from health class or sports that the first sign of dehydration is thirst and they should hydrate by drinking water. This connection to self is a hook to background knowledge. Students can build upon their understanding of new material by making a connection to what they already know.

Before students begin the procedure, we review their answers to the first three questions. We walk through the steps for the procedure and the safety concerns regarding the heating of substances in a test tube. 

I move around the room to light the candles and make sure that students are recording their observations. Safety is a high priority with my young chemists, so I am vigilant especially when students are handling the test tube over a flame.

Students are encouraged to attempt the chemical reaction table on their own. Students correct their work when we fill these out together after all students groups finish the procedure. The equation table is a great way for students to connect their visual observations with the language of a chemist.  

In this video, I explain how the lesson is setup for student success as we look look at a sample of student work.



Connecting the Learning

5 minutes

We completed the Decomposing Sucrose lesson just before this one so students were prepared for their aha moment.

We take time to review the reactant/products table at the end of the lesson. I particularly focus on the chemical equation and the drawing of the chemical equation.

What do you notice about the number of water molecules in the reactant and the product? The number of water molecules are the same in the reactant and the product.

What about the number of copper sulfate molecules? The number of copper sulfate molecules are also the same.

When chemist talk about balancing equations, what do you think they mean? Both sides of the equation should have the same number of elements. The equation should be balanced.

Even though we changed the appearance of the original substance, did we gain or lose any elements? No, elements are gained or lost.

What general statement can we make about elements in a chemical reactions? Elements are neither gained or lost during a chemical reaction.

We are working on not using "it" or other vague references when answering questions. Students should respond with the vocabulary used in the question. This strategy provides practice using the vocabulary of scientist.