A Colorful Decomposition of a Carbonate

Demonstration Kit

Introduction

Challenge students to deduce the reactions responsible for the dramatic color changes observed when a sample of copper(II) carbonate is heated.

Concepts

  • Decomposition reaction
  • Properties of carbonates
  • Gas solubility

Materials

Bromthymol blue indicator solution, 0.04%, 10 mL*
Copper(II) carbonate, Cu2CO3(OH)2, 5 g*
Erlenmeyer flask, borosilicate glass, 250-mL
Hot plate
Rubber stopper, size 6, 1-hole*
Student worksheet*
Wine airlock*
*Materials included in kit. 

Safety Precautions

Copper(II) carbonate is slightly toxic by ingestion and inhalation. Turn off the hot plate immediately after demonstration to avoid burns or hot plate damage. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The powdered contents of the flask and any unused copper(II) carbonate may be disposed of according to Flinn Suggested Disposal Method 26a. The airlock solution may be disposed of according to Flinn Suggested Disposal Method 26b.

Prelab Preparation

  1. Insert the stem of the wine airlock into the size 6 1-hole rubber stopper until the stem is completely through the stopper. Note: Check to verify cover is off the top of the airlock.
  2. An optional student worksheet for testing student understanding of the demonstration is included. If using these worksheets, pass out copies of the worksheet to the students.

Procedure

  1. Add 5 g of copper(II) carbonate, Cu2CO3(OH)2, to a 250-mL Erlenmeyer flask.
  2. Add 10 mL of bromthymol blue indicator solution to the wine airlock.
  3. Attach the wine airlock rubber stopper to the flask (see Figure 1). 
    {13937_Procedure_Figure_1}
  4. Set the flask on the hot plate and begin heating the apparatus. Note: Put the heating level initially to a medium setting, then set higher or lower to adjust the rate of reaction. As the flask is heated, bubbles will be forced through the solution into the airlock.
  5. Upon further heating, the bromthymol blue solution in the airlock changes from blue to yellow.
  6. The powder in the flask also changes from green to black.
  7. Break the seal between the flask and the rubber stopper before removing the Erlenmeyer flask from the hot plate.

Student Worksheet PDF

13937_Student1.pdf

Teacher Tips

  • This kit contains enough copper(II) carbonate, 50 g, and bromthymol blue indicator solution, 100 mL, to perform the demonstration at least seven times.
  • Bromthymol blue is an acid–base indicator that is yellow in solution when the pH is <6.0, blue when the pH is >7.6 and various shades of green in the transition pH range of 6.0–7.6.
  • This lab can be used to test the students’ understanding of a decomposition reaction. What are the bubbles being produced? Why is the indicator solution changing color? Why has the powder changed color?
  • Do not perform this demonstration with calcium carbonate, CaCO3. The calcium oxide, CaO, produced is very reactive with water.
  • Make sure the bromthymol blue indicator solution starts out blue—if not, add a drop of 0.01 M NaOH.
  • The bromthymol blue solution may appear more orange than yellow at the end of the demonstration due to possible impurities.

Correlation to Next Generation Science Standards (NGSS)†

Science & Engineering Practices

Constructing explanations and designing solutions

Disciplinary Core Ideas

MS-PS1.B: Chemical Reactions
HS-PS1.B: Chemical Reactions

Crosscutting Concepts

Stability and change

Performance Expectations

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-PS1-6: Undertake a design project to construct, test, and modify a device that either releases or absorbs thermal energy by chemical processes.
HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.

Answers to Questions

Copper(II) carbonate [Cu2CO3(OH)2(s)] undergoes decomposition in this reaction.

  1. What are some observations that indicate a chemical reaction has taken place?
    • The color of the solid changes from green to black.
    • Gases are produced.
    • The gases cause a color change in the indicator solution.
  2. What are the bubbles being produced?

    Looking at the formula of copper(II) carbonate and knowing the reaction is a decomposition, the two most likely gases produced would be carbon dioxide CO2(g) and water vapor H2O(g).

  3. Why is the indicator solution changing color? The indicator is a blue color in a basic solution and yellow in an acidic solution.

    Some of the carbon dioxide bubbling through the solution dissolves.

    CO2(g)→ CO2(aq)

    The dissolved carbon dioxide reacts with water to form carbonic acid, a weak acid.

    {13937_Answers_Reaction_1}

    This weak acid dissociates in water to form an acidic solution.

    {13937_Answers_Reaction_2}

    This acidic condition produces the yellow color in the indicator solution.

  4. If the final solid product is CuO(s), write the balanced equation for this reaction. Note: See Questions 2 and 3 for additional products.

    Cu2CO3(OH)2(s) → 2CuO(s) + CO2(g) + H2O(g)

  5. If 5.0 g of copper(II) carbonate, Cu2(CO3)(OH)2, are reacted, how many moles of copper(II) oxide, CuO, are produced? (Assume a 100% yield.)

    Molar mass of Cu2(CO3)(OH)2 = 221.11 g/mole
    Molar mass of CuO = 79.54 g/mole
    Moles Cu2(CO3)(OH)2 reacted = 5.0 g/221.11 g/mole = 2.3 x 10–2 moles Cu2(CO3)(OH)2
    Moles CuO produced = 2 moles CuO/1 mole Cu2(CO3)(OH)2 x 2.3 x 10–2 moles Cu2(CO3) (OH)2
                                     = 4.5 x 10–2 moles CuO
    Grams CuO produced = 4.5 x 10–2 moles x 79.54 g/mole = 3.6 g

Discussion

When a compound undergoes decomposition, it breaks apart into two or more simpler substances. Copper(II) carbonate will decompose to form copper(II) oxide, carbon dioxide and water upon heating. Copper(II) carbonate does not exist as CuCO3 but as Cu2CO3(OH)2.

In this demonstration, an acid–base indicator (bromthymol blue) solution, held in a winemaker’s airlock, changes color as the carbon dioxide produced during the decomposition of copper(II) carbonate passes through the airlock. Furthermore, the greencolored copper(II) carbonate powder changes to the black copper(II) oxide.

{13937_Discussion_Equation_1}
Initially, as the apparatus is heated, the air in the flask is forced through the bromthymol blue solution in the airlock due to the air expanding upon heating. This will be observed as bubbles moving through the airlock.

As the apparatus is further heated, the bromthymol blue solution color changes from blue to yellow to indicate the presence of an acid. As the carbon dioxide gas formed from the decomposition reaction dissolves in the bromthymol blue aqueous solution, it forms carbonic acid.
{13937_Discussion_Equation_2}
This weak acid dissociates in solution to produce an acidic solution.
{13937_Discussion_Equation_3}

References

Special thanks to Borislaw Bilash II, chemistry teacher, Pascack Valley High School, Hillsdale, NJ, for providing Flinn Scientific with the instructions for this demonstration.

†Next Generation Science Standards and NGSS are registered trademarks of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.