In Chemical Mystery #16 the colors red, white, and blue are all produced from the same solution that is poured into three separate bottles. You can view this experiment and how it is carried out in Video 1.

**Video 1:** Tommy Technetium YouTube Channel, Published 6/11/19 (accessed 6/12/19)

This Chemical Mystery can be tied to a quantitative discussion of aqueous chemical equilibria salt solubility. The white color produced in the second bottle appears as a result of the reaction between calcium ion (contained in the sponge soaked in 2 M calcium chloride) and carbonate ion (from sodium carbonate solution) to form calcium carbonate, a white solid:

**Ca ^{2+}_{(aq)} + CO_{3}^{2-}_{(aq)} à CaCO_{3(s) } K_{sp} = 5 x 10^{-9}**

We can calculate the reaction quotient, Q, for a mixture of 1 M Na_{2}CO_{3} (in the bottle) and the expected calcium concentration if all Ca^{2+} in the sponge is transferred to the sodium carbonate solution upon shaking. A total of 1 mL of 2 M CaCl_{2} added to a sponge equates to 0.002 moles of Ca^{2+ }added to the sponge:

**(2 mol Ca ^{2+}/ L) x 0.001 L = 0.002 mol Ca^{2+}**

About 250 mL of 1 M sodium carbonate is added to each bottle during this experiment. If all 0.002 mol of the calcium in the sponge is transferred to the solution of sodium carbonate by shaking, this would result in a 0.008 M solution of Ca^{2+}:

**0.002 mol Ca ^{2+} / 0.250 L = 0.008 M Ca^{2+}**

The reaction quotient, Q, that results from a mixture of 0.008 M Ca^{2+} and 1 M CO_{3}^{2-} comes out to 0.008, which is greater than K_{sp}. Thus, a precipitate is expected to form:

**Q = (0.008)(1) = 0.008 > K _{sp}**

The “secret” of saturating small sponges with various solutions and then stuffing the prepared sponges into bottle caps allows for a wide variety of modifications on this chemical mystery. For one simple possibility, consider the mixing of a solution of Na_{2}CO_{3} with a hidden sponge that has been soaked in a concentrated solution of CuSO_{4}. Might this form a blue precipitate? What other colors could be produced and types of chemical reactions be demonstrated using this “secret” procedure? I’d love to hear of different combinations that you and your students might suggest – or better yet try out on your own!

Happy experimenting!

## Safety

### General Safety

General Safety

**For Laboratory Work:** Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).

**For Demonstrations:** Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.

#### Other Safety resources

RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

## NGSS

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.

Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.

Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses to using algebraic thinking and analysis, a range of linear and nonlinear functions including trigonometric functions, exponentials and logarithms, and computational tools for statistical analysis to analyze, represent, and model data. Simple computational simulations are created and used based on mathematical models of basic assumptions. Use mathematical representations of phenomena to support claims.

Students who demonstrate understanding can 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.

*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.

Students who demonstrate understanding can 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.

Assessment is limited to chemical reactions involving main group elements and combustion reactions.

Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.

Students who demonstrate understanding can refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.

*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.

Students who demonstrate understanding can refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.

Assessment is limited to specifying the change in only one variable at a time. Assessment does not include calculating equilibrium constants and concentrations.

Emphasis is on the application of Le Chatelier’s Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level. Examples of designs could include different ways to increase product formation including adding reactants or removing products.

Students who demonstrate understanding can use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.

Students who demonstrate understanding can use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Assessment does not include complex chemical reactions.

Emphasis is on using mathematical ideas to communicate the proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Emphasis is on assessing students’ use of mathematical thinking and not on memorization and rote application of problem - solving techniques.

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## Comments 1

## Thank you.

Tom - As always...you are the master! Love the demonstration. Looking forward to saying hellow at ChemEd. Please keep them coming.