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:
Ca2+(aq) + CO32-(aq)à CaCO3(s) Ksp = 5 x 10-9
We can calculate the reaction quotient, Q, for a mixture of 1 M Na2CO3 (in the bottle) and the expected calcium concentration if all Ca2+ in the sponge is transferred to the sodium carbonate solution upon shaking. A total of 1 mL of 2 M CaCl2 added to a sponge equates to 0.002 moles of Ca2+ added to the sponge:
(2 mol Ca2+/ L) x 0.001 L = 0.002 mol Ca2+
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 Ca2+:
0.002 mol Ca2+ / 0.250 L = 0.008 M Ca2+
The reaction quotient, Q, that results from a mixture of 0.008 M Ca2+ and 1 M CO32- comes out to 0.008, which is greater than Ksp. Thus, a precipitate is expected to form:
Q = (0.008)(1) = 0.008 > Ksp
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 Na2CO3 with a hidden sponge that has been soaked in a concentrated solution of CuSO4. 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!