In Chemical Mystery #17, shavings of Dial soap are added to samples of Aquafina and Evian drinking water. Upon blowing into each mixture with a straw, only the Aquafina water produced stable bubbles. Why is this the case?

To understand the difference, first consider that Aquafina water is highly purified, so it contains almost no dissolved ions.¹ On the other hand, Evian water contains dissolved Ca²⁺ (80 mg L^-1) and Mg²⁺ (26 mg L^-1) ions.² It is the presence of these ions in Evian drinking water that prevents the formation of bubbles.

Dial soap is made in part of sodium laurate (NaC₁₂H₂₃O₂),³ which is a soluble salt of a fatty acid. Sodium laurate dissolves readily water, forming free sodium ions and free laurate (C₁₂H₂₃O₂^-) ions:

NaC₁₂H₂₃O₂(s) g Na⁺(aq) + C₁₂H₂₃O₂^-(aq)          Eq. 1

So mixing soap shavings with water causes C₁₂H₂₃O₂^- to become dissolved in the water; it is the freely dissolved C₁₂H₂₃O₂^- ions that support bubble formation upon blowing into soapy water through a straw.

Many doubly-charged ions react with C₁₂H₂₃O₂^- to form a precipitate. Consider, for example, the reactions between Ca²⁺(aq) and Mg²⁺(aq) and C₁₂H₂₃O₂^-(aq):

Ca²⁺(aq) + 2 C₁₂H₂₃O₂^-(aq) g Ca(C₁₂H₂₃O₂)₂ (s)      Eq. 2

Mg²⁺(aq) + 2 C₁₂H₂₃O₂^-(aq) g Mg(C₁₂H₂₃O₂)₂ (s)    Eq. 3

Because Evian water contains both Ca²⁺(aq) and Mg²⁺(aq), any C₁₂H₂₃O₂^-(aq) that dissolves into Evian water forms Ca(C₁₂H₂₃O₂)₂ and Mg(C₁₂H₂₃O₂)₂ precipitates. Thus the C₁₂H₂₃O₂^- does not remain dissolved in Evian water, which prevents bubble formation. On the other hand, the absence of these ions in Aquafina water means that C₁₂H₂₃O₂^- remains freely dissolved and available to support bubble formation.

In the video below you can view some experiments that further demonstrate these explanations, and also provide a suggestion or two for further investigation:

Video 1: Tap Water vs. Distilled Water: Sudsing action, Tommy Technitium YouTube Channel, 4/23/20.

These experiments can be related to the concept of solubility of inorganic salts. For example, the reverse of Equation 2 may be written in a form that is familiar when doing K_sp calculations:  

Ca(C₁₂H₂₃O₂)₂ (s)  g Ca²⁺(aq) + 2 C₁₂H₂₃O₂^-(aq)              Eq. 4

K_sp = [Ca²⁺][C₁₂H₂₃O₂^-]² = 6 x 10^-13                                       Eq. 5 (Table 1)

The low value of K_sp for Ca(C₁₂H₂₃O₂)₂ allows for the prediction that even at low concentrations of Ca²⁺ and C₁₂H₂₃O₂^-, precipitation is likely to occur. For example, if enough NaC₁₂H₂₃O₂ dissolves in Evian water to form a 0.001 M solution of NaC₁₂H₂₃O₂, will precipitation occur?

First we recognize that 80 mg Ca²⁺ L^-1 (the concentration of Ca²⁺ in Evian water) corresponds to a 0.002 M Ca²⁺ solution:

Using the reaction quotient, Q, associated with Equation 5, it is calculated that a saturated solution of Ca(C₁₂H₂₃O₂)₂ results under the stated conditions:

Q = [0.002][0.001]² = 2 x 10^-9            Q >> K_sp

Therefore, it is no surprise that Ca(C₁₂H₂₃O₂^-)₂ precipitate forms when Dial soap is mixed with Evian water. In the case of Aquafina water, [Ca²⁺] ~ 0. Therefore, in this case:

Q = [0][0.001]² = 0     Q <  K_sp

 And no precipitate is expected.

Bar soaps are comprised of sodium salts of a variety of fatty acids. K_sp values for calcium salts of some fatty acids are listed for the interested reader in Table 1. I was unable to find K_sp values for the corresponding magnesium salts. I would be very interested if someone could alert me to a source that lists K_sp values for magnesium salts of fatty acids or additional calcium salts of fatty acids. However, what I am really interested in is results from variations on this experiment. What other brands of bottled water produce bubbles – or not – when conducting this experiment with Dial bar soap? What happens if you use Ivory bar soap? Do you get the same results when you use dish detergent? If you and your students find something interesting, please let me know in the comments.

Happy experimenting!

Table 1: K_sp values for various calcium salts of fatty acids.⁴

References:

1. https://www.aquafina.com/en-US/faq.html

2. https://www.evian.com/en_us/what-is-spring-water/water-attributes/

3. https://www.in.gov/fssa/thehub/files/dial%20bar%20soap%20with%20vitamins.pdf

4. Peter R. Garret, The Science of Defoaming: Theory, Experiment, and Applications. 2014, CRC Press, Boca Raton, FL, pp. 243-244.