“Cations, anions:
Test in the lab if
Their aqueous combo
Yields chemical ‘storm.’
(Charts can be voluble,
Re: rules insoluble.
Key to observe:
Does precipitate form?)”
The 26 October 2020 Twitter poem provided an overview of qualitative analysis, a classic chemistry lab experiment that builds on the concept of the precipitation reaction. It employs the pseudo-double-dactyl form increasingly commonly found in this space.
“Cations, anions: /
Test in the lab if /
Their aqueous combo /
Yields chemical ‘storm.’”
Ionic compounds consist of positively charged ions (cations) bonded to negatively charged ions (anions) through electrostatic forces: the attraction between opposite charges. The resulting compounds are classified as water-soluble or water-insoluble, depending on whether they dissolve in water. While water is polar and excellent at dissolving many ionic compounds (since its own partial charges can repel and attract the charges present in the ionic compounds), certain cations and anions are attracted so strongly to one another that the compounds they form do not dissolve in water.
In a typical lab experiment, students are given a series of “unknown solutions” (unidentified ionic compounds dissolved in water) and discern which elements are present in the unknowns, by combining the unknown solutions with known reagents.
Two water-soluble compounds [denoted by (aq), for “aqueous”] exchange their ions. If either “post-exchange” compound is then water-insoluble [denoted by (s), for “solid”], it forms a precipitate, as shown here [AD (s)]:
AB (aq) + CD (aq) → AD (s) + CB (aq)
The solid’s crashing out of solution is designated poetically as a “chemical storm,” describing the observed behavior via another precipitation definition.
“(Charts can be voluble, /
Re: rules insoluble. /
Key to observe: /
Does precipitate form?)”
Charts of solubility rules provide students with guidelines for which combinations of cations and anions form precipitates. Using these lengthy (“voluble”) sets of rules, along with their lab data, students predict what ions must have been present in the unknown solutions.
These experiments are termed “qualitative analysis” because they involve analysis by way of qualitative (non-quantitative/non-calculation-based) observations of the reaction: most simply, does a solid precipitate form or not?
Chemphasis 