I sometimes wonder why some call precipitation reactions "double decomposition". It is a term I was brought up with in the 1960s. I note that it is still very common in the States. I suppose two (double) salts are sort of splitting apart (decomposing?) and then reforming with other radicals. But a solvent (usually water) is necessary to achieve the desired effect. But is adding water to a salt really decomposition? After all, the ions in salt are surrounded by ions of opposite charge and in solution the ions are surrounded with water molecules (solvation). So there has been bond breaking and reforming but no change in the identities of the ions. If I heat wood to get charcoal, then that really is decomposition.

Keith Taber in his online article entitled "Conceptual confusion in the chemistry curriculum: exemplifying the problematic nature of representing chemical concepts as target knowledge" (click here to access the online article) dissects part of the English National Curriculum on Chemistry. In this article he discusses the idea of precipitation. Taber writes, "...precipitation can be understood to be a kind of shift in the electron distribution around the ions as they form into a lattice". Dissolving, of course, is a kind of shift in the electron distribution around the ions as water surrounds the ion, aka solvation. The system reaches a dynamic equilibrium when the potential energy reaches a minimum. This is the "solubility product", which is no longer in our A level curriculum. So I thought I would challenge myself to see if I could turn the world upside down by precipitating sodium chloride from two Group 1 salts. This was certainly a job for microscale chemistry as the solutions are very concentrated and use a lot of material. Conclusion: It worked. Precipitation and solubility should be taught together!

Below is the method/procedure in graphic form with photos too. You can see the precipitation result on my YouTube channel at https://youtu.be/GDWJKEbIlKo.

References

Taber, K.S. Found Chem (2019). https://doi.org/10.1007/s10698-019-09346-3

Materials

Lithium Chloride

Lithium Nitrate

Sodium Chloride

Sodium Nitrate

Glass vials (min. 10mL capacity)

Plastic transfer pipette

Background

Procedure

Source: Bob Worley (author of post)

Questions

Concepts

Solubility of Group 1 salt

Solvation

Time Required

10-15 minutes to prepare the aqueous solutions and make observations.

Preparation

Standard teacher preparation for non-hazardous aqueous salt solutions. 

Attribution

Bob Worley

Credits