Double Decomposition and Dualism

Frame (21)

This slide provides a general explanation of "double decomposition" reactions. Early 19th century scientists (such as Davy in the previous slide) explained double decomposition with the concept of dualism, which states that substances have one of two charges (positive or negative) and opposites attract. Thus, it is explained by electricity. In "double decomposition", these substances can change partners as long as they pair up with another oppositely charged substance. As will be shown in Frame 23, the double decomposition equation, along with charges, is A(+)B(-) + C(+)D(-) => A(+)D(-) + C(+)B(-). This displays a clear trend of decomposition (e.g. breaking up of AB into A and B) for both compounds and the reformation of separate compounds through oppositely charged attractions.

Frame (22)

Back in 1774, before the invention of the current Latinized literal chemical symbols, chemists used traditional and rather arcane pictoral symbols to represent various chemicals, as shown by the list of Swedish chemical symbols. One such example of these symbols and nomenclature was Vitriolum cupri. (According to Merriam-Webster, "vitreolum" is a Latin adjective for glassy and has an association in French with stained glass windows. Somehow this resulted in "vitriol" becoming the name for sulfuric acid. More obviously, "cuprum" in Latin is copper. So Vitriolum cupri was copper sulfate.) The following word, "caeruleum", is a Latin word for dark blue. Thus, despite the seemingly cryptic naming, the terms are both descriptive and fitting in this situation.

The Vitriolum cupri was the blue solution of CuSO4 that Professor McBride showed during the lecture. When it was mixed with caustic soda ("corrosive sodium") – NaOH – the two solutions reacted. The solution changed to a lighter blue color, and a precipitate formed. Adding more NaOH would yield a darker blue solution. This reaction would be considered an example of "double decomposition", which could be explained in those days by the concept of dualism.

Frame (23)

AB + CD => AD + CB

Charges: A(+)B(-) + C(+)D(-) => A(+)D(-) + C(+)B(-)

Specific Reaction: 2NaOH + CuSO4 => Na2SO4 + Cu(OH)2 (precipitate)

A Double Decomposition reaction is one where two compounds change their partners. It is called a double decomposition because both compounds appear to decompose from their original formats once in the solution. An example is the reaction between sodium hydroxide and copper II (cupric) sulfate. Scientists wondered why compounds would partner with some compounds, but not others. This was explained by the idea of dualism, that there are two different kinds of matter or chemicals - positive and negative (as shown in frame 21). One can see by the charge labeling that positive and negative charges seem to go together. So, these reactions were explained by electricity, which has a lot of truth to it for ionic reactions of salts.

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