Precipitate Reactions

In chemistry, precipitate reactions cannot be used to determine solubility rules. These reactions prove that one of the substances created in the double replacement reaction is soluble and one is solid, but not which is which, meaning that without knowing the solubility rules, the soluble substance cannot be determined. This relates to our latest chemistry experiment, which involved numerous chemical reactions, some of which produced a precipitate.

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This lab consisted of a laminated reaction sheet, on which we performed the experiments using different chemicals. Repeating our processes a total of sixty six times, we were able to produce consistent results and confirm previous observations. In addition, our data allowed us to ensure presence of certain substances. Some reactions produced a precipitate, while others did not. After our experiment, we practiced converting between molecular and ionic reactions, and, where applicable, net ionic reactions. In reactions where a precipitate was not produced, the net ionic equation was the same as the ionic equation because all substances were aqueous, meaning none cancelled in the reaction to create a more simple equation. An example of  conversion between molecular, ionic, and net ionic equations is shown below.

Molecular:    Na2CO3 (aq) + CaCl2 (aq) = 2 NaCl (aq) + CaCO3 (s)

Ionic: Na(1+)aq + CO3(2-)aq + Ca(2+)aq + Cl(2-)aq = 2Na(1+)aq + 2Cl(1-)aq + Ca(2+)aq + CO3(2-)aq

Net Ionic: CO3(2-)aq + Ca(2+)aq = CaCO3 (s)

The presence or absence of particular cations and anions determined what level of reaction occurred. If a cation or anion was the same in both substances, then no precipitate was created because the substances produced were similar in chemical composition to the reactants. In other words, the solubility of all substances involved was aqueous, because of the combination of chemicals via double replacement. Substances in a sample could be qualitatively identified by the production of a precipitate. In our case, a precipitate presented as a darker section of coloring, or cloudiness in the substance produced. The solubility rules allowed us to predicted what reactions would produce precipitates, making them incredibly useful.

Resources:

Small Scale Precipitate Reactions Lab

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