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Predicting Precipitates

Demonstrates how solubility constants can be used to predict the formation of precipitates.

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How's the Weather Looking?

Are our predictions about precipitation reactions always accurate?

Credit: Bobbi Jones Jones
Source: http://www.publicdomainpictures.net/pictures/30000/nahled/cloud-cloud-everywhere-a-cloud.jpg
License: CC BY-NC 3.0

[Figure1]

You may have looked up at the sky this morning to see clouds. Whether it’s the fall, winter, spring, or even summer, you may have wondered whether or not there would be some form of water falling from the sky. Rain? Snow? Perhaps even hail? By observing the conditions of a certain day (temperature, clouds, etc.), we could probably make a good guess about the rain, snow, or hail we may (or may not) encounter. There aren’t many clouds today and the sun is up, so a picnic in the park sounds like a possibility! The clouds are gray, hanging pretty low, and completely covering the sky -- umbrellas out!

Using a table or our knowledge of solubility rules, we can identify the ions in a reaction and predict what product will precipitate, or end up being insoluble in water. For instance, in the following reaction -- \begin{align*}\mathrm{{BaCl}_2{(aq)}+{Na}_2{SO}_4{(aq})\rightarrow{?}}\end{align*} -- we can determine the products, separate out the ions, and using the solubility rules identify the spectators and the precipitate. In this case, the precipitate is \begin{align*}\mathrm{{BaSO}_4{(s)}}\end{align*}.

Sometimes, however, our predictions of the weather are not entirely accurate -- it may end up drizzling in the middle of July! So in order to be better prepared, we would probably check the news or the Internet for expert weather forecasts.

Similarly, the solubility rules come equipped with a series of exceptions. Not all tables are complete with every single exception, so we often have to cross check with tables from other sources or even research Ksp values.

Creative Applications

  1. True or false: NH4Cl is insoluble, and forms a precipitate. You may use the links below in the "Resources" section.
  2. Determine the expected products for this reaction, and whether or not a precipitate will be formed (If so, what is it?): A solution of potassium chloride is mixed with a solution of lead (II) nitrate. Look up the named ions and solubility rules if necessary, or use the links in the "Resources" section below. 
  3. Determine the expected products for this reaction, and whether or not a precipitate will be formed (If so, what is it?): A solution of sodium chloride is mixed with a solution of iron (II) nitrate. Look up the named ions and solubility rules if necessary, or use the links in the "Resources" section below.

 Resources

http://www.csudh.edu/oliver/chemdata/solrules.htm

http://www.sciencegeek.net/tables/PT_ions.pdf

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Image Attributions

  1. [1]^ Credit: Bobbi Jones Jones; Source: http://www.publicdomainpictures.net/pictures/30000/nahled/cloud-cloud-everywhere-a-cloud.jpg; License: CC BY-NC 3.0

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