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Predicting Precipitates Using Solubility Rules

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Close Enough or On The Money?
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Close Enough or On The Money?

Credit: Cpl Ashley Santy
Source: https://commons.wikimedia.org/wiki/File:U.S._Marine_Corps_Cpl._Kenneth_R._Storvick,_a_tool_room_noncommissioned_officer_in_charge_with_Marine_Support_Squadron_271,_measures_wood_at_Camp_Leatherneck_in_Helmand_province,_Afghanistan,_April_1,_2013_130401-M-BU728-001.jpg
License: CC BY-NC 3.0

There’s an old saying in carpentry and other areas where accurate measurement is important – “measure twice and cut once”. The idea behind this saying should be obvious to anybody who has ever made something. Make sure your measurements are accurate before take the irreversible step of cutting the board. If the measurement is not correct, the board will not fit where it needs to go in the project.

Why It Matters

  • In the historic community of Old Salem (located in Winston-Salem, North Carolina), crafts are demonstrated the way they were done in the 1700s when the Moravians first settled that area. The shoemaker demonstrates the making of shoes the old-fashioned way. He points out that shoes could be made to the size of a person’s foot, but both shoes were the same – there was no right and left foot. The feet of the person wearing the shoes would gradually shape them into the specific foot.
  • Credit: Andrew Gould
    Source: http://www.flickr.com/photos/andrewgould/3643719405/
    License: CC BY-NC 3.0

    A restored 18th-century German settler's house, with handcrafted fence [Figure2]

     

  • The shoes were made to a “close enough” standard. They did not fit exactly, but were good enough to get the job done. Many times, “good enough” is all that is needed. If you need a hole dug to plant a tree, as long as the roots fit in, you don’t need an accurate measurement, just something in the general area of the right size. Solubility rules that predict solubilities as “mostly soluble” or “somewhat insoluble” are qualitative because they do not give accurate numbers.
  • When building a complex machine, you want a higher level of accuracy and precision. A piston may need to be machined to a tolerance of 0.1 mm in order to fit properly into a cylinder. “Close enough” won’t get it when you’re building a house. You want the doors to match the door openings and you want the floors level. Solubility calculations using solubility product information allow a much more precise description of what is happening in a given process.
  • Watch a video and the link below and learn a solubility song:

https://www.youtube.com/watch?v=EUZoaUUuINk

Show What You Know

Use the links below to learn more about qualitative and quantitative measurements. Then answer the following questions.

  1. What two factors affect aqueous solubility of a compound?
  2. Is magnesium carbonate soluble or insoluble?
  3. Does barium hydroxide really dissociate completely in water?
  4. What does the plot of solubility vs. temperature tell us?

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