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Solubility test lab

(Think of ions like boys and girls - girls are the cations, positively charged, and they go first.  Boys are the anions, negatively charged and they go last!)

 
Precipitation reactions take two liquids and produce a solid.  How does this happen?? Precipitation reactions are always double displacement reactions. These are reactions where one ion will displace another and replace it. 

 
If a certain combination of ions will precipitate or not is based on solubility rules - you will be making your own up during this lab.

 
Coulomb's law predicts whether or not an ionic compound will precipitate.  F = charge 1 * charge 2 / constant * (radius)^2. 

 
The charges are important!!  If two charges are the same charge, for example two anions, they repel.  Think of two polarized magnets - when you bring the south end of two magnets together they repel. (For ions and our boy/girl analogy, think of two boys on a bus - will they ever sit in one seat together?  NO, because they repel each other!)

 
If the two charges are different charge, for example a cation and an anion, they attract. (For ions and out boy/girl analogy again, think of a boy and girl who like each other - they attract one another.)

 
The same thing is at work here with cations and anions.  As the force gets bigger and more negative - the more likely things will precipitate out.  This means that when you bring big oppositely charged particles together, they will precipitate. 

 
So which ions should always be soluble, meaning they will not precipitate out?  These are ones with small charges, such as Na+.  Can you think of any others?

 
Remember, however, that you always need a positive charged particle (cation) and a negatively charged particle (anion) to make an ionic compound.  You can NEVER have two anions in an ionic compound.

This is a cool website that shows animation of what happens in a precipitation reaction!

You will need Shockwave to run it.

 
Number 4b - prelab question amendment

 NO3- is nitrate, and you will be looking at it in the lab.  It is always soluble, and it is what the cation's salts are.  They are all nitrates.  

Posted at 09:35AM Jul 11, 2006 by CHRISTIAN, CAROLINE in General  |  Comments[1]