Precipitates?

Question

How do you know whether a precipitation forms out of:
Lead (II) nitrate and potassium iodide
Explain

Answer 1

Is either of the two possible products of the double replacement reaction insoluble? Yes (the PbI2).

You start out with something like this,
Pb(NO3)2 + KI -->
You are not sure whether a reaction actually occurs or not. You have two possibilities....either nothing happens or a double replacement reaction occurs where the cations and anions switch. If no reaction occurs, there will be no precipitate (since both substances start out in solution and the formation of a precipitate is an indicator of a chemical reaction).

Try out out the possibility of a double replacement reaction,
Pb(NO3)2 + 2 KI --> PbI2 + 2 KNO3
Is Lead (II) Iodide or KNO3 insoluble in water? If one is, it would form a precipitate and cause a net reaction to occur.
*note, if both happen to be soluble in water, then there is no net ionic reaction.

It just so happens that Lead (II) Iodide is insoluble (or, strictly speaking, has a VERY low solubility). So there is a net ionic reaction which occurs.
If you did not already know this, you need to look at the solubility rules for ionic compounds and/or the solubilities for the potential products of the chemical reaction.

Pb(NO3)2 (aq) + 2 KI (aq) --> PbI2 (s) + 2 KNO3 (aq)
Pb+2 (aq) + 2 I- (aq) --> PbI2 (s)

The Potassium Nitrate is soluble and remains behind in solution.

The Lead (II) Iodide forms a bright yellow precipitate which is one of the favorite chemical reactions chemistry text books like to have pictures of (I actually had a text book with the reaction on the cover once).

Answer 2

You will get a precipitate of lead iodide. If you're studying first semester general chemistry, you should be given some guidelines as to what salts are soluble and which aren't. For iodides, like chlorides and bromides, the lead, mercurous, and silver salts are the only insoluble ones.

If you're in second semester, you should be able to find a Ksp value for lead iodide. If you can't find a Ksp value for a salt, good chance it is soluble.