Identifying the Formation of a Precipitate
First, I would like you to look at each beaker above and see if you can explain what is happening, or has
happened, inside of the beaker. KI(aq) means that solid potassium iodide was dissolved in water, and
singe it is aqueous, that must mean that KI is a soluble compound. But what if you weren’t sure
potassium iodide was soluble, where would you look? You would need to refer to your solubility
guidelines. You need to memorize that all sodium, potassium, ammonium, and nitrate salts are soluble
in water, but other than that, solubility guidelines will have to be provided to you if needed to answer a
problem. Since KI is soluble, this means that both potassium ions and iodide ions will be present in
solution, as shown in beaker 1 above. Lead(II) nitrate is a nitrate salt, so it too is soluble in water, and
there are lead(II) ions and nitrate ions present in beaker 2.
Now, I would like you to identify the type of reaction that exists when the contents in the two beakers
shown above are mixed. If you said a double replacement reaction, you’d be correct. As a reminder,
during a double replacement reaction, the cations of both compounds, switch, and then depending on
the new compounds, they may or may not decompose further. When the contents are mixed all ions
are present in solution until the reaction happens…which will occur pretty much instantaneously. So
does a reaction occur? Well we have to see if something other than an aqueous compound is formed.
KI(aq) + Pb(NO3)2(aq)  KNO3(?) + PbI2(?)
As you can see from this third beaker, when the contents are mixed, two new compounds are formed,
potassium nitrate, KNO3(aq), which is soluble in water, and at the bottom of the beaker, you can see a
much more organized particle diagram of lead(II) iodide. What does this mean? This is showing that
lead(II) iodide, PbI2(s), is insoluble in water, and is in solid state. Check your solubility guidelines to see if
this makes sense. A solid that forms out of solution is known as a precipitate.