precipitation reactions

Precipitation
reactions are one of

the three main
classes of reactions
that we will study in

this course.

Imagine two aqueous
solutions are mixed:

PbሺNO3ሻ2ሺ ሻ + KIሺ ሻ

Which ions would be
present in solution?

These ions will be
present in the two
solutions:

(Solution 1)

Pb2+ሺ ሻ + NO3− ሺ ሻ

these ions will be present in a ratio of 1:2

(Solution 2)

K+ሺ ሻ + I− ሺ ሻ

these ions will be present in a ratio of 1:1

What do you expect
these two solutions to
look like?

Clear solutions are ones that you can see
through. Dissolved ions will form clear solutions
that may be colorless (as here) or have color.

When these two solutions are
mixed, the following is the result:

A yellow solid is observed. Solid formation is
indicated by cloudiness. What is the solid?

There are 4 possible combinations of
cation + anion:

Two of these combinations were the
formulas that we started with. Since we
know those combinations are soluble, they
can be eliminated as solid candidates.

Examine the remaining two
combinations. Consult a solubility
table to see whether one
combination would produce an
insoluble product.

When iodide is mixed with lead(II),
an insoluble solid forms.

Precipitate:

solid that forms
upon mixing of two
aqueous solutions

This is an example of a
precipitation reaction,
because a precipitate
was formed.