Qualitative Chemical Analysis (Ref: pg. 344 section 9.2)
Purpose: Qualitative analysis identifies elements, ions, or compounds in a sample.
Review of qualitative tests
There are 3 important processes involved in the chemical analysis of solutions:
a) observation of colour of solution  some ions are coloured (very limited)
b) Subject solution to a series of double displacement reactions which result in the
precipitation of the low-solubility product
c) Flame tests (to identify metallic ions in a solution, confirm their presence, or narrow
possibilities for testing)
Because you add each precipitating solution in a particular order/sequence, the
above process is called sequential chemical analysis.
Sequential Chemical Analysis
(Involving differences in solubility)
What’s needed to perform a sequential chemical analysis?
 Solubility chart (to determine a suitable reagent which will precipitate out the ion that
may be present, usually a highly soluble reagent.
 Use of a flow chart to show/determine the sequence of steps, and the sequence has
to be in the precise order of the test sequence.
 An effective method to separate the precipitate formed from the other substances
present (usually filtration)
 Filtrate is the clear liquid remained after a mixture has been filtered.
Qualitative Chemical Analysis
Example 1: Does a sample solution contain Ag+, and/or Zn2+ ions?
Strategy:
1) Decide on which solutions will precipitate out each of the above ions by using the
solubility table . Identify an anion that may be used to precipitate each of the cations in
the solution
According to the solubility table, sulfide ions precipitate both silver and zinc. However,
chloride will precipitate silver but not zinc so it should be added first.
2) Determine the order /sequence in which you’ll use each of the above solution to
precipitate ions
Silver ions must be precipitated fist by adding a solution of souble chloride compound
such as as sodium chloride, NaCl(aq).
If the test is positive, filter the mixture to remove the silver chloride precipitate. Adding a
solution of sodium sulfite should prduce a precipitate of zinc sulfide, if zinc ions are
present.
3) show your steps in a flow chart
Example 2: A solution is suspected to contain Pb2+ ions and/or Ba2+ ions. Perform a
qualitative analysis of a solution that could be used to verify the presence of the ions.
(Construct a flow chart like the one above to illustrate the test sequence)
Practice: Construct flow chart for the qualitative analysis of a solution that may contain
each of the following pairs of ions:
(a) Sr2+(aq) and/or Mn2+(aq)
(b) Cu+(aq) and/or Fe3+(aq)
(c) OH-(aq) and/or C2H3O2-(aq)
INQUIRY TASK
You are given a solution that may contain any or all of acetate, phosphate, and lead
ions. Design a flowchart to illustrate your testing for the presence of these ions.