Qualitative analysis – the identification of the specific substances present Quantitative analysis – the measurement of the quantity of a substance present

Qualitative Analysis Many substances can be identified by their unique physical or chemical properties: Observation of colour of solution For cations that are usually colourless in solution and highly soluble, a flame test may be used (only works when a single cation is present) For gases, perform a gas test to identify if the gas produced is H2, O2, or CO2 Other possible qualitative tests Reactions with other substances to produce a new colour and/or precipitate

1. Qualitative Analysis – Colour of Solution

2. Qualitative Analysis – Flame Test The majority of solids are white powders and difficult to distinguish between just by looking. Flame tests can give a good indication of what metal ion it contains.

3. Qualitative Analysis – Gas Test Most gases are clear, colourless and odourless.  So it's not easy to distinguish between them with a simple chemical test.

Test for Hydrogen Hydrogen is less dense than air and explosive when mixed with Oxygen. A test-tube filled with hydrogen can be safely held up-turned since no Hydrogen can escape by sinking into denser air.

Oxygen Test Oxygen has a similar density to air, so needs to be contained in a "right-side up" container So, if we light a splint and blow out the flame so that the end is still glowing (the reason the flame does not reappear is that 79% of the air around it is not Oxygen) Placing the glowing splint in pure Oxygen means that every gas molecule that collides with the glowing ember is Oxygen. This increases the rate of reaction, providing enough heat to re-light the flame. 

Carbon Dioxide Test Carbon Dioxide puts out fires. But so do most other gases. So the test for Carbon Dioxide must be something that only Carbon Dioxide does. Only Carbon Dioxide turns Lime-water milky white The white colour is a precipitate of Calcium Carbonate ( like chalk).

4. Other possible tests Conductivity – to determine if solution is ionic or covalently bonded pH – to determine id the solution is acidic or basic Universal indicator (neutral is green, red is acid, blue is base) Bromothymol blue (neutral is green, yellow is acid, blue is base) Phenolphthalein (pink is base)

Melting point/boiling point Solubility in water – to determine polar or non polar

5. Qualitative Analysis – Reactions in Solution This is a type of test where we set up a double displacement reaction using one unknown solution and one known solution In this test, we would predict that if a precipitate forms then a certain ion must have been present in the unknown solution

Indicate the test being performed Identify a Positive Test What Observations did you see? Analysis – what does this mean? Identify a Negative Test

To Complete A Sequential Analysis Locate the possible cations on the solubility table. Determine which anions would precipitate the possible cations. Plan a sequence of precipitation reactions that would use anions to precipitate a single cation at a time. Use filtration between the steps to remove cation precipitates that might interfere with subsequent additions of anions. Draw a flow chart to assist testing & communication.

Example 1 No Lead (II) ions present A solution known to contain Pb2+ Positive test is a white precipitate Solution contained Lead(II) ions, precipitated as PbCl2 Negative test is no precipitate No Lead (II) ions present Add NaCl

Example 2 A solution known to contain Sr2+ Positive test is a white precipitate Solution contained Sr2+ ions, precipitated as SrSO4 Negative test is no precipitate No Sr2+ ions present Add Na2SO4

Example 3 A solution known to contain Cu2+ Positive test is a white precipitate Solution contained Copper (II) ions, precipitated as Cu(OH)2 Negative test is no precipitate No Copper (II) ions present Add NaOH

To Complete A Sequential Analysis Locate the possible cations on the solubility table. Determine which anions would precipitate the possible cations. Plan a sequence of precipitation reactions that would use anions to precipitate a single cation at a time. Use filtration between the steps to remove cation precipitates that might interfere with subsequent additions of anions. Draw a flow chart to assist testing & communication.

You try! Design a sequential analysis for each of the ions below, to determine if it is present in the solution. Ag+ Ca2+ Mg2+

Example with Two Ions A solution is thought to contain lead (II) ion or magnesium ion. Design a sequential qualitative analysis to determine whether the solution contains either or both of these ions

Solution known to contain Pb2+ (aq) or Mg2+ (aq) If Solution contains Pb2+ ions, a ppt will form of PbCl2 Solution contained Pb2+ ions, precipitated as PbCl2 If Solution does not contain Pb2+ ions then no ppt will form No Lead (II) ions present Add NaCl – the Na+ is always soluble, the Cl- will ppt with Pb2+, the Cl- is soluble with Mg2+

If a positive test occurred, filter off the ppt and run a second test for the second cation

If a white ppt forms, the Mg2+ ions present in form if MgCO3 Add Na2CO3 – again, Na+ is always soluble, but the CO32- will ppt with Mg2+ If a white ppt forms, the Mg2+ ions present in form if MgCO3 If no ppt forms then no Mg2+ was present

Another Example with Two Ions A solution is thought to contain calcium ion and mercury (II) ion. Design a sequential qualitative analysis to determine whether the solution contains either or both of these ions

Solution known to contain Ca2+ (aq) or Hg2+ (aq) If Solution contains Hg2+ ions, a ppt will form of HgCl2 Solution contained Hg2+ ions, precipitated as HgCl2 If Solution does not contain Hg2+ ions then no ppt will form No Hg2+ ions present Add NaCl – the Na+ is always soluble, the Cl- will ppt with Hg2+, the Cl- is soluble with Ca2+

If a positive test occurred, filter off the ppt and run a second test for the second cation

If a white ppt forms, the Ca2+ ions present Add Na2SO4 – again, Na+ is always soluble, but the SO42- will ppt with Ca2+ If a white ppt forms, the Ca2+ ions present If no ppt forms then no Ca2+ was present

You Try!! Design a sequential analysis for each of the pairs of ions below, to determine if one or both are present in the solution. Ba2+ and Pb2+ Ag+ and Zn2+ Sr2+ and Mn2+ Cu+ and Fe3+

Example containing three ions! Solution known to contain Ag+1aq or Cu2+aq or Ba2+ ion Chose an anion that will produce a ppt for one of the cations Add NaCl – the Na+ is always soluble, but the Cl- will ppt with Ag1+ If Solution contains Ag1+ ions, a ppt will form of AgCl If Solution does not contain Ag1+ ions then no ppt will form

If a positive test occurred, filter off the ppt and run a second test for the second cation

If a ppt forms, the Cu2+ ions present in the form of Cu(OH)2 Add NaOH – This causes basic conditions and OH- ion will ppt with Cu2+ (Ba2+ is soluble with OH-) If a ppt forms, the Cu2+ ions present in the form of Cu(OH)2 If no ppt forms then no Cu2+ was present

If a positive test occurred, filter off the ppt and run a third test for the third cation

Add Na2SO4 –Na+ is always soluble and Ba2+ ion will ppt with SO42- ion If a ppt forms, the Ba2+ ion present If no ppt forms then no Ba2+ was present

You should perform the colour of solution test and the PPT tests to narrow down which ions you think are present. You should also determine the colours that each ion would produce in a flame test before performing the flame test The flame test is not used to confirm or eliminate the presence of a particular ion.