Qualitative Analysis is a fancy name for identifying the components of a compound or mixture using a series of tests. We use the qualities of certain compounds and substances to determine whether they are present in a mixture or compound.
For example, you might use your sense of smell to identify if a certain spice is in your food, or you might taste something or observe it closely to determine if it is spoilt. In Chemistry, we are doing essentially the same thing (but we won't be tasting anything in a lab!) We are simply observing changes that occur when we interact with a compound or mixture in some way.
Well, now that the basics are out of the way, the rest of qualitative analysis as a topic is memorizing the results of a variety of tests when certain components are present. CSEC only requires that you know qualitative analysis of ionic compounds (and how to recognize a few common gases as well).
Identifying Cations
There are several methods of identifying cations, which are usually used together since some cations produce similar results from the same tests.
Reaction with Sodium Hydroxide (NaOH) Solution
Metal cations (apart from sodium and potassium) will form insoluble metal hydroxides when they react with sodium hydroxide solution. This can be shown in the ionic equation:
Mⁿ⁺(aq) + nOH⁻(aq) → M(OH)n(s)
We can use this reaction to identify certain cations:
Make a solution of the compound to be identified in distilled water.
Add Sodium Hydroxide solution dropwise (using a pipette) and observe the colour of any precipitate that may form.
Continue to add sodium hydroxide solution in excess and note whether or not the precipitate dissolves.
If no precipitate forms, warm the mixture gently and test for ammonia gas.
The results of the reaction for the different cations can be seen below:
Why do these results occur?
The hydroxides of calcium, copper, iron (II) and iron (III) are all basic, and do not react with sodium hydroxide. So, they remain solid and do not dissolve.
The hydroxides of aluminium, zinc and lead (II) are amphoteric, so they react with the alkali sodium hydroxide to form soluble salts that dissolve.
Ammonium hydroxide is soluble, so it does not form a precipitate.
Reaction with Ammonium Hydroxide Solution
All of the metal cations except for the calcium ion form insoluble hydroxides with ammonium hydroxide solution.
Mⁿ⁺(aq) + nOH⁻(aq) → M(OH)n(s)
We can use this reaction to identify certain cations:
Make a solution of the compound to be identified in distilled water.
Add Ammonium Hydroxide solution dropwise (using a pipette) and observe the colour of any precipitate that may form.
Continue to add ammonium hydroxide solution in excess and note whether or not the precipitate dissolves.
As you can see, the procedure is the same as it is for sodium hydroxide solution, except that no test can be carried out for the ammonium ion since it is already present in the solution.
The results of the reaction for the different cations can be seen in the diagram below:
Why do these results occur?
The hydroxides of aluminium, lead (II), iron (II) and iron (III) do not react with ammonium reaction, so they remain insoluble.
Zinc hydroxide and copper (II) hydroxide react with ammonium hydroxide to form complex soluble salts, so the precipitates dissolve.
Using the past two tests, you will be able to distinguish between all of the cations given except for two: the aluminium ion and the lead (II) ion. They both form white precipitates in both reactions. Both of their precipitates are soluble in excess sodium hydroxide solution and insoluble in excess ammonium hydroxide solution.
So, we need a particular test to distinguish between these two ions if necessary.
Reaction with Potassium Iodide (KI)
The reaction between potassium iodide and the lead (II) ion can be used to distinguish between the lead (II) ion and the aluminium ion:
Make a solution of the compound to be identified in distilled water.
Add a few drops of potassium iodide (using a pipette) and look for the formation of a precipitate.
The lead ion will form a bright yellow precipitate of lead (II) iodide while the aluminium ion will form no precipitate.
Identifying Anions
Heating
Heat causes the decomposition of some compounds, and by testing for the presence of certain gases, we can deduce the identity of the anion present:
Heat a small amount of the solid in a test tube
Test the gas(es) evolved
Identifying the Nitrate Ion
To a sample of the solid in a test tube, add concentrated sulphuric acid and copper turnings (strips of copper metal). If the nitrate ion is present, a blue solution forms and brown nitrogen dioxide gas is released.
Reaction with Dilute Acid
The carbonate and sulphite ions can be distinguished based on the compound's reaction with dilute acid (hydrochloric acid or nitric acid):
Add dilute HCl or HNO3 to a sample of the solid in a test tube
Heat if necessary
Test the identity of the gas evolved
Reaction with Concentrated Sulphuric Acid
We can distinguish between certain anions based on the reaction of their compounds with concentrated sulphuric acid and testing the gas evolved:
Add concentrated sulphuric acid drop-wise to a small quantity of the solid in a test tube
Test the gas evolved
Reaction with Silver Nitrate then Aqueous Ammonia (Ammonium hydroxide)
We can distinguish between the halides (halogen ions) by observing the precipitate formed with silver nitrate and then testing the solubility of that precipitate in aqueous ammonia (ammonium hydroxide). Since all silver halides are insoluble, we will always be able to observe a precipitate:
Dissolve the solid in dilute nitric acid
Add silver nitrate solution drop-wise and observe the colour of the precipitate
Look to see if the precipitate dissolves when aqueous ammonia is added
Reaction with Barium Nitrate (or Barium Chloride) Solution then Dilute Acid
You may have noticed that the very common sulphate ion has been missing from the last tests. Since it is so common, it is necessary that we have a test to differentiate it from other anions:
Dissolve the solid in distilled water
Add barium nitrate or barium chloride drop-wise and observe the precipitate
Add dilute hydrochloric or nitric acid and see if the precipitate dissolves
Test any gas evolved
Identifying Gases
Many of the tests mentioned above require that you test for the release of a certain gas. In addition, many reactions you will study and encounter during your CSEC course involve the evolution of a common gas (like the reaction between a metal and an acid releases hydrogen gas).
So, you'll definitely need to remember tests for identifying these common gases:
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