Core Practicals 07 & 15 - Analysis of unknown compounds

How can a flame test be used to identify metal ions?

1. Clean wire loop with H2SO4 or HCl
2. Dip the loop into the sample.
3. Place the loop into a blue flame (using a bunsen
burner).
4. Record the colour of the flame.

group 1 metal ion flame colours?

Lithium, Li+ : Red
Sodium, Na+ : Yellow
Potassium, K+ : Lilac
Rubidium, Rb+ : Red
Caesium, Cs+ : Blue

group 2 metal ion flame colours?

Calcium, Ca2+ : Orange-red
Barium, Ba2+ : Green
Strontium, Sr2+ : Crimson
Magnesium, Mg2+ : None
Beryllium, Be2+ : None

How can you identify cations?

1.Dissolve the unknown substance in water.
2. Add aqueous NaOH drop by drop, until in excess and no further change occurs.

What are the results from the reactions between cations and NaOH?

chromium(III) [Cr(H2O)6]3+
Violet solution forms green precipitate
excess:Precipitate dissolves to form a green solution

iron(II) [Fe(H2O)6]2+
Green solution forms green precipitate
turning brown on exposure to air
excess:Precipitate is insoluble

iron(III) [Fe(H2O)6] 3+
Yellow solution forms red-brown
precipitate
excess: Precipitate is insoluble

cobalt(II) [Co(H2O)6]2+
Blue precipitate turning pink on standing
excess:Precipitate is insoluble

copper(II) [Cu(H2O)6]2+
Blue solution forms blue precipitate
excess:Precipitate is insoluble

How do you test for a saturated hydrocarbon?

Add bromine water to the sample.
If the solution turns from brown to colourless then a unsaturated hydrocarbon was present.

How do you test for an aldehyde?

● Add Tollen’s reagent to sample
Warm → silver mirror confirms an aldehyde was present.

● Add Fehling’s/Benedict’s solution → aldehyde turns clear
blue solution to brick red ppt

● Add dichromate ions (eg. K2Cr2O7 / Na2Cr2O7) acidified
eg. with H2SO4 & heat → turns orange solution green

How do you test for a carboxylic acid?

● Add an alcohol & acid (eg. H2SO4) → glue or
pear-drop smell (formation of an ester).

● Add a metal carbonate → effervescence as CO2 is
released (test with limewater)

● Add a reactive metal → effervescence as H2 is
released

● Add an acyl chloride → misty fumes (HCl)

How do you test for ammonium (NH4 + ) ions?

● Add an equal volume of NaOH to the sample.
Shake.
● Warm the solution in the test tube.
● Test the gas released with damp red litmus paper.
● If it goes blue, ammonium ions are present

How do you test for group 7 / halide ions?

To the compound being tested, add nitric acid and silver nitrate. Record observations.
● To samples of this solution, add dilute and then concentrated ammonia.

What are the results from the reactions of halide ions with silver nitrate and ammonia?

Cl-: White precipitate
Dilute NH3 White precipitate -> colourless
Concentrated NH3 colourless

Br-:Cream precipitate
Dilute NH3 :same
Concentrated NH3: Cream -> colourless)

I-:Yellow precipitate
Dilute NH3 : same
Concentrated NH3: Same

How do you test for carbonate (CO3 2-) and hydrogencarbonate (HCO3 - ) ions?

● Add dilute acid.
● If carbonate/hydrogencarbonate ions are present, the
solution will effervesce.
● You can bubble the gas given off through limewater and it
should go cloudy, confirming the presence of carbonate / hydrogencarbonate ions.

How do you test for sulfate (SO4 2-) ions?

● Add HCl and BaCl2 to the suspected sulfate solution.
● If sulfate ions are present, a white precipitate of BaSO4 will form

What is the order of testing ions? (Carbonate, halide and sulfate)

Carbonate → Sulfate → Halide
This prevents false positive results occurring i.e. Unexpected insoluble precipitates such as Ag2SO4 ,Ag2CO3 and BaCO3 could form.