Question

For a lab, I need to be able to identify an ionic compound out of 50 other ionic compounds. There is a long list of Cations and Anions that assemble for each compound. We have to develop a procedure to identify which of the ionic compound we have. I have no idea how to begin such a proceadure. Any help is appreciated.

Answer 1

The procedure to identify the cations and anions is as follows:

1. Identification of Cations

I. Flame tests

• lithium ion Li⁺gives a red-crimson (carmine–red) colour in the flame
• sodium ion Na⁺gives a yellow-orange colour in the flame
• potassium ion K⁺gives a lilac-purple colour in the flame
• calcium ion Ca²⁺gives a brick red colour in the flame
• copper ion Cu²⁺gives a blue–green colour in the flame

II. Chemical Test

A few drops of sodium hydroxide solution is added to the salt solution and the observations are as follows:

metal ion detected	colour of precipitate with NaOH	ionic equation for the reactions
calcium, Ca2+ colourless	white precipitate	Ca2+(aq) + 2OH–(aq) ==> Ca(OH)2(s)
copper(II), Cu2+ blue	blue precipitate	Cu2+(aq) + 2OH–(aq) ==> Cu(OH)2(s)
iron(II), Fe2+ pale green	dark green precipitate	Fe2+(aq) + 2OH–(aq) ==> Fe(OH)2(s)
iron(III), Fe3+ orange	orange–brown precipitate	Fe3+(aq) + 3OH–(aq) ==> Fe(OH)3(s)
zinc, Zn2+ colourless	white precipitate , which dissolves in excess to give a clear colourless solution	Zn2+(aq) + 2OH–(aq) ==> Zn(OH)2(s) Zn(OH)2(s) + 2OH–(aq) ==> Zn(OH)4]2–(aq)
aluminium, Al3+ colourless	white precipitate, which dissolves in excess, to give a clear colourless solution	Al3+(aq) + 3OH–(aq) ==> Al(OH)3(s) Al(OH)3(s) + OH–(aq) ==> [Al(OH)4]–(aq)
ammonium, NH4+ colourless	no precipitate formed, but ammonia gas released which you can smell, the gas turns damp red litmus paper blue	NH4+(aq) + OH–(aq) ==> H2O(l) + NH3(g)

2. Identification of anions

I. Test for halide ions

to test the presence of halide little dil. nitric acid and a few drops of silver nitrate solution are added to the salt solution

halide ion	Colour of precipitate with silver nitrate	Ionic equation to show precipitate formation
chloride Cl–	white precipitate of AgCl silver chloride (slowly darkens when exposed to light)	Ag+(aq) + Cl–(aq) ==> AgCl(s)
bromide Br–	cream precipitate of AgBr silver bromide	Ag+(aq) + Br–(aq) ==> AgBr(s)
Iodide I–	yellow precipitate of AgI silver iodide	Ag+(aq) + I–(aq) ==> AgI(s)

II. Test for Carbonate CO₃^2–

• Add dilute HCl to the salt
• The effervescence produced coforms the evolution of carbon dioxide gas.

The ionic equation is

CO₃^2–_(s) + 2H⁺_(aq) ==> H₂O_(l) + CO_2(g)

III. Test for the sulfate ion SO₄^2–

• Dissolve the salt solution in water
• add little dilute hydrochloric acid added followed by a few drops of barium chloride solution
• White precipitate formed conforms the presence of sulfate ions

barium ion + sulfate ion ==> barium sulfate

Ba²⁺_(aq) + SO₄^2–_(aq) ==> BaSO_4(s)

There are other tests for various other ions.