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Revision:Edexcel Chemistry Unit 3B - Laboratory Techniques - Test for Ions

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Tests for Ions

Contents

Specification

a) Recognise the results of reactions of compounds specifically mentioned in Units 1 and 2 of the specification and the results of tests for simple ions: carbonate, hydrogencarbonate, sulphate(VI), sulphite, chloride, bromide, iodide, nitrate, ammonium, lithium, sodium, potassium, magnesium, calcium, strontium and barium


Tests

Ion Formula Test Observations
Carbonate CO32- Test 1
Heat the solid in a test tube with a Bunsen burner.

It should decompose producing the oxide and carbon dioxide. E.g.
\displaystyle \mathsf{CuCO_{3(s)}} \longrightarrow \mathsf{CuO_{(s)}} + \mathsf{CO_{2(g)}}

Test for the gas using limewater solution.
Limewater should turn from colourless to cloudy in the presence of carbon dioxide due to precipitation of calcium carbonate.
Test 2
Add dilute HCl to the solid.
Test for the gas evolved using limewater solution.
Vigorous effervescence.
Limewater should turn from colourless to cloudy in the presence of carbon dioxide due to precipitation of calcium carbonate.
Hydrogencarbonate HCO3- Test
Add calcium chloride to a hydrogencarbonate solution.
No precipitate forms since calcium hydrogencarbonate is soluble.
Sulphate (VI) SO42- Test
Add barium chloride solution acidified with dilute HCl to the test solution.
White precipitate of barium sulphate forms.
Sulphite SO32- Test
Warm the sulphite with dilute HCl.
Test for gases using acidified potassium dichromate(VI) solution (or paper)
The solution turns green.
Chloride Cl- Test 1
Add concentrated sulphuric acid to the solid chloride.
White steamy acidic fumes are seen - HCl fumes.
Test 2
Add dilute nitric acid to a solution of a chloride to acidify the solution. This eliminates any carbonates or sulphites.
Add silver chloride to the solution.
Add dilute ammonia solution.


White precipitate of AgCl forms.

Solid dissolves.
Bromide Br- Test 1
Add concentrated sulphuric acid to the solid bromide.
Steamy brownish acidic fumes are seen.
Test 2'
Add dilute nitric acid to a solution of a bromide to acidify the solution. This eliminates any carbonates or sulphites.
Add silver chloride to the solution.
Add concentrated ammonia solution.


Cream precipitate of AgBr forms.

Solid dissolves.
Iodide I- Test 1
Add concentrated sulphuric acid to the solid iodide.
Purple acidic fumes are seen. The mixture turns to a brown slurry.
Test 2
Add dilute nitric acid to a solution of a iodide to acidify the solution. This eliminates any carbonates or sulphites.

Add silver chloride to the solution.

Add concentrated ammonia solution.
Yellow precipitate of AgI forms.

Solid is insoluble.
Nitrate NO3- Test 1
Heat solid nitrate.

If group 1 solid (not Li) then will decompose to give the nitrite and oxygen.

All other solid nitrates decompose to give the metal oxide, nitrogen dioxide and oxygen.




Oxygen gas is evolved that will relight a glowing splint.

Brown gas is seen (NO2). Oxygen gas is also evolved and will relight a glowing splint.
Test 2
Boil nitrate solution with aluminium/Devarda’s alloy, in sodium hydroxide solution.
Test vapour with red litmus paper.
Litmus paper turns blue in the presence of ammonia.
Ammonium NH4+
Test
Warm ammonium compound with NaOH.
Test vapours immediately using damp red litmus paper.
NH
3
turns the litmus paper blue.
Lithium Li+ Test
Dip nichrome wire in HCl.

Dip wire in solid.

Heat wire in centre of flame.

Observe colour of flame.





A carmine red flame is seen.
Sodium Na+ Test
Dip nichrome wire in HCl.

Dip wire in solid.

Heat wire in centre of flame.

Observe colour of flame.




A yellow flame is seen.
Potassium K+ Test
Dip nichrome wire in HCl.

Dip wire in solid.

Heat wire in centre of flame.

Observe colour of flame.
A lilac flame is seen.
Magnesium Mg2+ Test
Add NaOH solution to the magnesium solid.
A white solid forms which is insoluble in excess NaOH(aq). This is Mg(OH)2(s)
Calcium Ca2+ Test
Dip nichrome wire in HCl.

Dip wire in solid.

Heat wire in centre of flame.

Observe colour of flame.
A brick red flame is seen.
Strontium Sr2+ Test
Dip nichrome wire in HCl.

Dip wire in solid.

Heat wire in centre of flame.

Observe colour of flame.
A crimson red flame is seen.
Barium Ba2+ Test
Dip nichrome wire in HCl.

Dip wire in solid.

Heat wire in centre of flame.

Observe colour of flame.
A apple green flame is seen.


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