Chemistry - Tests for Ions and Radicals

precipitate

an insoluble solid

precipitation reaction

one in which an insoluble product is produced from soluble reactants

Test for sulfate and sulfite ions

use the reagent or barium chloride solution (BaCl2) to form a white precipitate of barium sulfate/sulphite

(i) sulphate:

Na2SO4 + Bacl2 → BaSO4 (white ppt.) + 2NaCl

(ii) sulphite:

Na2SO3 + BaCl2 → BaSO3 (white ppt.) + 2NaCl

distinguishing between which is BaSO4 + & BaSO3

add the reagent of dilute HCl (aq)

(i) BaSO4 + HCl → no reaction takes place, hence ppt. stays as barium sulphate is insoluble in dilute acid

(ii) BaSO3 + HCl → BaCl2 + H2O + SO2 (white precipitate disappears & sulphur dioxide gas is given off)

testing for nitrate ions

Brown Ring Test:

reagents needed →

• freshly made FeSO4 solution (Iron (ii) suphate)…made fresh as it Fe2+ reacts with O2 in the air to oxidise to become Fe3+

• concentrated H2SO4 (sulphuric acid solution)…this is quite viskus

Method:

• add sodium nitrate solution to a test tube with water (NaNO3)

• add fresh FeSO4 solution (a greenish colour)

• tilt the test tube to the side and slowly add the concentrated H2SO4 solution carefully down the inside of the test tube…this is as you don’t want the solution at the bottom to e mixing with the sulfuric acid

• the sulfuric acid then falls to the bottom as it is more dense

• a brown ring should then form at the junction between the two substances to show the presence of a nitrate ion in aqueous solution 

• forms FeSO4.NO

test for chloride ion

reagents: silver (i) nitrate (AgNO3) + few drops of concentrated nitric acid solution (HNO3)

Method:

add silver nitrate solution to an aqueous solution of sodium chloride and a few drops of concentrated nitric acid solution

reaction:

• immediate white precipitate of formed (ionic reaction & is instantaneous/extremely fast)

• this proves a chloride ion in aqueous solution is present

NaCl + AgNO3 -(hno3)→ NaNO3 + AgCl (white ppt.)

to confirm the presence of chloride ion

add dilute ammonia solution to the white precipitate and it disappears

this is due to silver chloride being soluble in dilute ammonia solution

test for phosphate ion

reagents: ammonium molybdate & nitric acid (for acidic conditions)

method:

• add ammonium molybdate solution to a test tube containing phosphate ions & very carefully add 5 drops of concentrated HNO3

• heat gently (no heat = no reaction)

• result: a yellow precipitate of ammonium phosphomolybdate is formed

test for presence of carbonate or hydrogencarbonate ions in aqueous solutions

reagent: add dilute acid (HCl) to the solution containing ions.

result: effervescence due to release of carbon dioxide gas

• Na2CO3 + 2HCl → 2NaCl + H2O + CO2

• NaHCO3 + HCl → NaCl + H2O +  CO2

to test for carbon dioxide gas

Bubble the carbon dioxide gas through a colourless solution of limewater, in which it then turns to a milky white colour

*limewater is an aqueous solution of calcium hydroxide

CO2 + Ca(OH) → CaCO3 + H2O

*if you continue to bubble the carbon dioxide through the solution, the precipitate will dissolve as it’ll become calcium hydrogencarbonate

to distinguish between a carbonate & a hydrogencarbonate

reagent used: add magnesium sulfate solution to both (MgSO4)

• if white precipitate is produced, a carbonate is present as the white precipitate is magnesium carbonate (insoluble in aqueous solutions)

Na2CO3 + MgSO4 → MgCO3 + Na2SO4

• if no precipitate is produced, hydrogencarbonate is present as magnesium hydrogencarbonate is soluble in aqueous solutions

2NaHCO3 + MgSO4 → Mg(HCO3)2 + Na2SO4

*if you boil the clear and colourless hydrogencarbonate solution, we get a white precipitate as hydrogencarbonates decompose on heating