7. test for ions

Question 1

test for carbonates CO3 2-

1. reagents
2. equation for reaction
3. e.g equation CaCO3(s) + HCl(aq) ->
4. observation

Answer 1

1. dilute acid e.g. HCl
2. CO3 2- (s) + 2H+(aq) -> CO2(g) + H2O(l)
3. CO2(g) + H2O(l) + CaCl2(aq)
4. bubble gas through limewater. if it turns cloudy carbonate present (CO2 turns limewater cloudy

Question 2

test for sulfates SO4 2-

1. reagents
2. equation for reaction
3. e.g. equation Na2SO4(aq) + BaCl2(aq) ->
4. observation

Answer 2

1. add dilute HCl followed by barium chloride solution BaCl2
2. Ba2+ + SO4 2- (aq) -> BaSO4(s)
3. BaSO4(s) + NaCl(aq)
4. barium sulfate = white precipitate

Question 3

test for ammonium compounds

1. reagents
2. conditions
3. equation for reaction
4. e.g. equation NH4Cl(aq) + NaOH(aq) ->
5. observation

Answer 3

1. add sodium hydroxide (NaOH)
2. warm the mixture. litmus paper needs to be damp so ammonia gas can dissolve and make colour change.
3. NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l)
4. NH3(g) + H2O(l) + NaCl(aq)
5. damp piece of red litmus paper. ammonia present = blue (ammonia gas is alkaline)

Question 4

what order should you do tests?

Answer 4

1. test for carbonates. no CO2 ->
2. test for sulfates. no precipitate ->
3. test for halides .

Question 5

why test for carbonates before sulfates

Answer 5

as well as barium sulfate BaSO4 , barium carbonate BaCO3 insoluble (make precipitate)

Question 6

why test for sulfates before halides

Answer 6

sulfate ions also produce a precipitate with silver nitrate (need to rule out sulfate ions first)

Question 7

how to get rid of any anions (negative ions) that you dont want e.g. sulfite SO2 -3

Answer 7

add dilute acid to test solutions