Topic 17 - Chemical Testing

Question 1

Hydrogen Test

Answer 1

Lit splint

2H2 (g) + O2 (g) -> 2H2O (g)

Exothermic (combustion)

Question 2

Hydrogen Result

Answer 2

Squeaky pop

Top of the tube resonating and the gas expands quickly through it

Question 3

Oxygen test

Answer 3

Glowing splint

Wood + oxygen -> smoke

Question 4

Oxygen result

Answer 4

Relights

Oxygen concentration is multiplied by 5, so the rate of reaction increases

Question 5

Test for water that determines purity

Answer 5

Measure MP and BP

Question 6

Result for water test that determines purity

Answer 6

MP = 0°C 
BP = 100°C

Question 7

Tests (2) for water which do not determine purity

Answer 7

anhydrous CuSO4 (s)

anhydrous CoCl2 (s)

Question 8

Results (2) for water tests which do not determine purity

Answer 8

white (s) -> blue (s)

blue (s) -> pink (s)

Question 9

Test for carbon dioxide

Answer 9

Bubble through limewater

Saturated Ca(OH)2 (aq)

Question 10

Result for carbon dioxide test

Answer 10

Turns cloudy -> precipitation

Ca(OH)2 (aq) + CO2 (g) -> CaCO3 (s) + H2O (l)

Question 11

test for chlorine

Answer 11

Damp blue litmus

Question 12

Result for chlorine

Answer 12

Turns white (via red because a solution of chlorine is acidic)

Cl2 (g) + H2O (l) -> HCl (aq) + HClO (aq)

Goes white because chloric acid (HClO) acts as a bleach

Question 13

Test for ammonia

Answer 13

Damp red litmus

Damp because the ammonia gas needs to dissolve to make the colour change

Question 14

Result for ammonia

Answer 14

Turns blue

NH3 (aq) + H2O (l) -> NH4+ (aq) + OH-

OH- is an alkali!

Question 15

Test for carbonate

Answer 15

1) add dilute HCl (aq)
2) limewater test

2H+ + CO32- -> CO2 + H2O

Question 16

Result for carbonate test

Answer 16

Fizzing

Limewater turns cloudy

Question 17

Test for sulphate

Answer 17

1) do the carbonate test (adding HCl gets rid of any traces of carbonate or sulfite ions which would both produce a precipitate, confusing the results)
2) add a few drops of BaCl2 (aq)

Ba2+ (aq) + SO42- (aq) -> BaSO4 (s)

Question 18

Result for sulphate

Answer 18

No fizzing 
White precipitate (barium sulphate) formed

Question 19

Test for halides

Answer 19

1) add dilute HNO3 (aq) to get rid of carbonate or sulfite ions
2) add AgNO3 (aq)
To provide Ag+, and NO3 prevents false positives because there are no other precipitates

Question 20

Chloride result

Answer 20

White precipitate

Ag+ (aq) + Cl- (aq) -> AgCl (s)

Question 21

Bromide result

Answer 21

Cream precipitate

Ag+ (aq) + Br- (aq) -> AgBr (s)

Question 22

Iodide result

Answer 22

Yellow precipitate

Ag+ (aq) + I- (aq) -> AgI (s)

Question 23

Describe test for lithium, sodium, potassium, calcium and copper

Answer 23

1) clean a platinum wire loop by dipping it into some dilute HCL
2) dip platinum wire into substance
3) put wire into non-luminous flame
4) check colour

Question 24

Lithium result

Answer 24

Red flame

Question 25

Sodium result

Answer 25

Yellow flame

Question 26

Potassium result

Answer 26

Lilac flame

Question 27

Calcium result

Answer 27

Orangey-red flame

Question 28

Copper flame result

Answer 28

Blue-green flame

Question 29

Describe the test used for Iron (II), Iron (III) and Copper

Answer 29

Add NaOH

look at precipitate formed

Question 30

Iron (II) result

Answer 30

Sludgy green precipitate

Question 31

Iron (III) result

Answer 31

Reddish brown precipitate

Question 32

Copper precipitate result

Answer 32

Blue precipitate

Question 33

Test for ammonium

Answer 33

1) add NaOH

2) test for ammonia

Question 34

Result for ammonium

Answer 34

Damp red litmus goes blue