CHEMICAL TEST
Learning Objectives
▪ Describe tests for these gases:
▪ Hydrogen
▪ Oxygen
▪ Carbon dioxide
▪ Ammonia
▪ Chlorine
▪ Describe how to carry out a flame test
▪ Know the colours formed in flame tests for these cations:
▪ Li+ (red)
▪ Na+ (yellow)
▪ K+ (lilac)
▪ Ca2+ (orange –red)
▪ Cu2+ (blue-green)
Learning Objectives
▪ Describe tests for these cations:
▪ NH4+ using sodium hydroxide solution and identifying the gas evolved.
▪ Cu2+, Fe2+ and Fe3+ using sodium hydroxide solution.
▪ Describe tests for these anions:
▪ Cl-, Br- and I- using acidified silver nitrate solution.
▪ SO42- using acidified barium chloride solution.
▪ CO32- using hydrochloric acid and identifying the gas evolved
▪ Describe a test for the presence of water using anhydrous copper (II)
sulfate.
▪ Describe a physical test to show whether or not a sample of water is pure.
Test for Gases
➢ Hydrogen gas
➢ Oxygen gas
➢ Carbon dioxide
➢ Chlorine gas
➢ Ammonia Gas
➢Sulfur dioxide
Test For Hydrogen Gas
❖ Burning candle near the source of hydrogen.
❖On doing so, hydrogen gas burns with a squeaky pop sound.
❖Hydrogen gas is recognized by the 'pop' when it burns.
To test for hydrogen,
use a burning splint.
The gas will explode
with a squeaky ‘pop’.
Test For Oxygen Gas
To test for
oxygen, use a
glowing splint.
The gas will
cause the splint
to re-light.
Test For Carbon Dioxide
To test for carbon dioxide,
bubble the gas through
limewater. After a short
while the limewater will go
milky-white.
If carbon dioxide continues to be
bubbled through limewater, the
liquid will eventually go clear
again.
Test For Chlorine Gas
Chlorine gas will turn moist litmus
paper from BLUE to RED, and will
then bleach it
Safety: Wear eye protection. Avoid
inhaling or ‘shiffing’ chlorine or
ammonia, especially if you have a
breathing problem such as asthma.
Test for Ammonia
Ammonia gas will turn moist litmus paper or universal
indicator paper from RED to BLUE
Test for Sulfur dioxide
Test
Litmus paper - The moist blue litmus paper turned red.
- The moist red litmus paper unchanged
Acidified potassium nanganate (VII) - the colour turned from
purple to colourless.
https://www.youtube.com/watch?v=zhI7wUBxxeA
SUMMARIZE
Testing For Ions
❖ Anions ( negative charged ions)
❖ Cations ( positive charged ions)
A flame test is used to show the presence of certain metal ions
(cations) in a compound
Cations
▪ Cations are positively charged ions. ( Na+ and Mg 2+
▪ Group 1 metals form monovalent cations. ( Lithium forms Li+ )
▪ Group 2 metals form divalent cations. (Eg. Calcium forms Ca2+ )
▪ Group 3 metals form trivalent cations. (Eg. Aluminium forms Al3+ )
Common Cations
Monovalent Divalent Trivalent
Lithium Li+ Barium Ba2+ Aluminium Al3+
Potassium K+ Calcium Ca2+ Iron(III) Fe3+
Sodium Na+ Magnesium Mg2+
Copper(I) Cu+ Zinc Zn2+
Silver Ag+ Iron(II) Fe2+
Hydrogen H+ Tin(II) Sn2+
Lead(II) Pb2+
Copper(II) Cu2+
Anions
▪ Anions are negatively charged ions. (Chloride ions, Cl-, and oxide ions, O2-)
▪ Group 6 elements form divalent anions. (Eg. sulphide forms S2- )
▪ Group 7 elements form monovalent anions. (Eg. fluoride forms F- )
Common anions
Monovalent Divalent Trivalent
Bromide Br- Oxide O2- Phosphate PO43-
Chloride Cl- Carbonate CO32-
Iodide I- Sulphate SO42-
Hydroxide OH- Sulphite SO32-
Nitrate NO3- Sulphide S2-
Li+, Na+, K+, Ca2+ using flame
tests
Instructions for a flame test:
1. Clean the flame test metal loop wire by dipping it into hydrochloric acid and then
holding it in a hot Bunsen flame.
2. Repeat this until the wire doesn't produce any colour in the flame.
3. When the wire is clean, moisten it again with some of the acid and then dip it into a
small amount of the solid you are testing so that some sticks to the wire.
4. Place the wire back in the flame again.
5. If the flame colour is weak, it is often worthwhile to dip the wire back in the acid
again and put it back into the flame as if you were cleaning it. You often get a very
short but intense flash of colour by doing that.
4. Place the wire back in the flame again.
5. If the flame colour is weak, it is often worthwhile to dip the wire back in
the acid again and put it back into the flame as if you were cleaning it.
You often get a very short but intense flash of colour by doing that.
Test Results
https://www.youtube.com/watch?v=IJkLuwt8GWU
Test for NH4+ :
All ammonium salts react with dilute alkalis, such as sodium hydroxide,
to give ammonia.
NH4Cl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) + NH3(g)
Because it is an alkaline gas, ammonia will turn red litmus blue.
Testing For Cations using Sodium Hydroxide solution
▪ Test for cations (Cu2+, Fe2+ and Fe3+ ) using sodium hydroxide
solution.
Test for Cu2+
▪ When a copper(II) salt is added to sodium hydroxide, a
pale blue precipitate of copper(II) hydroxide is formed.
Copper + Sodium → Copper + Sodium
sulphate hydroxide hydroxide sulphate
CuSO4 + NaOH → Cu(OH)2 + Na2SO4
▪ Formation of a blue precipitate shows the presence of
Blue precipitate of
copper (II) ions. copper (II) hydroxide
Test for Fe2+
▪ When an iron(II) salt is added to sodium hydroxide, a dirty
green precipitate of iron(II) hydroxide is formed.
Iron + Sodium → Iron + Sodium
sulphate hydroxide hydroxide sulphate
FeSO4 + 2NaOH → Fe(OH)2 + Na2SO4
▪ Formation of an green precipitate shows the presence of
iron (II) ions.
Test for Fe3+
▪ When an iron(II) salt is added to sodium hydroxide, a
orange -brown precipitate of iron(III) hydroxide is formed.
Iron + Sodium → Iron + Sodium
sulphate hydroxide hydroxide sulphate
FeSO4 + 2NaOH → Fe(OH)2 + Na2SO4
▪ Formation of a orange-brown precipitate shows the
presence of iron (III) ions
Testing for SO42-
SO42- , using dilute hydrochloric acid and barium
chloride solution
➢ Dilute hydrochloric acid is added to a solution of the sulphate and then
barium chloride solution is added.
Barium sulphate precipitate
A white precipitate shows the presence of a sulphate.
BaCl2(aq) + Na2SO4(aq)→ BaSO4(s) + 2NaCl(aq)
Testing for Chloride ions (Cl- , Br-, I-)
Add dilute nitric acid to a solution of the halide, and then add silver nitrate
solution.
White ppt
Pale- yellow ppt
Cream ppt
