CHEM SPA!
1. Note COLOUR and ODOUR of gas.
2. Test the gas evolved with MOIST RED AND BLUE LITMUS PAPERS.
3. SPECIFIC TESTS
Note COLOUR and ODOUR of gas.
Colourless and odourless
Colourless and pungent
Greenish-yellow and pungent
*colour isn’t very obvious – hold against white surface to see!
H2, O2, CO2
NH3, SO2
Cl2
Test the gas evolved with MOIST RED AND BLUE LITMUS PAPERS.
No colour change to the moist red and blue litmus paper is observed H2, O2
Gas evolved turns moist blue litmus paper red
CO2, SO2
*CO2 cannot really be observed!
Gas evolved turns moist red litmus paper blue
NH3
Gas evolved turns moist blue litmus paper red and bleaches it
Cl2
Gas evolved bleaches moist red litmus paper
Gas is neutral
Gas is acidic
Gas is alkaline
Gas evolved has
bleaching properties,
gas evolved is chlorine
SPECIFIC TESTS
Hold a lighted splint near the mouth of the test tube
Insert a glowing splint into the test tube
Bubble the gas evolved into limewater
 Use 2cm of limewater
 Hold limewater test tube on right, solution on left
 Heat solution – in and out of flame
 Limewater must be removed before solution to prevent
“suck back”
Hold a strip of filter paper which has been dipped in acidified
potassium dichromate (VI) solution, at the mouth of the test
tube
Hold a strip of filter paper which has been dipped in acidified
potassium manganate (VII) solution, at the mouth of the test
tube
Hold a piece of moist starch-potassium iodide paper near the
mouth of the test tube
Gas evolved extinguishes lighted
splint with a “pop” sound
Gas evolved relights glowing splint
Gas evolved forms a white
precipitate with limewater
H2
Gas evolved turns acidified
K2Cr2O7 solution from orange to
green
Gas evolved turns acidified
KMnO4 solution from purple to
colourless
Gas evolved turns moist starchpotassium iodide paper blue-black
SO2
O2
CO2
SO2
Cl2
Hydrogen
1. Colourless and odourless
2. No colour change to the moist red and blue litmus papers is observed = Gas is neutral
3. Hold a lighted splint near the mouth of the test tube
= Gas evolved extinguishes the lighted splint with a “pop” sound
Oxygen
1. Colourless and odourless
2. No colour change to the moist red and blue litmus papers is observed = Gas is neutral
3. Insert a glowing splint into the test tube
= Gas evolved relights the glowing splint
Carbon Dioxide
1. Colourless and odourless
2. Gas evolved turns moist blue litmus paper red (cannot really be observed) = Gas is acidic
3. Bubble gas evolved into limewater
= Gas evolved forms a white precipitate with limewater
Sulphur Dioxide
1. Colourless and pungent
2. Gas evolved turns moist blue litmus paper red = Gas is acidic
3. Hold a strip of filter paper that has been dipped in acidified potassium dichromate (VI) solution near
the mouth of the test tube
= Gas evolved turns acidified potassium dichromate solution from orange to green
4. Hold a strip of filter paper that has been dipped in acidified potassium manganate (VII) solution near
the mouth of the test tube
= Gas evolved turns acidified potassium manganate solution from purple to colourless
Ammonia
1. Colourless and pungent
2. Gas evolved turns moist red litmus paper blue
= Gas is alkaline
Chlorine
1. Greenish-yellow and pungent
2. Gas evolved turns moist blue litmus paper red and bleaches it, gas evolved bleaches moist red litmus
paper
= Gas evolved has bleaching properties, gas evolved is Chlorine
3. Hold a strip of moist starch-potassium iodide paper near the mouth of the test tube
= Gas evolved turns moist starch-potassium iodide paper blue-black
Bunsen Burner
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Close air hole
Adjust until non-luminous flame is obtained
Adjust gas tap to obtain desired strength of flame
Turn off flame when not in use (non-luminous flame is INVISIBLE)
Heating
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Wear safety glasses
When heating a solid, must use clean and dry test tube
Use a test tube holder! About ¼ length from top of test tube
Point mouth of test tube away from people
Gently shake solution as it is being heated
Move test tube in and out of flame
LOOK at the test tube and the flame throughout
Place all chemicals away from flame
Do not touch hot test tube
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Cap bottles
Handle apparatus properly
Do not place moist litmus papers on table/wherever
Amount of solutions used should be 2cm
Gas Collection
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Depends on physical properties of gas
o Solubility in water
o Density compared to air
Not soluble (hydrogen) or sparingly soluble (oxygen) in water – collected by downward
displacement of water
Soluble/react with water – air in a container is displaced by these gases.
o Gas has density greater than density of air – displace air from bottom to top of upright
gas jar (eg chlorine, carbon dioxide, sulphur dioxide, nitrogen dioxide) = downward
delivery
o Gas has density lower than density of air – displace air from top to bottom of inverted gas
jar (eg ammonia) = upward delivery