1. Properties of air mixture of different gases: colourless (無色)
odourless (無味)
mixture of different gases:
nitrogen
(氮)
78 %
oxygen
(氧)
21 %
carbon dioxide
(二氧化碳)
0.03 %
noble gases
(惰性氣體)
0.9 %
water vapour
(水蒸氣)
variable

2. Tests for gases O2 CO2 N2 relight red purple red yellow milky colour
colourless
smell
no smell
burning splint
relight (重燃)
goes out
glowing splint
relight
hydrogencarbonate
indicator
red purple
red yellow
remain red
lime water
milky

3. Nitrogen (N2) Percentage in air: Characteristics:
78 % (about 4/5 of the air)
Characteristics:
colourless
odourless
very un-reactive(stable) (穩定)
most in the air

4. Nitrogen (cont’d) Uses: Test(s):
used in electronic industry make electronic parts
cooling effect (-196 °C) fast freezing (急凍)
fertilizers (肥料)
fireworks (煙花)
anaesthetic (麻醉藥)
Test(s):
no simple test

5. Oxygen (O2) Percentage in air: Characteristics: 21 % colourless
odourless
support burning (助燃)

6. Oxygen (cont’d) Uses: Test(s):
need for all living things to keep alive (生存)
medical use baby in oxygen box
burning BBQ
diving (潛水) : the breathing (呼吸) of diver
welding (燒焊)
Test(s):
a glowing (半熄滅的) splint (木條) : relight

7. Carbon Dioxide (CO2) Percentage in air: Characteristics: 0.03 %
colourless
odourless

8. Carbon Dioxide (cont’d)
Uses:
need for plant photosynthesis (光合作用)
making soft drinks (汽水)
fire extinguishers (滅火器)
dry ice (乾冰)
Test(s):
hydrogencarbonate indicator : red to yellow
lime water : colourless to milky

9. Noble gases(inert gases)
Examples:
helium, neon, xenon, argon
Percentage in air:
0.9 %
Characteristics:
colourless
odourless
chemically stable

10. Noble gases (cont’d) Uses:
fill in light bulb prevent overheat (過熱) of filament
used in advertising signs (廣告牌) glow in different colour when electric current passed
helium replace hydrogen fill in airships or balloons
diving : mixture of helium & oxygen

11. Water vapour Percentage in air: Characteristics: Test(s): variable
colourless
odourless
condense to water droplets (水滴) on a cold surface
Test(s):
dry cobalt chloride paper : blue to pink
Can’t use our hands to touch the paper !!!

12. Relative humidity humidity : amount of water vapour in the air
max. amount of water vapour in the air at the same temp.

13. Breathed & unbreathed air
burning splint
goes out quickly
goes out slowly
hydrogencarbonate
indicator
red yellow
remain unchange(red)
cobalt chloride paper
blue pink
remain unchange

14. Summary of activities aim :
carry out simple tests to show the composition of air
conclusion :
oxygen relights a glowing splint & burns more intensely for a burning splint
carbon dioxide turns hydrogencarbonate indicator from red to yellow & turns lime water from colourless to milky
no simple test for nitrogen

15. Summary of activities (cont’d)
aim :
to prove the existence (存在) of oxygen in the air
conclusion :
there is oxygen in the air
air is not totally made up of oxygen

16. Summary of activities (cont’d)
aim :
find out the percentage of oxygen in the air
conclusion :
rise in water level
 100 %
= ~ 21 %
length of boiling tube
air contains about 21 % of oxygen

17. Summary of activities (cont’d)
aim :
to prove the existence of carbon dioxide in the air
conclusion :
air contains a small amount of carbon dioxide

18. Summary of activities (cont’d)
aim :
to prove the existence of water vapour in the air
conclusion :
air contains a small amount of water vapour

19. Summary of activities (cont’d)
aim :
find out the difference between breathed & unbreathed air
conclusion :
characteristics of breathed air :
less oxygen
more carbon dioxide
more water vapour
warmer

20. Hydrogencarbonate indicator
% of carbon dioxide
colour of indicator
> 0.03 (e.g. breathed air)
yellow
about 0.03 (e.g. atmosphere)
red
< 0.03
purple

21. Hydrogen v.s. Helium Hydrogen Helium light in mass
danger (may lead explosion)
safe (chemically stable)
not recommended to use
use : to fill airship or balloons