Properties of air

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
Tests for gases
O2
CO2
N2
colour
colourless
colourless
colourless
smell
no smell
no smell
no smell
burning splint
relight (重燃)
goes out
goes out
glowing splint
relight
goes out
goes out
hydrogencarbonate
red
indicator
lime water
purple red
colourless
yellow remain red
milky
colourless
Nitrogen (N2)

Percentage in air:
78 % (about 4/5 of the air)

Characteristics:
colourless
odourless
very un-reactive(stable) (穩定)
most in the air
Nitrogen (cont’d)

Uses:
1.
2.
3.
4.
5.

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
Oxygen (O2)

Percentage in air:
21 %

Characteristics:
colourless
odourless
support burning (助燃)
Oxygen (cont’d)

Uses:
1.
2.
3.
4.
5.

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
Carbon Dioxide (CO2)

Percentage in air:
0.03 %

Characteristics:
colourless
odourless
Carbon Dioxide (cont’d)

Uses:
1.
2.
3.
4.

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
Noble gases(inert gases)

Examples:
helium, neon, xenon, argon

Percentage in air:
0.9 %

Characteristics:
colourless
odourless
chemically stable
Noble gases (cont’d)

Uses:
1.
2.
3.
4.
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
Water vapour

Percentage in air:
variable

Characteristics:
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 !!!
Relative humidity


humidity : amount of water vapour in the air
relative humidity :
amount of water vapour in the air
max. amount of water vapour in the air at the same temp.
Breathed & unbreathed air
burning splint
hydrogencarbonate
indicator
cobalt chloride paper
Breathed air
Unbreathed air
goes out quickly
goes out slowly
red
blue
yellow remain unchange(red)
pink
remain unchange
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
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
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
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
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
Summary of activities (cont’d)
aim :
find out the difference between breathed & unbreathed air
conclusion :
characteristics of breathed air :
1.
less oxygen
2.
more carbon dioxide
3.
more water vapour
4.
warmer
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
Hydrogen v.s. Helium
Hydrogen
Helium
light in mass
light in mass
danger (may lead explosion)
safe (chemically stable)
not recommended to use
use : to fill airship or balloons