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

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

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

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

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

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

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

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

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

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

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 !!!

Relative humidity 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 goes out quickly goes out slowly hydrogencarbonate indicator red yellow remain unchange(red) cobalt chloride paper blue 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 : less oxygen more carbon dioxide more water vapour 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 danger (may lead explosion) safe (chemically stable) not recommended to use use : to fill airship or balloons