Properties of air

1. Properties of air • colourless (無色) • odourless(無味) • mixture of different gases:

2. Tests for gases

3. 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

4. Nitrogen (cont’d) • Uses: • 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: • 21 % • Characteristics: • colourless • odourless • support burning (助燃)

6. Oxygen (cont’d) • Uses: • 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: • 0.03 % • Characteristics: • 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: • 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 !!!

12. 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.

13. Breathed & unbreathed air

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 existenceof carbon dioxide in the air conclusion : air contains a small amount of carbon dioxide

18. Summary of activities (cont’d) aim : to prove the existenceof 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 • morecarbon dioxide • morewater vapour • warmer

20. Hydrogencarbonate indicator

21. Hydrogen v.s. Helium