2. Air Chapter 31

3. Air Air is a mixture of gases which forms a blanket around the earth. Another name for air is the Atmosphere.

4. Composition of Air –Nitrogen 78% –Oxygen 21% –Inert gases and Carbon Dioxide 1% The main gases in air are:

5. Proof that Air is a mixture Air is a mixture because: –Its composition varies from place to place. –The different gases condense as liquids at different temperatures. N 2 -196 o C O 2 -183 o C –When the gases are mixed in the right proportions the result is air.

6. Experiment: To measure the % of Oxygen in air 1. Set up apparatus as shown in Diagram. 2. 100 cm3 of air is passed backward and forward over the heated copper.

7. To measure the % of Oxygen in air Results Volume of air before heating= 100cm 3 Volume of air after heating = 79cm 3 Calculations % Oxygen = decrease in volume of air x 100 = 21 x 100 = 21% initial volume of air 100 Conclusion: Percentage of Oxygen in Air = 21%

8. Experiment: To show that Carbon dioxide is present in air. 1. Set up apparatus as shown in the diagram. 2. Allow air to be drawn through the limewater for about half. an hour. Result The limewater turns milky Conclusion This shows us that carbon dioxide is present in air

9. Experiment: To show that water vapour is present in air. 1. Set up apparatus as shown in diagram. 2. Test the liquid with the cobalt chloride paper and observe the copper sulphate. Results The liquid turns the white anhydrous copper sulphate blue The blue cobalt chloride paper turns pink. Conclusion Water vapour is present in air

10. Oxygen Oxygen makes up 21% of the air. It is prepared in the Laboratory by the breakdown of Hydrogen Peroxide(H 2 O 2 ) into Oxygen(O 2 ) and Water. 2H 2 O 2 --> 2H 2 O + O 2 Manganese Dioxide acts as a catalyst. A catalyst is a substance that changes the speed of a chemical reaction but is not used up in the reaction itself

11. Preparation of Oxygen 1. Set up apparatus as shown in diagram 2. Allow the Hydrogen peroxide to fall on the Manganese Dioxide catalyst. 3. Collect the gas given off

12. Properties of Oxygen Oxygen re-lights a glowing splint Test for Oxygen Properties of Oxygen

13. Formation of Oxides Oxygen is a reactive element and it combines with other elements to form oxides. Reaction with Carbon C + O 2 --> CO 2 Carbon + Oxygen --> Carbon Dioxide Carbon forms an Acidic Oxide

14. Reaction of Oxygen with Magnesium Magnesium burns in Oxygen to form Magnesium Oxide. 2Mg + O 2 --> 2MgO Magnesium forms a basic Oxide

15. Uses of Oxygen 1. Breathing 2 Welding 3.Burning 4. Medicine 5. Manufacture of Steel 6. Space-rockets

16. Carbon Dioxide Carbon dioxide makes up less than 1% of the air but is a very important gas. It is needed by plants for photosynthesis. Without Carbon Dioxide plants would not grow. Without plants, animals would not survive

17. Preparation of Carbon Dioxide Carbon Dioxide is prepared in the lab by the reaction between Hydrochloric acid(HCl) and marble chips(CaCO 3 ) 2HCl + CaCO 3 --> CaCl 2 + H 2 O + CO 2

18. Properties of Carbon Dioxide Does not support combustionHeavier Than Air

19. Test For CO 2 Carbon Dioxide turns Limewater Milky. Limewater + Carbon Dioxide --> Chalk + Water Ca(OH) 2 + CO 2 --> CaCO 3 + H 2 O

20. Carbon Dioxide does not support combustion

21. Uses of Carbon Dioxide Photosynthesis Fizzy Drinks Fire Extinguishers Refrigeration Stage Shows(Dry Ice)