Air
Chapter 31
Air
• Air is a mixture of
gases which forms a
blanket around the
earth.
• Another name for
air is the
Atmosphere.
Composition of Air
The main gases in air are:
– Nitrogen
78%
– Oxygen
21%
– Inert gases and
Carbon Dioxide 1%
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. N2 -196o C O2 -183o C
– When the gases are mixed in the right
proportions the result is air.
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.
To measure the % of Oxygen in air
• Results
Volume of air before heating= 100cm3
Volume of air after heating = 79cm3
• 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%
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
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
Oxygen
Oxygen makes up 21% of the air.
It is prepared in the Laboratory by the breakdown of
Hydrogen Peroxide(H2O2) into Oxygen(O2) and Water.
2H2O2 --> 2H2O + O2
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
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
Properties of Oxygen
Test for Oxygen
Oxygen re-lights a
glowing splint
Properties of Oxygen
Physical Properties
Colourless, Odourless, tasteless
gas
Slightly soluble in water
Slightly heavier than air
Chemical Properties
Supports combustion
Reacts with most elements to
form oxides
No effect on litmus
Formation of Oxides
Oxygen is a reactive element and it combines with other
elements to form oxides.
Reaction with Carbon
C + O2 --> CO2
Carbon + Oxygen --> Carbon Dioxide
Carbon forms
an Acidic Oxide
Reaction of Oxygen with Magnesium
Magnesium burns in Oxygen to form Magnesium
Oxide.
2Mg
+ O2 --> 2MgO
Magnesium forms a basic Oxide
Uses of Oxygen
1. Breathing
2 Welding
3 .Burning
4. Medicine
5. Manufacture of Steel
6. Space-rockets
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
Preparation of Carbon Dioxide
2HCl + CaCO3 --> CaCl 2 + H2O + CO2
Carbon Dioxide is prepared in
the lab by the reaction between
Hydrochloric acid(HCl) and
marble chips(CaCO3)
Properties of Carbon Dioxide
Physical Properties
Colourless Odourless Tasteless gas
Slightly Soluble in Water
Heavier than Air
Does not support combustion
Chemical Properties
Does not supports Combustion
Turns limewater milky
Dissolves in water to form acidic
solutions
Heavier Than Air
Test For CO2
Carbon Dioxide turns Limewater Milky.
Limewater + Carbon Dioxide
--> Chalk
Ca(OH)2
--> CaCO3 + H2O
+
CO2
+ Water
Carbon Dioxide does not support combustion
Uses of Carbon Dioxide
•
•
•
•
•
Photosynthesis
Fizzy Drinks
Fire Extinguishers
Refrigeration
Stage Shows(Dry Ice)