Composition of Air • Investigate the % of oxygen in the air,
using a night light (MP)
• Main steps– Floating night light, lit, graduated cylinder
over it–Water rises– Allow to cool, measure volume of gas left– Not very accurate, air is oxygen
Issues with experiment
• Night light goes out without all oxygen being used up
• Graduated cylinder does not measure volume as accurately as gas syringe
Composition of air
• Reaction: Copper + Oxygen Copper oxide– Start with 100cm3 of air (100%)– Volume at end of reaction 79cm3 (79%)– Volume of oxygen reacted with copper =
21cm3 21% oxygen
Why it’s more accurate• More oxygen removed from air• Gas syringe is more accurate at
measuring volume
• What is the significance of:cobalt chloride
limewaterlitmus
glowing splinta lighting splint going out?
Composition of Air• To show that carbon dioxide & water
vapour are present in air (MP)
– Cobalt chloride paper goes from blue to pink in presence of water
– Limewater goes milky/cloudy in presence of CO2
Preparation of oxygenDecomposition of hydrogen peroxide
Word equationHydrogen peroxide water + oxygen
Chemical equation (HL)2H2O2 2H2O + O2
–This reaction requires a catalyst–Substance which speeds us a chemical reaction but is not used up in the reaction itself–Catalyst in this reaction is manganese dioxide/MnO2
MnO2
Preparation of Oxygen (MP)
• Main steps– H2O2 dropped on to catalyst– Oxygen produced is collected over water as it
is only slightly soluble
Properties of oxygen (MP)• Oxygen is neutral• No change in colour of
red litmus or blue litmus paper
• Oxygen relights a glowing split
Properties of oxygen (MP)• Burning carbon in
oxygen• Another name for
burning is combustion• Rxn: Carbon + oxygen
carbon dioxide• C + O2 CO2
• Limewater turns milky• Carbon dioxide +
water turns blue litmus paper red acidic gas
• Carbon dioxide quenches a burning split
• Denser then air
Properties of Oxygen (MP)• Burning magnesium in oxygen– Burns much brighter in oxygen then in air–Magnesium + oxygen magnesium oxide
Properties of Oxygen (MP)• Magnesium oxide and waterWord equation:Magnesium oxide + water
magnesium hydroxide– Red litmus turns blue a base is formed
Properties of Oxygen - summary
Physical Properties Chemical PropertiesColourless, odourless, tasteless gas
Supports combustions, substance burn more vigorously
Slight soluble in water Reacts with most elements to form oxides
Slightly heavier then air No effect on litmus
Summary of reactions with O2
Element Reaction with O2
Compound formed
Effect of compound on
moist litmus paper
Carbon Glows brightly
Carbon dioxide
Blue litmus turns red (acidic)
Magnesium Burns vigorously
Magnesium oxide
Red litmus turns blue (basic)
• What is the significance of:cobalt chloride
limewaterlitmus
glowing splinta lighting splint going out?
Uses of Oxygen• Breathing– Stored in cylinders– Used in hospitals and by mountain climbers
• Burning and welding– Acetylene in O2 reaches 3000C
– Flame can cut through melting or melt metal and join them together
• Space rockets – Need to bring own oxygen to space for
combustion