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Composition of Air | Mandatory Practical Activity 14 | Mandatory Practical Activity 15 | Catalyst | Properties of Oxygen

Air and Oxygen 

Air is a Mixture of Gases
Air is an important mixture of gases in everyday life.
It is the source of oxygen for aerobic respiration and burning of fossil fuels.
It is also the source of carbon dioxide for photosynthesis.

Composition of Air

Gas                                          %
Nitrogen (N2 )                            78%
Oxygen (O2 )                             21%
Noble Gases (Ar)                         1%
Carbon Dioxide (CO2 )           0.004%

Water vapour is also present in the air but varies widely from 0% to 4%.

Pollutant gases like sulphur dioxide, nitrogen oxides and ozone are also present but again their percentage is low but varies.

Mandatory Practical Activity 14

(a) Show that Approximately One Fifth of the Air is Oxygen.
Cut the base from a plastic drinks bottle.
Place the open bottle over a lit night light that is floating in a trough of water.
Screw the bottle cap on airtight.
Mark, on the outside of the bottle, the level of the water in the bottle.
The water level rises in the bottle, the night light extinguishes.
Let the air in the bottle cool back to room temperature.
Pour water into the trough to equalise the levels inside and outside the bottle.
Mark new level.
Use the marks to calculate the volume of the original air and the volume of the air that remained after burning.
Calculation will show that burning reduced the air by about 20%.

(b) Show that CO2 is in Air
Carbon dioxide turns limewater milky.
Draw air through clear limewater.
The limewater eventually goes milky.
Therefore air has carbon dioxide because the air turned limewater milky.

(c) Show that Water Vapour is in air.
Place ice inside a beaker.
Note that a colourless liquid condenses on the outside of the beaker where only the air is in contact.
Test this liquid with dry blue cobalt chloride paper.
The paper turns from blue to pink.
Therefore the liquid that condensed from the air is water.

Mandatory Practical Activity 15

Prepare a Sample of Oxygen by Decomposing H2O2 Using MnO2 as a Catalyst.
1. Manganese dioxide black power catalyst is in the flat-bottomed flask.
2. A dropping funnel holds the clear solution of hydrogen peroxide (H2O2 ).
3. A delivery tube leads from the flask to a trough of water where oxygen gas will be collected over water in gas jars.
4. Very slowly dribble the hydrogen peroxide solution into the flask so oxygen gas is produced quietly and steadily.
5. First collect test tubes of the emerging gas to test for oxygen using a glowing splint.
6. When the collected gas relights a glowing splint then pure oxygen is escaping from the flask – all the air has been driven out of the apparatus.
7. Now collect the gas in gas jars – for convenience of collection the gas jars of water stand on a beehive shelf as they are filled with the gas.

Hydrogen Peroxide       manganese dioxide      Water   + Oxygen     Word Equation

         H2O2                           MnO2               2H2O  +     O2       Chemical Equation


A catalyst is a substance that alters the rate of a chemical reaction without itself being used up in the reaction.

Most catalysts speed up reactions.
Manganese dioxide greatly speeds up the decomposition of hydrogen peroxide.

Some catalysts are used in the chemical industry to slow down contaminating reactions.

Properties of Oxygen

a) Physical
1. Colourless, odourless and tasteless.
2. Gas at room temperature.
3. Slightly denser than air.
4. Slightly soluble in water – about 3 cm3 per 100 cm3 of water.

b) Chemical
1. Supports combustion – many items burn in oxygen.
2. Relights a glowing splint – the classical test for oxygen.
3. Very active element forming oxides with many other elements.
4. Neutral – it has no effect on litmus.
5. Carbon burns with oxygen producing carbon dioxide.
6. Magnesium burns with oxygen producing magnesium oxide.

Uses of Oxygen (any two)
1. Aerobic respiration by the living cells of plants and animals.
2. Burning fossil fuels to supply energy.
3. Assisted Breathing – hospital patients, astronauts, climbers, divers.
4. Sewage treatment.
5. Steel manufacture.
6. Plastic manufacture. 

Investigate the ability of oxygen to support combustion in a wooden splint and a candle.
1. Light one end of a wooden splint using a Bunsen flame.
2. Note how well it burns.
3. Blow out the flame and see if it relights – it doesn’t.
4. Now place the glowing tip into a gas jar of oxygen – it relights and burns much more vigorously.
5. Repeat the above procedure with a night light candle – same results..

Demonstrate and Describe what Happens when a Wooden Splint is Burned in Air.
1. Light a thin strip of wood at one end in a Bunsen flame.
2. It catches fire producing smoke and visible flame of heat and light.
3. The burning continues to the far end.
4. All that remains is a thin sliver of black carbon.

Demonstrate and Describe what Happens when Magnesium is Burned in Air.
1. Light, using a Bunsen flame, one end of a short strip of magnesium ribbon.
2. Hold the burning magnesium over the centre of a wire gauze.
3. Do not look directly at the burning magnesium.
4. The magnesium burns extremely hot and bright with much ‘white smoke’.
5. The ‘white smoke’ settles to a white powder.
6. All the magnesium is converted to the white powder.

Burn Carbon in Oxygen and Test the Products using Moist Litmus.
1. Heat charcoal (graphite carbon) in a deflagrating spoon with a Bunsen flame until it starts to burn.
2. Then place the burning charcoal in the deflagrating spoon into a gas jar of oxygen that has a small layer of water.
3. When the burning stops swirl the water around the gas jar and then test the water with red and blue litmus paper.
4. The red stays red and the blue litmus goes red showing that carbon dioxide is an acidic oxide – it dissolved in the water forming carbonic acid.

If the above investigation is carried out with a shallow layer of limewater the limewater will go milky showing that carbon dioxide was produced by the burning of carbon in oxygen.

Burn Magnesium in Oxygen and Test the Products using Moist Litmus.
1. Heat s strip of magnesium ribbon that is wrapper around the spoon end of a deflagrating spoon until it starts to burn.
2. Then place the burning magnesium on the deflagrating spoon into a gas jar of oxygen that has a small layer of water – note that the magnesium burns much brighter and more vigorously with a rapid formation of white powder..
3. When the burning stops swirl the water around the gas jar to ‘dissolve’ the white powder and then test the water with red and blue litmus paper.
4. The blue stays blue and the red litmus goes blue showing that magnesium oxide is a basic oxide – it dissolved in the water forming magnesium hydroxide.



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