# Air

Air and Oxygen

Gas %

Nitrogen N2 78%

Oxygen O2 21%

Noble Gases (Ar) 1%

Carbon Dioxide (CO2 ) 0.004%

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.

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.

MandPractivity 14 (a) Show that Approximately One Fifth of the Air is Oxygen.

Cut the base from a plastic drinks bottle. Place the open bottle in a basin of water, mark on the outside of the bottle, the level of the water, now place this over a floating night light over a lit night light that is floating in a trough of water. Screw the bottle cap on airtight. 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.

MandPractivity 15 Prepare a Sample of Oxygen by Decomposing H2O2 Using MnO2 as a Catalyst.

1. Manganese Dioxide (MnO2) black powder 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 will be collected over water in gas jars.

4. Very slowly dribble 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.

Manganese dioxide

Hydrogen Peroxide Water + Oxygen Word Equation

MnO2

H2O2 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

Physical Chemical

Colourless Supports combustion – many items burn in oxygen

odourless Relights a glowing splint – the classical test for oxygen

tasteless Very active element forming oxides with many other elements

Gas at room temperature. Neutral – it has no effect on litmus

Slightly denser than air. Carbon burns with oxygen producing carbon dioxide

Slightly soluble in water Hydrogen burns with oxygen producing water!

Uses of Oxygen (any two) Investigate the ability of oxygen to support combustion in a wooden splint and a candle.

1. Aerobic respiration by the living cells.

2. Burning fossil fuels to supply energy.

3. Assisted Breathing - patients, astronauts, divers.

4. Sewage treatment.

5. Steel manufacture.

6. Plastic manufacture. 1. Light one end of a wooden splint using a Bunsen

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

Demonstrate and Describe what Happens when a Wooden Splint is Burned in Air. Demonstrate and Describe what Happens when Magnesium 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. 1. Light, using a Bunsen, a short strip of magnesium.

2. Hold the burning magnesium over a wire gauze.

3. Do not look directly at the burning magnesium.

4. The magnesium burns extremely hot and bright

5. The ‘white smoke’ settles to a white powder.

6. All the magnesium is converted to the white powder.

Burn Carbon and Test the Products using Litmus. Burn Magnesium in Oxygen and Test using 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 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.

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. 1. Heat a strip of magnesium ribbon that is wrapped 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 and then test the water with 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.

Carbon Dioxide

MandPractivity (a) Preparation of Carbon Dioxide

1. Marble chips of calcium carbonate (CaCO3) are in the flat-bottomed flask.

2. A dropping funnel holds the clear hydrochloric acid .

3. A delivery tube leads from the flask to a trough of water where the gas will be collected over water in gas jars or into a dry gas jar with a temporary cardboard lid for the collection of carbon dioxide by downward delivery.

4. Very slowly dribble the hydrochloric acid into the flask so carbon dioxide gas is produced quietly & steadily.

5. First collect test tubes of the emerging gas to test for carbon dioxide using limewater.

6. Bubble the gas through the limewater and when it goes milky then all the air has been driven out of the apparatus and pure carbon dioxide is coming out.

7. Now collect the gas in gas jars seal with a greased lid.

A lighted taper is used to test for fullness of carbon dioxide those filled by downward delivery – if the taper is extinguished at the top of the gas jar then the jar is full of carbon dioxide.

(b) Show that Carbon Dioxide does not Support Combustion

Insert a lighted taper into a gas jar of carbon dioxide – the taper is extinguished.

Insert a lighted taper into a gas jar of air – the taper is not extinguished.

Therefore carbon dioxide extinguished the taper and this shows that carbon dioxide does not support combustions.

Word Equation Calcium + Hydrochloric » Calcium + Water + Carbon

Carbonate Acid Chloride Dioxide

Chemical Equation CaCO3 + 2HCl » CaCl2 + H2O + CO2

The milkiness is caused by the formation of tiny solid pieces of white insoluble calcium carbonate commonly known as limestone or chalk.

Calcium Hydroxide + Carbon Dioxide » Calcium Carbonate + Water

Ca(OH)2 + CO2 » CaCO3 + H2 O

Physical Chemical

Colourless, odourless and tasteless. Does not support combustion, puts out fires

Much denser than air (one and a half times). Acidic with water, forms carbonic acid (a mild acid)

Fairly soluble in water. Turns limewater milky

That carbon dioxide is denser than air can be shown that it can be poured out of the gas jar and sinks in air.

a) Pour down onto a small candle flame that is then extinguished.

b) Poured onto one side of a balanced metre rule causing it to move down on that side.

Acidity of a Solution of Carbon Dioxide in Water

Bubble the carbon dioxide through blue litmus solution – the litmus goes red. Bubble the carbon dioxide through universal indicator solution – match the colour to one on the chart and note that the pH is less than 7.

Bubble the carbon dioxide through water, test it with red & blue litmus paper – both red the solution is acidic.

Conclusion: carbon dioxide is an acidic oxide.

Uses of Carbon Dioxide

1. Photosynthesis – manufacture of food by plants using light energy.

2. Fire extinguisher – carbon dioxide does not support combustion and it is heavier than air.

3. Fizzy drinks, sparkling water and sparkling wine – great quantities of CO2 can dissolve in water with pressure.

4. Dry Ice is solid CO2 (-78°C) and is used to keep items cold without wetting them. Dry ice turns directly into a gas on heating, it does not melt forming a liquid. & Special misty stage effects.