air as a mixture of gases

OC21 understand that air is a mixture of gases,

and state the composition of air (approx 78% N2 and 21% O2, with CO2, H2O & other gases making up the balance)

Note this is for Dry Air, water vapour can make up typically 1%-4% at surface


OC22 show that approximately one fifth of the air is oxygen;

There are many ways of doing this experiment, some are good some are bad. With some the equipment is not cheap, the following method is a reasonably good method and with easy to obtain materials, however the experiment should be left over a weekend, as it does take a little time.

This experiment is based on the fact that rusting requires oxygen


Steel wool, 2 graduated cylinders, a basin and retort stand.


  1. Place some steel wool in the bottom of a graduated cylinder a small ball will do. You may have to use a ruler to get it down the bottom.
  2. Pour in water until it gets to the 100ml mark, allow some time for air bubbles to rise to the top (tapping the side of the cylinder can help this).
  3. Top up the water until the bottom of the meniscus is at the 100ml mark. It should be obvious that the volume of water is not 100ml because the steel wool takes up space too.
  4. Pour out the water into the other graduated cylinder to find the volume of water. Write this value down.
  5. Invert the cylinder with the wool in a basin of water, with a little practice you can trap some water in here too.
  6. Repeat step 5 until the level of water is on the graduations but close to the 100ml mark, read the height of the water on the graduations, this is the Volume of air + the volume of steel wool.
  7. Clamp in position and leave for a few days, make sure the readings you have taken down are safe and will nor be lost
  8. The water will rise as the Oxygen in the tube get used up in the rusting.
  9. find the volume of air after these reactions have taken place.
  10. Find the difference between the volumes thus determine the percentage of Oxygen in air .... it should be about 20%


Volume of water poured out = cm3. (1)

100 - Vol of water poured out = cm3 = Volume of steel wool (2)

Air pressure =

Volume of air in upturned graduated cylinder at start = cm3 (3)

(Height of water in tube before - Vol steel wool)

Volume of air in upturned graduated cylinder at end = cm3 (4)

(Height of water in tube after - Vol steel wool)

Difference (3 - 4) = cm3 (5)


What is in the cylinder ?? Graduated ???

Total Volume = 100 ml

Vol of Steel Wool = 11

Vol of Water = 8

+ Vol of Air = X



Volume of Air in the Beginning ...... =

Vol of Air = 100 - (Vol of Steel Wool + Vol of water)

Volume of Air at the End ...... =

show that there is CO2 in Air

Characteristic test for Carbon Dioxide

To do this we need to find a substance that detects CO2, Such a substance is Limewater, it is clear when it is fresh, it goes Milky in colour when it comes in contact with Carbon Dioxide.

So now we need to get carbon dioxide into the limewater

If you have done the section on Respiration you might have noticed that sucking in did not cause the limewater to go milky, so we will need a more effective method, we could replace our lungs with a suction pump.

An good and simple alternative is to get a pump and to pump air through the lime water.

Characteristic test for Carbon Dioxide

Limewater goes milky in colour when it has CO2 passing through it !

Carbon Dioxide also extinguishes fire !

and is heavier than air !

how much CO2 is in the air .....

CO2 emissions League

Russia 32.9 Tonnes / hd/an

USA 19.5

Australia 18.4

7 France 11.9

8 Germany 10.4

11 UK 9.6

and water vapour in air

anhydrous copper sulphate

white anhydrous copper sulphate which will turn blue in the presence of water

often the container labelled anhydrous copper sulphate will have gone lightly blue, because air contains water vapour

cobalt chloride!

*Cobalt Chloride is on the banned substance list*

is blue when it is dry


pink when it is in the presence of water!

OC23 demonstrate and describe what happens when (i) a wooden splint and (ii) a piece of magnesium are burned in air

products of combustion of carbon and magnesium


OC24 prepare a sample of oxygen by decomposing H2O2 using MnO2 as a catalyst (word equation and chemical equation)

OC25 investigate the ability of oxygen to support combustion in a wooden splint and a candle, and state two uses of oxygen

There is enough oxygen in 2kg of moon dust to keep a human alive for a day!

OC26 burn carbon and magnesium in oxygen, and test the products using moist litmus paper


OC27 prepare carbon dioxide (word equation and chemical equation), and show that it does not support combustion

preparation and properties of carbon dioxide

density of CO2 (qualitative only) acidity of a solution of CO2 in water

Set up the apparatus as shown

By coating all the joins with a petroleum jelly (like vaseline) make sure the system is air tight.

Flood a gas jar with water in and place it with the mouth of the gas jar open above the hole in the beehive shelf.

Slowly drop the HCl on the marble chips.

Observe the reaction in the flask,

Check out the link below for the full description of what happens in the setup to prepare a sample of Carbon Dioxide (CO2) .

Hydrochloric Acid + Marble chips → Calcium Chloride + Water + Carbon Dioxide

Hydrochloric Acid + Calcium Carbonate → Calcium Chloride + Water + Carbon Dioxide

Gas Tests

Place a lit splint in to the gas jar. Write down your observations

The Splint stops burning

Place a glowing splint into the gas jar. Write down your observations

The Splint stops Glowing

Pour some Limewater in the Jar. Write down your observations

The Limewater turns MILKY

Pour the contents onto a lit candle Write down your observations.

The CO2 is denser than air and so flows down putting out the candle

Pour some water into the spent gas jar and test with Litmus papers Write down your observations

Litmus paper turns Blue, indicating a base

The first part of a pneumatic system may not sound like an actual part, the compressed gas itself. There are three main gases used in the construction of a robot pneumatic weapon system; Carbon Dioxide (CO2), Nitrogen (N2), and High Pressure Air (HPA). Nitrogen and HPA can be compressed in a tank to upwards of 5000 psi (pounds per square inch) but require larger tanks. CO2 on the other hand liquefies at around 850 psi which allows it to take up less space and therefore need a smaller tank (The actual pressure is dependant upon outside ambient temperature. The note below will explain why but for the purpose of this help section we'll use 850 psi). CO2 exhibits some really unique properties under pressure and when it is vented. Without going into the theories of thermodynamics let's suffice it to say that as pressure drops so does the temperature and lower temperatures create lower pressures to a point of equilibrium. In other words, a drop in pressure results in a drop in temperature but that same pressure will not come back until the ambient temperature rises back to the same point.

When you let some of the gas out of a CO2 tank (or any pressurized tank for that matter [see below]) the pressure inside the tank drops which makes the tank get cold (REAL cold). Well, now that there isn't as much pressure in the tank the liquid CO2 starts to boil. The boiling CO2 releases gas which increases the pressure until there is enough pressure to keep the liquid from boiling any more, which happens to be around 850 psi. Pretty cool, huh? So as long as you have liquid CO2 in the tank, and it is kept at the same temperature the pressure remains at a relative constant.

An Easy way to prepare CO2 would be to mix some bicarbonate of soda with some vinegar,

Use a funnel to place some vinegar in a small drinks bottle. Over the mouth of the bottle place a balloon with some bicarbonate of soda in the balloon, hold up the balloon, and Observe.

OC28 carry out simple tests on carbon dioxide involving its reaction with limewater (word equation and chemical equation), and with moist litmus paper

acidity of a solution of CO2 in water

Blue Limus

Stayed Blue

Red Litmus

Goes Bluish


pH = 6

OC29 investigate the density of carbon dioxide relative to air (qualitative only), and state two uses of carbon dioxide

Carbon Dioxide is more dense than air

for equal volumes of Air and Carbon Dioxide the Carbon Dioxide would have a greater mass

for equal masses of Air and Carbon Dioxide the Carbon Dioxide would less space

  • Dry Ice is Solid Carbon Dioxide (and thus cold)
  • Dry Ice does not Melt into a Liquid but SUBLIMES straight in to a gas.
  • Dry Ice is used as a stage effect (smokey stage?!?!)
  • Dry Ice was used as a way of keeping Ice Cream Cold
  • Carbon Dioxide is used in Fire Extinguishers
  • Carbon Dioxide is the bubbles in your Coke / Fanta / fizzy drink (it also makes the drink acidic, gives a bite to the taste)
  • Plants absorb CO2 and use it to make their own food in Photosythesis
  • (some) Fridges use Carbon Dioxide to remove heat from within the Fridge.


Test your knowledge at, you will do well

2. Give two uses of oxygen

5. Carbon dioxide is used for

6. Something that changes the rate of a chemical reaction without being used up is a ________________________

7. When water is present cobalt chloride paper turns ________________________

8. The percentage of carbon dioxide in air is __________________

9. The pH of oxygen is ______.

10. The pH of carbon dioxide is _____.

11. Draw a labelled diagram showing how you would show there is water vapour in air. (9)