Carbon Dioxide, Oxygen and Air

Air

Air is a mixture of gases.
It is a solution of oxygen, argon and carbon dioxide dissolved in nitrogen.

The composition of air
The composition of the air varies.

A typical sample would contain about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide.

Some do not consider water vapour to be part of the air as its boiling point  (100˚C) is much higher than the average temperature of the atmosphere (15˚C). The level of water vapour in the air varies depending on the humidity, from near 0% to 4%.

Carbon dioxide is given out by volcanic activity and by living organisms during respiration. Plants excrete carbon dioxide at night. Carbon dioxide is removed from the air during photosynthesis.

All the oxygen in the air was produced by photosynthesis. Even in poor light photosynthesis is faster than respiration. During the day plants produce more oxygen than they need for their respiration. The excess oxygen is excreted in the air. Plants are the ‘lungs of the Earth’.
 
The largest component of air is the very inactive gas nitrogen. This gas is stable and fairly unreactive and its percentage in air remains constant.

Limewater goes milky if air is bubbled through it. This shows that carbon dioxide is present in the air.

If a beaker of crushed ice and salt is suspended in the air a clear liquid condenses on the outside of the beaker. Dry blue cobalt chloride paper turns pink with this liquid - the liquid must be water. Therefore the air contains water vapour.

Air is a mixture: proofs

1. The composition of air varies.
2. The components of air can be separated by physical method e.g. fractional distillation: nitrogen boils off at –196˚C, oxygen at –183˚C.
3. Physical mixing of the component gases produces air without heat being lost or taken in i.e. heat is not involved in the production of air.

The composition of a compound does not vary, its components cannot be separated by physical methods and heat is involved in the formation of a compound.

Oxygen makes up about 20% of the air
Burn a candle or phosphorus in an enclosed volume of air e.g. in a bell jar that is sitting in a basin of water.
Note that the water level first goes down in the bell jar – the increase in temperature expands the air in the bell jar.
The water starts to rise – the water is moving in to take the place of the oxygen used up during burning.

The candle or phosphorus ‘goes out’ – all the oxygen has been used up.
The water continues to rise – cooling causes the air in the bell jar to contract.
When the temperature returns to room temperature the rise in water stops.
Pour water into the basin so the water levels inside and outside the bell jar are the same – the air pressure inside and outside the bell jar is then identical.
When the calculations are done it will be found that the volume of air reduced by about 20% because of the burning.
Therefore oxygen gas is about 20% of the air.

Oxygen

Produced in the laboratory by the breakdown of hydrogen peroxide.

Hydrogen peroxide (H₂O₂) is quite unstable, breaking down to water and oxygen.
The breakdown is too slow to be useful in the laboratory.
The breakdown can be greatly speeded up by the use of a catalyst.

Catalyst: a substance that alters the rate of a chemical reaction without itself being changed by the reaction.
In practice, catalysts are mostly used to speed up reactions.

Manganese dioxide (MnO₂) is the catalyst used to speed up the release of oxygen gas from hydrogen peroxide.
 2H₂O₂  + MnO₂  -> MnO₂  +  2H₂O  + O₂ 

              2H₂O₂ -> 2H₂O  + O₂

The manganese dioxide is in the form of a black powder in the flask.
Hydrogen peroxide solution is in a dropping funnel.
Slowly drip the hydrogen peroxide solution into the flask to maintain a steady quiet production of oxygen gas.
After a minute, collect the oxygen gas in gas jars of water placed upside down on a beehive shelf in a basin of water.
(The gas that first escapes from the flask and delivery tube is air pushed out by the advancing oxygen.)
Test the gas for purity: relights a glowing splint if it is pure oxygen.

Physical properties of oxygen
Colourless, odourless, tasteless gas at room temperature.
A little denser than air.
Slightly soluble in water, more soluble in cold water.

Chemical properties of oxygen
Relights a glowing splint (the classic test) – substances burn better in oxygen than in air.
Magnesium burns in oxygen forming magnesium oxide (Mg0) a white solid – this is a basic oxide dissolving in water forming magnesium hydroxide.
Carbon burns in oxygen forming carbon dioxide (C0₂) a colourless gas – this is an acidic oxide dissolving in water forming carbonic acid.
Sulphur burns in oxygen forming sulphur dioxide (S0₂) a colourless gas – this is an acidic oxide dissolving in water forming sulphurous acid.
(Carbon dioxide and sulphur dioxide are pollutants produced by the burning of fossil fuels.)

Some everyday uses of oxygen
Aerobic respiration: the controlled release of energy from food, using oxygen.
Breathing: high altitude climbers, astronauts, premature babies, people with damaged lungs, scuba divers.
Burning fossil fuels for heat and electricity.
Welding and steel making.
Making of nitrate fertilisers.
 

Carbon Dioxide

Produced in the laboratory by the action of an acid on a metal carbonate.
        Acid     +    Carbonate      ->    Salt  +  Water  +  Carbon Dioxide

Hydrochloric      Calcium  Calcium
     Acid        +    Carbonate     ->  Chloride    +  Water  +  Carbon Dioxide

  2HCl          +    CaCO₃           ->    CaCl₂       +     H₂O     +     CO₂ 

The calcium carbonate is usually in the form of marble chips in the flask.
The dilute hydrochloric acid is in a dropping funnel.
Add enough acid to cover the marble chips.
Add more acid if the production of carbon dioxide slows down.
At first, pass the escaping gas through limewater.
When the limewater turns milky then pure carbon dioxide gas is passing from the flask - the gas that first escapes from the flask and delivery tube is air pushed out by the advancing carbon dioxide.
Turning limewater milky is the classic test for carbon dioxide.
Since carbon dioxide is much denser than air it may be collected by the upward displacement of air (downward delivery).
Because it is not very soluble in water carbon dioxide may also be collected, like oxygen, in gas jars of water placed upside down on a beehive shelf in a basin of water.
Knowing when the gas jar is full of carbon dioxide is a bit of a problem because it is a colourless gas. Use a lighted taper to check the gas jar for fullness. The lighted taper will be extinguished at the mouth of gas jar when it is full of carbon dioxide. Carbon dioxide does not support combustion (does not allow burning).

Physical properties of carbon dioxide
Colourless, odourless, tasteless gas at room temperature.
Much denser than air – about one and a half times denser.
Slightly soluble in water.
Solid carbon dioxide (dry ice) sublimes at room temperature.

Chemical properties of carbon dioxide
Turns limewater milky – the classic test.
Does not burn and does not support combustion.
Reacts with water when it ‘dissolves’ forming a weak carbonic acid.
Absorbed by sodium hydroxide (soda lime) – this property is used in some respiration and photosynthesis experiments.

Some everyday uses of carbon dioxide
Photosynthesis.
Fire extinguisher – especially for all indoor fires as it is safe and clean.
Fizzy or sparkling drinks.
Refrigeration: dry ice keeps items cold without wetting them because it sublimes.
‘Foggy effect’ for concert and stage productions.