Air Composition and Pollution (Senior 1 Chemistry): free lessons | Ganzaa

Air Composition and Pollution

Lessons

Senior 1 · Chemistry

Air Composition and Pollution

5 lessons

Lesson 1 of 5

Components of Air and Their Percentages

25 min

An invisible mixture

You cannot see air, smell it or catch it in your hand, yet your classroom holds more than 100 kg of it. Blow up a balloon and it gets heavier; leave a bowl of limewater open on the bench and a white skin forms on it by the next morning; stand a cold bottle of water in a Kigali kitchen and drops appear on the outside. Each of these tells you about a different gas in the air. What exactly is air made of, and in what proportions?

Experiment: finding the gases in air

Put a spatula of white anhydrous copper(II) sulfate in a dish and leave it uncovered for a day. It slowly turns blue: air contains water vapour (Unit 9, Lesson 2).
Leave a beaker of clear limewater open for a day. A white, chalky skin forms on the surface: air contains carbon dioxide, which reacts with calcium hydroxide to form insoluble calcium carbonate:

$\mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)}$

Stand a small candle in a dish of water, light it and cover it with a jar. The flame soon goes out: air contains a gas that burning needs, oxygen, and only a limited amount of it. (Lesson 2 shows how to measure that amount properly.)
After the oxygen has been used, the gas left in the jar does not let a burning splint burn: most of air is a gas that does not support burning, nitrogen.

Warning

Safety. Limewater and copper(II) sulfate irritate the eyes: wear goggles and wash your hands. Stand the candle in a heat-proof dish, keep hair and paper away from the flame, and let the jar cool before you lift it.

The composition of air

Careful measurements show that dry air (air with its water vapour removed) has almost the same composition all over the world:

Table 1.1: The composition of dry air by volume

Gas	Formula	Percentage by volume	Element or compound?
Nitrogen	$\mathrm{N_2}$	78%	element
Oxygen	$\mathrm{O_2}$	21%	element
Argon	$\mathrm{Ar}$	0.93%	element (noble gas)
Carbon dioxide	$\mathrm{CO_2}$	about 0.04%, and slowly rising	compound
Neon, helium, krypton, hydrogen and others		about 0.003% in total	elements
Water vapour (not in dry air)	$\mathrm{H_2O}$	varies, from 0 to about 4%	compound

Figure 1.1: The composition of dry air by volume.

Definition

Air: the mixture of gases that surrounds the Earth, made up of about 78% nitrogen, 21% oxygen, 0.93% argon, about 0.04% carbon dioxide and traces of other gases, together with a variable amount of water vapour. Example: the air in Kigali and the air over Lake Kivu have the same proportions of nitrogen and oxygen, but different amounts of water vapour.

Worked example

Problem. A classroom measures 8 m by 6 m by 3 m. What volume of (a) nitrogen, (b) oxygen and (c) argon does it contain?

Solution.

Volume of air $= 8 \times 6 \times 3 = 144\,\mathrm{m^3}$ .
(a) Nitrogen: $\frac{78}{100} \times 144 = 112.3\,\mathrm{m^3}$ .
(b) Oxygen: $\frac{21}{100} \times 144 = 30.2\,\mathrm{m^3}$ .
(c) Argon: $\frac{0.93}{100} \times 144 = 1.34\,\mathrm{m^3}$ .

Answer: about $112\,\mathrm{m^3}$ of nitrogen, $30\,\mathrm{m^3}$ of oxygen and $1.3\,\mathrm{m^3}$ of argon.

Worked example

Problem. Calculate the angle of each sector needed to draw Figure 1.1: nitrogen 78%, oxygen 21%, other gases 1%.

Solution. A whole circle is $360^\circ$ , so each sector's angle is its fraction of $360^\circ$ :

Nitrogen: $\frac{78}{100} \times 360^\circ = 280.8^\circ$ .
Oxygen: $\frac{21}{100} \times 360^\circ = 75.6^\circ$ .
Other gases: $\frac{1}{100} \times 360^\circ = 3.6^\circ$ .

Check: $280.8^\circ + 75.6^\circ + 3.6^\circ = 360^\circ$ . The argon and carbon dioxide sector is so thin that the chart needs a label to show it.

Air is a mixture, not a compound

Figure 1.2 shows air as particles. The nitrogen and oxygen molecules are not joined to one another; they simply move about together. The evidence that air is a mixture is summed up in Table 1.2.

Figure 1.2: Air drawn as particles.

Table 1.2: Why air is a mixture

Test	Air (a mixture)	A compound, e.g. carbon dioxide
Composition	varies slightly: more water vapour on a rainy day, more carbon dioxide beside a fire	always fixed: one carbon atom to two oxygen atoms
Properties	each gas keeps its own properties: the oxygen in air still supports burning	different from those of its elements
Separation	separated by a physical method, fractional distillation of liquid air	only by a chemical reaction
Formula	cannot be written	$\mathrm{CO_2}$
Making it	mixing nitrogen and oxygen gives no heat or light	forming it gives out or takes in energy

Separating air

Industry obtains pure oxygen, nitrogen and argon by fractional distillation of liquid air, the same method you used for ethanol and water in Unit 4. Air is filtered to remove dust, and water vapour and carbon dioxide are removed because they would freeze and block the pipes. The air is then compressed and cooled until it becomes a liquid at about $-200\,^\circ\mathrm{C}$ . As the liquid warms slowly in a fractionating column, each gas boils off at its own boiling point:

Table 1.3: Boiling points of the main gases in air

Gas	Boiling point	Order of boiling off
Nitrogen	$-196\,^\circ\mathrm{C}$	first (lowest boiling point); collected at the top
Argon	$-186\,^\circ\mathrm{C}$	second
Oxygen	$-183\,^\circ\mathrm{C}$	last; stays liquid longest, collected at the bottom

Warning

Common mistake: saying that air is "mostly oxygen". Nearly four-fifths of air is nitrogen. And carbon dioxide, which people talk about a great deal, is only about 0.04%: four particles in every ten thousand.

Tip

"Four-fifths, one-fifth." Air is roughly four-fifths nitrogen and one-fifth oxygen; everything else together is less than 1%.

Royal Society of Chemistry: Gases from the Air (4 min). Watch how liquid air is separated in a fractionating column, and note which gas leaves first.

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Practice

Name the two gases that make up about 99% of dry air, with their percentages.
Which gas in air turns limewater milky? Write the equation.
How could you show that air contains water vapour?
Give two pieces of evidence that air is a mixture.
Why is water vapour left out of tables of the composition of dry air?
A laboratory holds $250\,\mathrm{m^3}$ of air. What volume of oxygen does it contain?
Liquid air warms slowly. Using Table 1.3, which gas boils off first, and why?
A pie chart of dry air is drawn with an oxygen sector of $90^\circ$ . What mistake has been made?

Show answersHide answers

Nitrogen, 78%, and oxygen, 21%.
Carbon dioxide: $\mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)}$ .
Leave anhydrous copper(II) sulfate exposed to air: it turns from white to blue (or cobalt(II) chloride paper turns pink).
Any two: its composition varies; each gas keeps its own properties; it can be separated by fractional distillation; it has no formula; no energy change happens when the gases mix.
Its amount varies from place to place and with the weather, from 0 to about 4%.
$\frac{21}{100} \times 250 = 52.5\,\mathrm{m^3}$ .
Nitrogen, because it has the lowest boiling point, $-196\,^\circ\mathrm{C}$ .
$90^\circ$ is a quarter of the circle, 25%. The sector should be $\frac{21}{100} \times 360^\circ = 75.6^\circ$ .

Sources and credits

Atmosphere of Earth (the composition of dry air by volume, and the variable water vapour), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Air separation (fractional distillation of liquid air, the removal of water and carbon dioxide first, and the boiling points of nitrogen, argon and oxygen), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Image: Liquidnitrogen.jpg by Cory Doctorow, via Wikimedia Commons, licensed under CC BY-SA 2.0.
Video: Gases from the Air, Royal Society of Chemistry, embedded from YouTube.
The diagrams, worked examples, Practice items and exercises were written and drawn by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1An invisible mixture

2Experiment: finding the gases in air

3The composition of air

4Air is a mixture, not a compound

5Separating air

An invisible mixture

Experiment: finding the gases in air

The composition of air

Air is a mixture, not a compound

Separating air