2.3Unit 2 · Inorganic Chemistry
Gases in the atmosphere — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 7 parts Lesson 3 of 4 1 quiz
Lesson summary
State the approximate percentages of the main gases in dry air.Dry airabout 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide.Copper turns from pink/orange to black2Cu + O2 → 2CuO.Iron rusting also removes oxygeniron + oxygen + water → hydrated iron(III) oxide.2Mg + O2 →…
- Composition of air
- Composition of the atmosphere
- Practical – % oxygen with copper
- Practical – % oxygen with iron
- Combustion of elements
- Acid–base character of oxides
Section 1 of 7Gases in the atmosphere
Essential knowledge
Composition of air
Composition of air dry air is a mixture of gases. By volume it is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide, with traces of other noble gases. Water vapour varies, so values are quoted for dry air.
Composition of the atmosphere
Composition of the atmosphere these proportions stay fairly constant because processes that remove gases (e.g. respiration and combustion using oxygen) are balanced by processes that add them (e.g. photosynthesis releasing oxygen). Human activity has slowly raised CO2.
Practical – % oxygen with copper
Practical – % oxygen with copper two gas syringes are joined by a silica tube containing copper turnings.
- 100 cm3 of air is passed repeatedly over the heated copper.
- Oxygen reacts to form black copper(II) oxide, so the volume falls to about 79 cm3.
- After cooling, the decrease (about 21 cm3) gives the % oxygen.
Practical – % oxygen with iron
Practical – % oxygen with iron damp iron wool is pushed into the end of a measuring cylinder inverted in water. Over several days the iron rusts, using up oxygen, so water rises up the tube. The fraction of the original air volume the water rises gives the % oxygen. Phosphorus or pyrogallol can also be used to absorb oxygen.
Combustion of elements
Combustion of elements many elements burn in oxygen to form oxides. Magnesium burns with a dazzling white flame forming white magnesium oxide. Hydrogen burns to form water. Sulfur burns with a blue flame to form sulfur dioxide. Carbon burns to form carbon dioxide (or carbon monoxide if oxygen is limited).
Acid–base character of oxides
Acid–base character of oxides metal oxides that dissolve form alkaline solutions (e.g. MgO gives a solution with pH above 7). Non-metal oxides such as SO2 and CO2 dissolve to form acidic solutions (pH below 7). Universal indicator shows this.
Carbon dioxide from thermal decomposition
Carbon dioxide from thermal decomposition many metal carbonates break down when heated to give a metal oxide and carbon dioxide, e.g. CuCO3 → CuO + CO2. The gas can be bubbled through limewater, which turns milky. Calcium carbonate needs strong heating: CaCO3 → CaO + CO2.
The greenhouse effect
The greenhouse effect the Sun's short-wavelength radiation passes through the atmosphere and warms the Earth's surface.
- The warm surface emits longer-wavelength infrared radiation.
- Greenhouse gases (CO2, methane, water vapour) absorb some of this infrared and re-emit it in all directions,
- including back towards Earth, keeping the planet warm enough for life.
Enhanced greenhouse effect
Enhanced greenhouse effect burning fossil fuels and deforestation increase CO2 levels; farming (cattle, rice) and landfill increase methane. More infrared is trapped, raising average global temperature. Consequences include melting ice caps, rising sea levels and more extreme weather (climate change).
Practical – % oxygen with phosphorus
Practical – % oxygen with phosphorus burn a small piece of phosphorus in a bell jar standing in water. As it uses up the oxygen, the water level inside the jar rises. Phosphorus burns until all the oxygen is gone, but it is toxic, so it must be handled with great care.
Practical – iron in a burette
Practical – iron in a burette put damp iron filings in a burette, invert it in water and read the level now and after a few days. Include the unmarked volume below the tap. E.g. dead space 4.0 cm3, levels 3.0 then 13.0: air 51.0 then 41.0 cm3, so oxygen = 10.0 ÷ 51.0 × 100 ≈ 19.6%.
Combustion as oxidation
Combustion as oxidation combustion is a reaction with oxygen that forms oxides and always releases heat, so it is exothermic. Because oxygen is gained, combustion is also an oxidation reaction.
Which carbonates decompose
Which carbonates decompose carbonates of metals low in the reactivity series, such as copper, break down easily on heating. Carbonates of more reactive metals need much stronger heating, and sodium and potassium carbonates do not decompose in a Bunsen flame.
Sources of carbon dioxide
CO2 is added to the air by burning wood and fossil fuels, by respiration, by heating carbonate rocks (e.g. limestone for cement and lime) and by acids reacting with carbonates.
Natural vs enhanced effect
Natural vs enhanced effect the natural greenhouse effect keeps Earth's mean surface temperature at about 15 °C, which life needs. The enhanced effect is the extra warming caused by human increases in greenhouse gases.
Section 2 of 7Process & sequence
1
Set up a known volume of air (e.g. 100 cm3) in a gas syringe connected to a tube of heated copper, or trap air over water with damp iron wool.
2
Record the starting volume of air.
3
Heat the copper (or leave the iron wool for about a week) so that oxygen reacts to form an oxide.
4
Push the air back and forth over the hot copper until the volume stops decreasing.
5
Allow the apparatus to cool to room temperature before reading the final volume.
6
Calculate % oxygen = (volume decrease ÷ starting volume) × 100.
Measuring oxygen in airwrite these steps in this order; each step is a marking point.
1
Measure 100 cm3 of air
2
Pass over hot copper
3
Oxygen forms copper oxide
4
Cool to room temperature
5
Read final volume
6
Calculate percentage decrease
Section 3 of 7Compare
| Gas | Approx. % by volume | Note |
|---|---|---|
| Nitrogen, N2 | 78 | Very unreactive (strong triple bond) |
| Oxygen, O2 | 21 | Supports combustion and respiration |
| Argon, Ar | 0.9 | Noble gas, unreactive |
| Carbon dioxide, CO2 | 0.04 | Greenhouse gas; slowly increasing |
| Water vapour | variable | Removed when we say 'dry air' |
Figures are rounded; learn 78, 21, 0.9 and 0.04.
| Element | Observation | Oxide | Oxide in water |
|---|---|---|---|
| Magnesium | Bright white flame, white powder | MgO | Basic (alkaline, pH above 7) |
| Hydrogen | Pale blue flame, squeaky pop in tests | H2O | Neutral |
| Sulfur | Blue flame, choking gas | SO2 | Acidic (pH below 7) |
| Carbon | Glows orange/yellow | CO2 | Weakly acidic |
Metal oxides are generally basic; non-metal oxides are generally acidic.
Section 4 of 7Worked examples
1
A student passed 80.0 cm3 of air over heated copper. After cooling, the volume was 63.4 cm3. Calculate the percentage of oxygen.
- Volume of oxygen removed = 80.0 − 63.4 = 16.6 cm3. % oxygen = (16.6 ÷ 80.0) × 100 = 20.75% ≈ 20.8%.
- This is close to the accepted 21%.
2Explain why the apparatus must cool before the final volume is read.
- Gases expand when hot.
- If the reading were taken while hot, the gas volume would be larger than at room temperature,
- so the calculated decrease (and % oxygen) would be too small.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Atmosphere · Nitrogen · Oxygen · Argon · Combustion · Oxide · Thermal decomposition · Greenhouse gas · Climate change
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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