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(c) Gases in the atmosphere

Syllabus
2024
Topic
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Level
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Learning objectives

2.9Gas composition of airKnow the approximate percentages by volume of the four most abundant gases in dry air2.10Testing oxygen percentageUnderstand how to determine the percentage by volume of oxygen in air using experiments involving the reactions of metals (e.g. iron) and non-metals (e.g. phosphorus) with air2.11Combustion productsDescribe the combustion of elements in oxygen, including magnesium, hydrogen and sulfur2.12Hydrocarbon combustion practicalDescribe the formation of carbon dioxide from the thermal decomposition of metal carbonates, including copper(II) carbonate2.13Carbon dioxide increaseKnow that carbon dioxide is a greenhouse gas and that increasing amounts in the atmosphere may contribute to climate change2.14Oxygen percentage practicalPractical: determine the approximate percentage by volume of oxygen in air using a metal or a non-metal

Know the composition of dry air

Dry air is a mixture dominated by nitrogen and oxygen. Its four most abundant gases have approximate percentages by volume that add to about 100%.

Gas Approximate percentage by volume in dry air
nitrogen, NX2\ce{N2} 78%
oxygen, OX2\ce{O2} 21%
argon, Ar\ce{Ar} 0.9% (about 1%)
carbon dioxide, COX2\ce{CO2} 0.04%

In a 100 cm3100\text{ cm}^3 sample of dry air, this corresponds approximately to 78 cm378\text{ cm}^3 nitrogen and 21 cm321\text{ cm}^3 oxygen, with less than 1 cm31\text{ cm}^3 argon and a much smaller carbon dioxide volume.

'Dry air' excludes variable water vapour. The percentages are approximate: 78% nitrogen and 21% oxygen are not 79% nitrogen and 20% oxygen unless the question explicitly permits that rounding.

Measure oxygen by removing it from trapped air

To determine the oxygen percentage, trap a known volume of air and react its oxygen completely with a substance while the other main gases remain unreacted. The decrease in gas volume represents the oxygen removed.

Reactant How oxygen is removed Evidence reaction is complete
wet iron wool or filings iron rusts and binds oxygen in a solid oxide gas volume stops decreasing
heated copper air is passed repeatedly over hot copper, forming black copper(II) oxide repeated volume readings become constant
burning phosphorus phosphorus consumes oxygen and forms a solid oxide burning stops and the apparatus cools before the final reading

\text{oxygen percentage}=\frac{\text{initial gas volume}-\text{final gas volume}}{\text{initial gas volume}}\times100

If the trapped air starts at 100 cm3100\text{ cm}^3 and ends at 79 cm379\text{ cm}^3, the decrease is 21 cm321\text{ cm}^3, so oxygen is 21/100×100=21%21/100\times100=21\% by volume.

Use the total initial air volume, including connecting tubes, when required. Compare readings at the same temperature and pressure after cooling; leaks, incomplete reaction or a hot final gas volume make the result unreliable.

Describe combustion of magnesium, hydrogen and sulfur

Combustion in oxygen forms an oxide. Magnesium, hydrogen and sulfur give different observations and products, so each reaction must be identified precisely.

Element Observation in oxygen Product
magnesium intense bright white flame; white solid forms magnesium oxide, MgO\ce{MgO}
hydrogen pale blue flame; water vapour forms and may condense water, HX2O\ce{H2O}
sulfur blue flame; colourless choking gas forms sulfur dioxide, SOX2\ce{SO2}

\ce{2Mg + O2 -> 2MgO}\qquad\ce{2H2 + O2 -> 2H2O}\qquad\ce{S + O2 -> SO2}

A white magnesium product is a solid or ash, not a precipitate. Hydrogen combustion forms water, not hydrogen peroxide; sulfur normally forms sulfur dioxide under these conditions.

Form carbon dioxide by heating a metal carbonate

Thermal decomposition breaks one compound into simpler substances by heating it. Many metal carbonates form a metal oxide and carbon dioxide.

\ce{metal carbonate ->[heat] metal oxide + carbon dioxide}

When green copper(II) carbonate is heated, it forms black copper(II) oxide and carbon dioxide: CuCOX3(s)→heatCuO(s)+COX2(g)\ce{CuCO3(s) ->[heat] CuO(s) + CO2(g)}.

Evidence Conclusion
green solid becomes black copper(II) oxide has formed
gas turns limewater milky/cloudy carbon dioxide has formed

The gas is released because the carbonate decomposes; it is not combustion, because oxygen is not a reactant in the equation. 'Thermal' means heat is supplied, not that the reaction necessarily gives heat out.

Connect increasing carbon dioxide to climate change

Carbon dioxide is a greenhouse gas. It absorbs some infrared radiation emitted by Earth's surface and re-emits it, reducing the rate at which energy escapes to space.

If the amount of atmospheric carbon dioxide increases, more outgoing infrared radiation can be absorbed. This strengthens the greenhouse effect and may raise average global temperatures, contributing to climate change.

Climate change can alter rainfall patterns and increase the likelihood of some extreme conditions; warming also contributes to melting land ice and sea-level rise. These are consequences of a changing climate, not the definition of a greenhouse gas.

The natural greenhouse effect keeps Earth warm enough for life. The concern is an enhanced effect caused by increasing greenhouse-gas concentrations. Carbon dioxide may contribute to climate change; it is not correct to describe it as the sole influence on climate.

Determine oxygen percentage with wet iron wool

A practical way to estimate oxygen in air is to trap a measured column of air with wet iron wool. Water is required for rusting, and oxygen is removed from the trapped gas as solid rust forms.

Stage Action
1 Push wet iron wool into one end of a graduated tube and record the initial trapped-air length or volume.
2 Invert the open end in water so the air remains sealed; leave the apparatus until the reading becomes constant.
3 Record the final air length or volume at the same temperature and pressure.
4 Subtract final from initial; divide by the initial value and multiply by 100.

\text{oxygen percentage}=\frac{84-69}{84}\times100=17.9%\approx18%

Use excess finely divided iron to provide enough surface area, make airtight connections and wait for a constant reading. A value below 21% may mean not all oxygen reacted; leaks, temperature or pressure changes, or inaccurate meniscus readings can shift the result.

Wear eye protection and handle rusty or sharp iron wool with tools. Keep the apparatus stable and do not force a sealed plunger; the volume change is caused by oxygen removal, not by nitrogen being produced.