MYP Chemistry · Topic 3
The atmosphere
Air is a mixture you cannot see. Know what is in it, how to test for it, how we take gases out of it, and what we put back in.
Topic: The atmosphere (characteristics of gases; atmospheric composition, testing and treatment; extraction, emission and environmental implications). The chapter explains how gas particles behave, what air is made of, how common gases are identified, how gases are separated from air, which pollutants human activity adds, their effects, and how emissions are treated.
Prior knowledge: particles in the three states (Topic 4); acids and pH (Topic 7); balancing equations (Topic 6).
Learning objectives
Objectives and contextOverview
By the end of this chapter you should be able to:
- describe the characteristics of gases and explain them using the particle model, including why light gases diffuse faster A
- state the approximate composition of dry air and identify it as a mixture A
- outline the laboratory tests for oxygen, hydrogen, carbon dioxide and water A
- explain how nitrogen, oxygen and argon are extracted from air by fractional distillation A
- explain how pollutants form, their effects, and how emissions are reduced or treated A D
- evaluate fuels and technologies in terms of their impact on the atmosphere D
In December 1952 a thick fog settled over London for five days. It was mixed with smoke from millions of coal fires, and the coal contained sulfur. Universal indicator paper held in that fog would have turned red: the fog was acidic. At least four thousand people died of lung disease. The disaster changed the law on burning coal in cities, and it shows the whole of this topic in one event: the atmosphere is a mixture, human activity adds substances to it, chemistry explains what those substances do, and chemistry also provides ways to remove them.
02 / Gas particles
Characteristics of gasesCharacteristics of gases
Gases have properties that set them apart from solids and liquids. A gas has no fixed shape or volume: it spreads out to fill any container. It can be compressed into a smaller volume. It has a much lower density than the same substance as a liquid or solid. It spreads through other gases by diffusion, and it exerts pressure on every surface it touches.
All of these follow from one particle picture. In a gas the particles are far apart compared with their own size, they move rapidly and randomly in straight lines until they collide, and the forces between them are negligible. Because there is so much empty space, a gas can be squeezed and has a low density. Because the particles move freely, the gas fills its container. Pressure is the result of particles colliding with the walls: each collision pushes on the wall, and the total push per unit area is the pressure.
Diffusion: the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion.
Gas pressure: the force per unit area produced by gas particles colliding with a surface.
At a given temperature, all gas particles have the same average kinetic energy. Kinetic energy depends on both mass and speed, so lighter particles move faster. A helium atom (relative mass 4) moves on average about 2.6 times faster than a nitrogen molecule (relative mass 28) at the same temperature. Lighter gases therefore diffuse faster, and they escape faster through tiny gaps.
That explains why a helium balloon goes down within a day while an air-filled balloon lasts much longer. The rubber is full of microscopic gaps. Air is a mixture that is mostly nitrogen and oxygen, whose molecules are heavier than helium atoms and move more slowly, so they pass through the gaps less often. The fast, light helium atoms escape more quickly.
Escape race: helium and nitrogen
Two identical balloons, one of helium and one of nitrogen, at the same temperature. Particles move at a speed proportional to 1/√M (M = relative molecular mass) and escape when they reach a gap in the wall. Model: equal average kinetic energy for both gases; the gap size and number are the same for both.
Heating a gas makes its particles move faster. In a rigid container the faster particles collide with the walls more often and harder, so the pressure rises. In a flexible container such as a balloon, the gas expands instead, so the same mass of gas occupies a bigger volume and its density falls. That is why a hot-air balloon rises (Topic 4 develops kinetic theory further).
03 / What air is
The composition of airAtmospheric composition
Air is a mixture, not a compound. Its components are not chemically bonded together, their proportions can vary slightly, and each keeps its own properties — which is why the oxygen in air can still react with a hot metal. Dry air has the composition shown in Figure 3.1.
Each component matters in its own way. Nitrogen (N2) is unreactive because of the very strong triple bond between its atoms; it dilutes the oxygen so that burning is controlled. Oxygen (O2) is needed for respiration and combustion, and it causes corrosion. Argon, a noble gas, is unreactive. Carbon dioxide is used in photosynthesis and absorbs heat radiated by the Earth. Water vapour varies with the weather.
When air enters an argon-filled light bulb, the oxygen in the air reacts with the hot filament. Answers that treat air as if it were inert, or as if it were only nitrogen, miss this. Always ask which component of the mixture is doing the chemistry.
04 / Identifying gases
Testing for gasesTesting
Most common gases are colourless, so they are identified by what they do. Each test uses a property that is characteristic of one gas.
| Gas | Test | Positive result | Why it works |
|---|---|---|---|
| Oxygen, O2 | Collect the gas in a test tube; insert a glowing splint | The splint relights | Oxygen supports combustion much more strongly than air |
| Hydrogen, H2 | Hold a lit splint at the mouth of the tube | A squeaky pop | A small explosion: 2H2 + O2 → 2H2O |
| Carbon dioxide, CO2 | Bubble the gas through limewater (calcium hydroxide solution) | Limewater turns milky (cloudy) | Insoluble calcium carbonate forms: CO2 + Ca(OH)2 → CaCO3 + H2O |
| Water vapour, H2O | Blue cobalt(II) chloride paper | Turns pink | The cobalt compound changes colour when it takes up water |
Two steps earn the marks: collect the gas (in a test tube), then show it relights a glowing splint. A lit splint is the hydrogen test and was not accepted for oxygen. An oxygen probe was accepted as an alternative.
Gas detective
Read the observation and decide which gas was present.
05 / Extraction
Extracting gases from airExtraction
Nitrogen, oxygen and argon are extracted from air on a huge scale: oxygen for hospitals and steel-making, nitrogen for making ammonia and for keeping food fresh, argon for welding and filling light bulbs. Because air is a mixture, its components can be separated by a physical process that uses a difference in physical properties — here, their boiling points.
- Air is filtered to remove dust.
- Water vapour and carbon dioxide are removed, because they would freeze to solids and block the pipes.
- The air is compressed and cooled until it becomes a liquid at about −200 °C.
- The liquid air is allowed to warm up slowly in a fractionating column. Each gas boils off at its own boiling point: nitrogen first (−196 °C), then argon (−186 °C), then oxygen (−183 °C).
Liquid air: what boils off?
Move the temperature. The model compares it with each boiling point (nitrogen −196 °C, argon −186 °C, oxygen −183 °C) and shows which components are still liquid and which have boiled off.
Argon and oxygen boil only 3 °C apart, which is why obtaining pure argon needs a second, careful distillation. The same principle — separating a mixture by boiling point — is used for crude oil (Topic 5).
06 / Emissions
Where pollutants come fromEmission
Most atmospheric pollution comes from burning fuels. The products depend on what is in the fuel and how much oxygen is available.
| Pollutant | How it forms | Main effect |
|---|---|---|
| Carbon dioxide, CO2 | Complete combustion of any fuel containing carbon | Greenhouse gas: contributes to global warming and climate change |
| Carbon monoxide, CO | Incomplete combustion (too little oxygen) | Toxic: reduces the ability of the blood to carry oxygen; colourless and odourless |
| Carbon particles (soot) | Very incomplete combustion | Blackens buildings; damages lungs |
| Sulfur dioxide, SO2 | Sulfur impurities in coal and oil burn: S + O2 → SO2 | Dissolves in water to form acid rain; irritates lungs |
| Nitrogen oxides, NO and NO2 (NOx) | Nitrogen and oxygen from the air react at the very high temperatures in engines and furnaces | Acid rain; photochemical smog; lung irritation |
| Unburned hydrocarbons (VOCs) | Fuel that escapes combustion | React in sunlight with NOx to form ozone at ground level (photochemical smog) |
Notice that nitrogen oxides do not come from the fuel. Nitrogen is normally unreactive, but inside an engine the temperature is high enough for it to combine with oxygen. Any high-temperature combustion in air produces some NOx, whatever the fuel.
Pollutants do not stay where they are released. The gas particles move randomly and spread by diffusion, and winds carry them further, so emissions from one city or one volcano affect a wide area. Explaining this spread in terms of particle movement and diffusion is exactly what examination feedback found students tended to leave out.
Volcanoes are a natural source of the same gases. Volcanic gases are mixtures of water vapour, carbon dioxide, sulfur dioxide, hydrogen sulfide and hydrogen chloride. These acidic gases dissolve in the water of crater lakes, which is why such lakes can have pH values close to 0.
07 / Effects
Environmental implicationsEnvironmental implications
Acid rain
Rain is naturally slightly acidic because carbon dioxide dissolves in it. Acid rain has a lower pH than this, because sulfur dioxide and nitrogen oxides dissolve and react to form sulfuric and nitric acids:
4NO2(g) + O2(g) + 2H2O(l) → 4HNO3(aq)
Acid rain damages forests, makes lakes and rivers too acidic for fish, and corrodes limestone buildings and metal structures. Lakes can be treated by adding powdered limestone (calcium carbonate), a base that neutralises the acid — though too much makes the water alkaline.
The greenhouse effect and climate change
The Sun's energy reaches the Earth mostly as visible light and warms the surface. The warm surface radiates energy as infrared radiation. Greenhouse gases — carbon dioxide, methane and water vapour — absorb some of this infrared and re-emit it, keeping the lower atmosphere warmer than it would otherwise be. The effect itself is natural and makes the planet habitable. The problem is its enhancement: burning fossil fuels and cutting down forests (which removes plants that take in carbon dioxide by photosynthesis) have increased the carbon dioxide concentration, and the average temperature of the Earth has risen. Predicted consequences include droughts in some regions, floods in others, and effects on food supplies.
photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2
Smog
The London smog of 1952 was smoke and sulfur dioxide mixed with fog. Today most city air pollution is photochemical smog: sunlight drives reactions between nitrogen oxides and unburned hydrocarbons that produce ground-level ozone and other irritants.
Two different ideas are often mixed up. A fuel is renewable if it can be replaced as fast as it is used (solar, wind, biofuels); fossil fuels are non-renewable. A fuel is clean if its combustion products do not pollute. Hydrogen burns to water only: 2H2 + O2 → 2H2O, so it releases no CO2, SO2 or soot. Whether hydrogen is also renewable depends on how it was produced. When a question asks you to outline why one energy source is better, compare the two sources directly on these points.
08 / Treatment
Treating emissionsTreatment
There are three strategies: use less fuel, use cleaner fuel, or remove pollutants before they reach the air.
- Removing sulfur from fuels before they are burned prevents SO2 forming.
- Scrubbing flue gases. In power stations and waste incinerators the exhaust gases are passed through calcium hydroxide or calcium carbonate. These bases neutralise acidic gases such as SO2 and HCl, trapping them as solid calcium compounds that can be removed. This is an example of turning a pollutant into a product that binds it.
- Catalytic converters in car exhausts contain platinum, rhodium and palladium spread thinly on a honeycomb with a very large surface area. They convert carbon monoxide to carbon dioxide, nitrogen oxides back to nitrogen, and unburned hydrocarbons to carbon dioxide and water: 2CO + 2NO → 2CO2 + N2. They work only when hot, and they do not reduce carbon dioxide.
- Regulation. Governments set limits on what may be burned and on what may leave a chimney — for example, banning materials that produce toxic substances when incinerated, and requiring emissions to be monitored and controlled with suitable equipment.
- Capturing carbon dioxide. Carbon dioxide can be reacted to form solids, for example iron(II) carbonate, which locks the carbon away; planting forests increases natural removal by photosynthesis.
A Composition of air, gas tests, pollutant sources, particle explanations. D Evaluating fuels, waste-to-energy plants and carbon capture: give advantages and disadvantages, compare options directly, and finish with a justified judgement.
09 / Practice
Examination practicePast examination tasks
These tasks are adapted from past on-screen examinations; the chemistry, data and marks are unchanged. Attempt each one before opening the marking guidance.
A Criterion A · Knowing and understanding
Party balloons can be filled with helium or with air. Helium balloons float more easily but deflate faster than air-filled balloons. Using the composition of air (Figure 3.1) and your knowledge of the characteristics of gases, explain why a helium balloon deflates faster than a balloon filled with air.
Marking guidance
- Air is a mixture of gases, mostly N2 (and O2).
- N2 and O2 molecules are bigger or heavier than He atoms, or they move more slowly.
- So helium diffuses through the balloon's membrane faster at the same temperature.
Why: the three marks follow the chain composition → particle mass/speed → rate of escape. Stopping at “helium is lighter” gives only part of the answer.
Hydrogen peroxide decomposes slowly to water and oxygen: 2H2O2(aq) → 2H2O(l) + O2(g). Outline how you would test that oxygen was produced.
Marking guidance
- Collect the gas in a test tube.
- It relights a glowing splint. A lit splint is not accepted; use of an oxygen probe is accepted.
In December 1952 a fog mixed with smoke and pollutants from burning sulfur-containing coal covered a large city. The pollutants included sulfur dioxide, SO2, and nitrogen dioxide, NO2.
Marking guidance
(a) 4NO2 + O2 + 2H2O → 4HNO3. First mark: any two coefficients correct. Second mark: all coefficients correct.
(b) It is acidic, or it contains an acid, or it has a low pH.
Check: N 4 = 4; H 4 = 4; O 8 + 2 + 2 = 12 = 12.
Volcanic gases are mixtures of water vapour and gases such as carbon dioxide, hydrogen sulfide, sulfur dioxide and hydrogen chloride. Suggest why volcanic crater lakes typically have pH values as low as 0.1.
Marking guidance
- The gases dissolve in the water
- to form acids (from SO2 or HCl), giving a pH of about 0.1.
In a waste-to-energy plant, waste is burned at very high temperature. The gases produced are treated with calcium hydroxide, and the heat is used to make high-pressure steam that drives a turbine.
Marking guidance
(a) To neutralise acidic gases (“remove toxic gases” also accepted).
(b) Any reasonable requirement, for example: waste should not contain materials that could produce hazardous, toxic or explosive substances when burned; or atmospheric emissions should be controlled with proper equipment.
B Criterion B · Inquiring and designing
Environmental pollution is causing changes to our climate. Climate change is a key factor in the increasing risk of wildfires. Early firefighters used water whereas today various chemicals are also used. Firefighting tools such as fire extinguishers have changed over time. An MYP student is investigating the effectiveness of a home-made fire extinguisher that produces carbon dioxide to extinguish flames. The below shows an example of this type of extinguisher. The method the student used is given below. 1. Measure 100 cm3 of vinegar and place it in a teapot. 2. Measure 1 spoon of baking soda (sodium bicarbonate, NaHC03). 3. Add the baking soda to the teapot. 4. Pour the carbon dioxide that is produced in the reaction over lit candles without spilling any of the liquid inside the teapot. 5. Record how many lit candles can be extinguished with the carbon dioxide produced. 6. Repeat steps 1 to 5 using 2 spoons, 3 spoons, 4 spoons and 5 spoons of baking soda.
Identify the independent variable, the dependent variable and one control variable in the student’s method. Independent variable: Dependent variable:
Control variable: ____________
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
Independent variable: mass of baking soda (number of spoons). Dependent variable: number of lit candles extinguished. Control variable (any one): volume of vinegar (100 cm3); type or concentration of vinegar; the same teapot; size and spacing of the candles.
Formulate a hypothesis for the student’s investigation.
If: ____________
then: ____________
because: ____________
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
If the mass of baking soda added to the vinegar increases, then the number of candles extinguished will increase, because more sodium hydrogencarbonate reacts with the acid to produce more carbon dioxide. Carbon dioxide is denser than air and does not support combustion, so a larger volume can cover more flames and keep oxygen away from them.
A strong answer also notices that the 100 cm3 of vinegar is fixed: once all the acid has reacted, adding more baking soda produces no more gas, so the trend may level off.
C Criterion C · Processing and evaluating
Identify two limitations in the student’s method.
Limitation 1: ____________
Limitation 2: ____________
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
Any two, for example: a spoon does not measure mass accurately (use a balance); pouring the gas by hand is hard to control and some carbon dioxide escapes into the air; the candles may differ in size or distance from the spout; only one trial is carried out for each amount; the vinegar may run out (become the limiting reactant) at higher amounts of baking soda.
Suggest a different independent variable to extend the student’s investigation.
Suggested answer
Not from an official marking scheme — a worked answer written for these notes.
For example: the volume or concentration of vinegar; the type of acid; the temperature of the vinegar.
D Criterion D · Reflecting on the impacts of science
Hydrogen can be used as a fuel. Outline why the combustion of hydrogen is better for the environment than the combustion of petrol.
Marking guidance
- The only product is water, or no CO2 / NOx / SOx is produced.
- The product is non-toxic / not a pollutant; or, the other way round, combustion of petrol produces a pollutant gas.
References to energy density were not accepted — the question is about the environment.
It has been over forty years since the publication of a book called “Diet for a Small Planet.” The author of this book suggests that society needs to consider the environmental impact of farming. She argues that a meat-based diet is the main contributor to world hunger and that more effective methods of food production and processing may provide the answers to food shortage. The choice between a vegetable-based or meat-based diet is only one of the lifestyle choices that we have to make. This choice will not only affect us as individuals but will affect wider society also. Different types of food will emit different masses of gases in their production. This mass of gas emitted is called the carbon footprint. The carbon footprint of the food includes emissions from farming, transport, and distribution. The table shows the carbon footprint ranked for 1 kg of different foods, but different foods have very different energy content. For example, 1 kg of beef will contain 5260 kJ whereas 1 kg of broccoli contains 1460 kJ. If we consider the energy value of the food rather than the mass, the foods are ranked in a different order according to their carbon footprint. Switching to a vegetable-based diet is not a straightforward choice. The meat farming industry contributes 40 % of global agricultural gross domestic product and provides jobs for 1.3 billion people but at the same time contributes around 14 % of the harmful emissions contributing to global warming. 2 kJ for each food. The unit for measuring carbon footprint is kg CO2e or kg equivalent CO2. This mass includes all emissions of greenhouse gases such as methane and dinitrogen oxide in addition to carbon dioxide.
Adapted: the bar chart is not reproduced. From it, the carbon footprint of potato is 2.9 kg CO2e per kg of food.
Complete the table using information from the bar chart. Calculate the carbon footprint for the meat-based meal to two significant figures.
Vegetable-based meal (energy content approximately 3000 kJ)
| Food | Mass of food / kg | Carbon footprint per kg of food / kg CO2e | Carbon footprint for the meal / kg CO2e |
|---|---|---|---|
| Rice | 0.18 | 2.7 | 0.5 |
| Tomato | 0.2 | 1.1 | 0.2 |
| Broccoli | 0.2 | 2.0 | 0.4 |
| Beans | 0.4 | 2.0 | 0.8 |
| Total | 1.9 |
Meat-based meal (energy content approximately 3000 kJ)
| Food | Mass of food / kg | Carbon footprint per kg of food / kg CO2e | Carbon footprint for the meal / kg CO2e |
|---|---|---|---|
| Potato | 0.24 | ||
| Broccoli | 0.18 | 2.0 | 0.4 |
| Beef steak | 0.38 | 27.0 | 10.3 |
| Total |
Marking guidance
- value of 2.9 ± 0.2 from graph
- value for carbon footprint for the potato in the range 0.65 – 0.74 ECF from first marking point
- value 11.32 ± 0.04 for total carbon footprint (accept any sig figs) ECF from second marking point
- final value given to two sig figs
Outline the environmental impact of the two meals. Use data from the tables to support your answer.
Marking guidance
- energy values are similar for both
- (but) environmental impact is higher for meat-based than for vegetable
- correct use of 11.32 (kgCO2e) for meat-based or 1.91 (kgCO2e) for vegetable-based Do not award this mark if no data is given, ECF from part a for meat-based meal
Climate change is a global threat recognized by the Intergovernmental Panel on Climate Change (IPCC). The cause of climate change is thought to be due to increased levels of greenhouse gases, mainly carbon dioxide. Increased levels of carbon dioxide have resulted in an increase in the average temperature of the Earth. In the future, severe droughts are predicted in some regions, while floods are predicted in other areas. Both situations will lead to famine and increased poverty.
In the natural carbon cycle, carbon dioxide comes from sources such as respiration, volcanic eruptions, fossil fuels and industrial processes such as cement making. One way that carbon dioxide can be removed from the atmosphere is by photosynthesis in plants. The plants act as a natural carbon dioxide sink. Extensive deforestation means that less carbon dioxide is trapped in trees so levels are increasing in the atmosphere.
Photosynthesis is a natural process in which carbon dioxide is removed from the atmosphere. The glucose is used by plants for growth. The plants can be used as foods for local communities. The word equation for photosynthesis is shown below: carbon dioxide + water → glucose + oxygen Using this equation and information from your wider MYP studies, suggest an impact on the community due to deforestation. Justify your answer.
Marking guidance
- Accept any reasonable suggestion, for example:
- lack of fruit for food/diet
- lack of medical products
- loss of species
- destruction of habitat
- an impact on respiratory health
- Justification
- (because) fewer plants, so less photosynthesis is taking place or less glucose is synthesized or an increase in the level of carbon dioxide
Retrieval: the atmosphere
Original practice questions for retrieval — not past examination items.
10 / Examination feedback
Examiner's overall observationEvidence from examination feedback
Examiner's overall observation
Students were well prepared to link the removal of pollutant gases to useful products that bind them, and to connect environmental impact with renewable and non-renewable energy sources. The weaknesses were in particle-level explanation. Most students could say that gas particles move randomly but few added the kinetic-energy part of the description, and most did not use the idea of diffusion when explaining how emissions spread pollution over a large area. Many could not explain what happens when a pressurised drink is opened — that lowering the pressure lets dissolved gas come out of solution as bubbles. Students also did not treat air as a mixture: they missed that air entering a hot light bulb brings oxygen, which then reacts with the filament. In “outline” questions comparing energy sources, a single statement such as “solar panels do not release pollutant gases” was not enough; a direct comparison with fossil fuels, including that they are non-renewable, was expected.
11 / Summary
Summary and knowledge organiserRevision
Essential knowledge
- Gas particles are far apart, move fast and randomly, and collide with walls (pressure). Lighter particles move faster at the same temperature, so they diffuse and escape faster.
- Dry air: about 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide; water vapour varies. Air is a mixture.
- Tests: O2 relights a glowing splint; H2 squeaky pop with a lit splint; CO2 turns limewater milky; H2O turns cobalt chloride paper blue to pink.
- Fractional distillation of liquid air separates N2 (−196 °C), Ar (−186 °C) and O2 (−183 °C) by boiling point.
- CO2 → enhanced greenhouse effect; CO → toxic; SO2 and NOx → acid rain; NOx + hydrocarbons + sunlight → photochemical smog.
- Treatment: desulfurise fuel; neutralise flue gases with Ca(OH)2/CaCO3; catalytic converters; regulation; cleaner fuels such as hydrogen.
Definitions
- Diffusion — net movement from high to low concentration by random motion
- Pressure — force per area from particle collisions
- Acid rain — rain made more acidic by dissolved SO2 and NOx
- Greenhouse gas — absorbs infrared radiated by the Earth
Equations
- S + O2 → SO2
- 4NO2 + O2 + 2H2O → 4HNO3
- 2CO + 2NO → 2CO2 + N2
- 2H2 + O2 → 2H2O
- CO2 + Ca(OH)2 → CaCO3 + H2O
Must-remember distinctions
- Glowing splint (O2) vs lit splint (H2)
- Renewable vs clean
- NOx comes from the air, not the fuel
- Air is a mixture — its oxygen still reacts
Examination checklist
- Explain spread of pollution with diffusion
- Mention kinetic energy when describing gas motion
- Compare, don't just list, when evaluating fuels
- Name the specific pollutant and its effect
Other chapters: Criteria A–D · 1 · Periodic table · 2 · IUPAC naming · 3 · Atmosphere · 4 · Matter · 5 · Pure and impure · 6 · Bonding · 7 · Types of reaction
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