Earth’s atmosphere, climate and water treatment
By the end of this chapter you will be able to
- Give the approximate composition of the atmosphere and explain its history
- Explain the greenhouse effect and interpret data on carbon dioxide and temperature
- Compare the sources and effects of two atmospheric pollutants
- Describe the steps of water treatment in order
- Compare distillation and reverse osmosis for desalination, using energy as a criterion
You should already know
- Combustion and fuels (Chapters 12 and 15)
- Acids and neutralisation (Chapter 10)
- Percentages and ppm (Chapters 7 and 9)
Where this leads
- Finite resources and recycling (Chapter 19)
- Fertilisers and the Haber process (Chapter 14)
GCSE Chemistry. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe the composition of the atmosphere and how it changed over time, Explain the greenhouse effect, global warming and the evidence for climate change, Describe atmospheric pollutants, their sources and effects, including acid rain, Describe the steps needed to make water potable, and the potable water standard, Describe desalination methods and evaluate them, including their energy use.
Exercise P Do you remember?
Method. Multiply by one million and divide by 100.
- Multiply. 0.04 / 100 x 1 000 000 = 400.400
- Write the answer. 400
Check. 400 / 1 000 000 = 0.04%. ✓
Method. Recall the product of photosynthesis.
- Recall. Photosynthesis releases oxygen.oxygen
- Write the answer. oxygen
Check. Plants release oxygen in light. ✓
Method. One carbon and two oxygen atoms.
- Count. C and O2.CO2
- Write the answer. CO2
Check. Carbon dioxide is a greenhouse gas. ✓
Method. Recall the test.
- Recall. Carbon dioxide turns limewater milky.carbon dioxide
- Write the answer. carbon dioxide
Check. Chapter 17. ✓
Method. Add the three values.
- Add. 21 + 78 = 99, then 99 + 0.9 = 99.9.99.9
- Write the answer. 99.9
Check. The rest is about 0.1%. ✓
The composition and history of the atmosphere
The atmosphere is about 78% nitrogen and 21% oxygen, with about 0.9% argon, a small amount of carbon dioxide (about 0.04%), and a variable amount of water vapour.
The early atmosphere had little oxygen. It was formed from gases released by volcanoes, including carbon dioxide and water vapour. Ancient cyanobacteria carried out photosynthesis and released oxygen. Much of the carbon dioxide dissolved in the oceans and was locked up in sediments such as limestone, and the oxygen level gradually rose.
| Gas | Approximate percentage by volume |
|---|---|
| nitrogen | 78.0% |
| oxygen | 21.0% |
| argon | 0.9% |
| carbon dioxide | 0.04% |
| water vapour and other gases | about 0.06% |
| Stage | What happened |
|---|---|
| early Earth | volcanoes released carbon dioxide, water vapour and other gases |
| cyanobacteria | evolved and carried out photosynthesis, releasing oxygen |
| oceans | dissolved much of the carbon dioxide; it was locked up in sediments such as limestone |
| today | nitrogen and oxygen dominate; the carbon dioxide level is rising from burning fossil fuels |
Key ideas
- The atmosphere is about 78% nitrogen and 21% oxygen.
- The early atmosphere was made mostly of volcanic gases; it had very little oxygen.
- Cyanobacteria released oxygen by photosynthesis, which increased the oxygen level.
- Carbon dioxide was locked up in the oceans and in sediments such as limestone.
Method. Recall the composition.
- Recall. About 21% of the atmosphere is oxygen.21
- Check. Check the working against the definition or table.checked
- Write the answer. 21
Check. Nitrogen is about 78%. ✓
Method. Recall the organisms.
- Recall. Cyanobacteria carried out photosynthesis and released oxygen.cyanobacteria
- Decide. So the answer is cyanobacteria.cyanobacteria
- Write the answer. cyanobacteria
Check. They are ancient bacteria. ✓
Method. Think about where carbon dioxide went.
- Look. Carbon dioxide dissolved in the oceans.dissolved
- Conclude. It was locked up in sediments such as limestone.limestone
- Write the answer. it dissolved in the oceans and was locked up in sediments such as limestone
Check. This reduced the carbon dioxide level. ✓
Watch out
Watch out: the atmosphere is mostly nitrogen, not oxygen. Oxygen is about a fifth of it.
Exam tip
Exam tip. Name the gas and its approximate percentage, for example “about 78% nitrogen”.
Exercise 18A Composition and history of the atmosphere
Method. Recall the composition.
- Recall. About 78% of the atmosphere is nitrogen.78
- Write the answer. 78
Check. The most abundant gas. ✓
Method. Recall the composition.
- Recall. About 0.9% of the atmosphere is argon.0.9
- Write the answer. 0.9
Check. Argon is a noble gas. ✓
Method. Recall the composition.
- Recall. About 0.04% of the atmosphere is carbon dioxide.0.04
- Write the answer. 0.04
Check. This is about 400 parts per million. ✓
Method. Recall the volcanic gases.
- Recall. Volcanoes released carbon dioxide and water vapour.carbon dioxide
- Write the answer. carbon dioxide
Check. Oxygen was almost absent. ✓
Method. Recall the organisms.
- Recall. Cyanobacteria produced oxygen by photosynthesis.cyanobacteria
- Write the answer. cyanobacteria
Check. Ancient bacteria. ✓
Method. Recall the stores.
- Recall. Carbon dioxide dissolved in the oceans and was locked up in sediments.oceans and sediments
- Write the answer. in the oceans and in sediments such as limestone
Check. Limestone is made of calcium carbonate. ✓
Method. Recall the composition.
- Recall. About 21% of the atmosphere is oxygen.21
- Write the answer. 21
Check. Oxygen is the second most abundant gas. ✓
Method. Compare the percentages.
- Compare. Nitrogen is 78% and oxygen is 21%.78 and 21
- Decide. So nitrogen is more abundant.nitrogen
- Write the answer. nitrogen
Check. The atmosphere is mostly nitrogen. ✓
Method. 21% means 21 parts in 100.
- Read. 21% is 21 parts in every 100 parts of air.21
- Write the answer. 21
Check. Oxygen is about one part in five. ✓
Method. Think about the organisms.
- Organisms. Cyanobacteria carried out photosynthesis.photosynthesis
- Conclude. They released oxygen, which built up in the atmosphere.released oxygen
- Write the answer. Accept any sentence that says photosynthesis by cyanobacteria released oxygen into the air.
Check. The process took a very long time. ✓
Method. Think about where oxygen comes from.
- Source. Oxygen is produced mainly by photosynthesis.photosynthesis
- Conclude. Before cyanobacteria, little oxygen was released, so it was scarce.scarce
- Write the answer. Accept any sentence that says oxygen had not yet been produced in large amounts by photosynthesis.
Check. Volcanic gases dominated. ✓
Greenhouse gases and climate change
Sunlight passes through the atmosphere and warms the Earth’s surface. The Earth emits infrared radiation. Greenhouse gases, such as carbon dioxide, methane and water vapour, absorb some of this infrared and re-emit it in all directions, so the lower atmosphere and the surface are warmer than they would be otherwise. This is the greenhouse effect. The increase in greenhouse gases is causing global warming, and changes in climate.
| Period (approximate) | Carbon dioxide (ppm, approximate) |
|---|---|
| about 1850 (before large-scale fossil fuel use) | 280 |
| about 1960 | 317 |
| about 2000 | 370 |
Key ideas
- Greenhouse gases absorb and re-emit infrared radiation from the Earth.
- Carbon dioxide and methane are the main greenhouse gases from human activity.
- Global warming is the increase in the average temperature of the Earth.
- Evidence: rising carbon dioxide levels, measured over many years, and rising temperatures.
Method. Recall the greenhouse gases.
- Recall. Carbon dioxide, methane and water vapour are greenhouse gases.carbon dioxide and methane
- Check. Check the working against the definition or table.checked
- Write the answer. Accept any two of: carbon dioxide, methane, water vapour.
Check. Each absorbs infrared. ✓
Method. Subtract the two values.
- Subtract. 370 - 280 = 90 ppm.90
- Check. Check the working against the definition or table.checked
- Write the answer. 90
Check. 90 is about a third of 280. ✓
Method. Recall the process.
- Recall. Greenhouse gases absorb and re-emit infrared.absorb and re-emit
- Conclude. This warms the lower atmosphere.warms
- Write the answer. Accept any sentence that says greenhouse gases absorb infrared radiation from the Earth and re-emit it, warming the lower atmosphere.
Check. This is a natural process that keeps the Earth warm enough. ✓
Watch out
Watch out: the greenhouse effect is natural and keeps the Earth warm. The problem is the enhanced greenhouse effect from extra greenhouse gases, which is the cause of global warming.
Exam tip
Exam tip. Link the gas, the infrared radiation and the warming in each explanation.
Exercise 18B Greenhouse gases and climate change
Method. Recall the radiation.
- Recall. The Earth emits infrared radiation.infrared
- Write the answer. infrared
Check. Sunlight arrives as visible and other radiation. ✓
Method. Recall the term.
- Recall. The natural warming is called the greenhouse effect.greenhouse effect
- Write the answer. the greenhouse effect
Check. The natural effect keeps the Earth warm. ✓
Method. Recall the gas.
- Recall. Methane absorbs infrared and is a greenhouse gas.yes
- Write the answer. yes
Check. It is released from farming and from natural sources. ✓
Method. Recall the definition.
- Recall. Global warming is the rise in the average temperature of the Earth.rise in average temperature
- Write the answer. an increase in the average temperature of the Earth
Check. It is linked to rising greenhouse gases. ✓
Method. Subtract the earlier value from the later value.
- Subtract. 370 - 317 = 53 ppm.53
- Write the answer. 53
Check. The increase is steady over time. ✓
Method. Compare the two values.
- Compare. 370 ppm is larger than 280 ppm.370 > 280
- Decide. So the larger value is for about 2000.about 2000
- Write the answer. about 2000
Check. The concentration has risen over time. ✓
Method. Recall the gas.
- Recall. Burning fossil fuels releases carbon dioxide.CO2
- Write the answer. CO2
Check. Carbon dioxide is the main human-made greenhouse gas. ✓
Method. Think about the Earth without it.
- Without. Without greenhouse gases, the Earth would be much colder.colder
- Conclude. So the natural effect keeps temperatures suitable for life.suitable
- Write the answer. Accept any sentence that says it keeps the Earth warm enough for life to exist.
Check. The problem is the extra greenhouse gases. ✓
Method. Think about the timing.
- Timing. The rise in carbon dioxide has matched the increase in fossil fuel burning.matched
- Conclude. So the evidence links the rise to human activity.linked
- Write the answer. Accept any sentence that says carbon dioxide has risen as fossil fuels have been burned more.
Check. The scientific consensus is that human activity is the main cause. ✓
Atmospheric pollutants and acid rain
Burning fuels produces several pollutants. Carbon monoxide is poisonous and forms in incomplete combustion. Sulfur dioxide comes from fuels that contain sulfur, and oxides of nitrogen form at high temperatures in engines. Both dissolve in rain to form acid rain, which damages buildings, lakes and trees. Particulates (soot and dust) cause breathing problems.
Sulfur dioxide dissolves in water to form sulfurous and sulfuric acids; for example, SO2 + H2O -> H2SO3.
| Pollutant | Source | Effect |
|---|---|---|
| carbon monoxide | incomplete combustion of fuels | poisonous; binds to haemoglobin |
| sulfur dioxide | burning fuels containing sulfur | causes acid rain; breathing problems |
| oxides of nitrogen | car engines and power stations | cause acid rain and smog; breathing problems |
| particulates (soot, dust) | combustion and industry | cause breathing problems and respiratory disease |
Key ideas
- Carbon monoxide: poisonous, from incomplete combustion.
- Sulfur dioxide and oxides of nitrogen: cause acid rain.
- Particulates: cause breathing problems.
- Acid rain damages buildings, lakes and trees.
Method. Recall the pollutant.
- Recall. Carbon monoxide is poisonous and is formed in incomplete combustion.carbon monoxide
- Check. Check the working against the definition or table.checked
- Write the answer. carbon monoxide
Check. Chapter 15 covers this. ✓
Method. Recall the two pollutants.
- Recall. Sulfur dioxide and oxides of nitrogen cause acid rain.sulfur dioxide and oxides of nitrogen
- Check. Check the working against the definition or table.checked
- Write the answer. Accept sulfur dioxide and oxides of nitrogen.
Check. Both dissolve in rain. ✓
Method. Think about the material.
- Look. Acid rain reacts with the limestone or marble in buildings.reacts
- Conclude. So stone statues and buildings are worn away.worn away
- Write the answer. it wears away stone buildings and statues
Check. Acid rain also damages lakes and trees. ✓
Watch out
Watch out: carbon dioxide is a greenhouse gas, not the main cause of acid rain. Acid rain comes from sulfur dioxide and oxides of nitrogen.
Exam tip
Exam tip. For a pollutant, give the source, the effect and a reason, in that order.
Exercise 18C Atmospheric pollutants and acid rain
Method. Recall the source.
- Recall. Sulfur dioxide is released by burning fuels containing sulfur.sulfur
- Write the answer. burning fuels that contain sulfur
Check. Coal and some oils contain sulfur. ✓
Method. Recall the effect.
- Recall. Carbon monoxide is a poisonous gas.poisonous
- Write the answer. it is poisonous
Check. It binds to haemoglobin in the blood. ✓
Method. Recall the problem.
- Recall. They dissolve in rain to form acid rain.acid rain
- Write the answer. acid rain
Check. It damages buildings, lakes and trees. ✓
Method. Recall the pollutant.
- Recall. Particulates are small solid particles, such as soot and dust.particulates
- Write the answer. particulates
Check. They can be inhaled into the lungs. ✓
Method. Recall the source.
- Recall. They form at high temperatures in engines and power stations.engines
- Write the answer. car engines and power stations
Check. High temperatures combine nitrogen and oxygen. ✓
Method. Recall the pollutant.
- Recall. Sulfur dioxide dissolves in rain to form acids.yes
- Write the answer. yes
Check. It forms sulfurous and sulfuric acids. ✓
Method. Sulfur dioxide with water gives sulfurous acid.
- Equation. SO2 + H2O -> H2SO3.H2SO3
- Write the answer. H2SO3
Check. Sulfurous acid. ✓
Method. Recall the pollutant.
- Recall. Carbon monoxide is CO.CO
- Write the answer. CO
Check. It is not a greenhouse gas in the same way as CO2. ✓
Method. Think about the stone.
- Stone. Limestone and marble are made of calcium carbonate.calcium carbonate
- Conclude. The acid reacts with the carbonate and wears the stone away.reacts
- Write the answer. Accept any sentence that says the acid reacts with calcium carbonate in the stone.
Check. Acids react with carbonates (Chapter 10). ✓
Method. Think about the pH.
- pH. Acid rain makes the water more acidic, lowering its pH.lowers pH
- Conclude. Fish cannot survive at very low pH.harm fish
- Write the answer. Accept any sentence that says acid rain lowers the pH of the lake, which harms fish.
Check. Some lakes are affected more than others. ✓
Potable water
Potable water is water that is safe to drink. In the UK it must meet a standard: it must not contain harmful microbes, and dissolved substances must be below safe levels. It does not need to be pure: it can contain dissolved salts and minerals that are good for health. Water treatment uses four stages: screening removes large objects; sedimentation lets solids settle out; filtration removes smaller particles through sand and gravel; and disinfection kills microbes with chlorine or ozone.
Key ideas
- Potable water is safe to drink; it does not need to be pure.
- Screening, sedimentation, filtration and disinfection are the four main stages.
- Chlorine or ozone kills harmful microbes.
- Potable water may still contain some dissolved salts.
Method. Think about the order of treatment.
- Order. Screening removes large objects first.screening first
- Check. Check the working against the definition or table.checked
- Write the answer. screening
Check. Disinfection is the last stage. ✓
Method. Recall the disinfectant.
- Recall. Chlorine or ozone kills harmful microbes.chlorine or ozone
- Check. Check the working against the definition or table.checked
- Write the answer. Accept chlorine or ozone.
Check. Both are used in treatment works. ✓
Method. Think about the standard.
- Look. Potable water only needs to be safe to drink.safe
- Conclude. Small amounts of dissolved salts are acceptable and can be healthy.acceptable
- Write the answer. it only needs to be safe to drink, and dissolved salts are acceptable
Check. Pure water tastes flat and is not necessary. ✓
Watch out
Watch out: potable does not mean pure. Pure water has no dissolved substances at all; potable water has a safe level of them.
Exam tip
Exam tip. Give the four stages in order, and say what each one removes.
Exercise 18D Potable water
Method. Recall the definition.
- Recall. Potable water is water that is safe to drink.safe to drink
- Write the answer. water that is safe to drink
Check. It is not necessarily pure. ✓
Method. Recall the first stage.
- Recall. Screening removes large objects first.screening
- Write the answer. screening
Check. Large debris is removed. ✓
Method. Recall the stage.
- Recall. Sedimentation lets solids settle to the bottom.sedimentation
- Write the answer. sedimentation
Check. Settling tanks are used. ✓
Method. Recall the stage.
- Recall. Filtration passes water through sand and gravel.filtration
- Write the answer. filtration
Check. Smaller particles are removed. ✓
Method. Recall the disinfectant.
- Recall. Chlorine or ozone is used to kill microbes.chlorine
- Write the answer. Accept chlorine or ozone.
Check. This is the disinfection stage. ✓
Method. Recall the standard.
- Recall. Potable water needs to be safe, not pure.no
- Write the answer. no
Check. Dissolved salts can be present at safe levels. ✓
Method. Recall the order.
- Recall. Disinfection kills microbes at the end of treatment.disinfection
- Write the answer. disinfection
Check. Chlorine or ozone is used. ✓
Method. Use the order of treatment.
- Order. Screening, filtration, then disinfection.screening first
- Write the answer. screening
Check. The order is screening, filtration, disinfection. ✓
Method. Think about the risk.
- Risk. Water can contain microbes that cause disease.disease
- Conclude. Disinfection kills them, so the water is safe.kills
- Write the answer. Accept any sentence that says disinfection kills harmful microbes that cause disease.
Check. Filtration does not remove all microbes. ✓
Method. Think about the filter.
- Solids. Sedimentation removes most of the heavy solids.removes solids
- Conclude. So the filter does not get blocked too quickly.does not clog
- Write the answer. Accept any sentence that says it removes most of the solids first, so the filter does not clog.
Check. Each stage prepares the water for the next. ✓
Desalination and the use of water
Desalination makes fresh water from sea water. Distillation boils the water, and the vapour is condensed to give pure water, leaving the salt behind; it needs a lot of heat energy. Reverse osmosis forces water through a membrane that blocks the salt ions; it needs high pressure, and the membranes need maintenance. Water is also used by industry, for example for cooling and for processing, and this must be considered when water is treated.
| Method | How it works | Main drawback |
|---|---|---|
| distillation | water is boiled and the vapour condensed; salt is left behind | needs a lot of energy (heat) |
| reverse osmosis | water is forced through a membrane that blocks the salt ions | needs high pressure, so it uses energy and needs maintenance |
Key ideas
- Desalination: removing salt from sea water.
- Distillation: boil and condense; needs a lot of heat energy.
- Reverse osmosis: high pressure through a membrane; needs energy and maintenance.
- Evaluate a method by its cost, energy use, and environmental impact.
Method. Recall the method that boils water.
- Recall. Distillation boils the water and condenses the vapour.distillation
- Check. Check the working against the definition or table.checked
- Write the answer. distillation
Check. It is energy-intensive. ✓
Method. Recall the method with a membrane.
- Recall. Reverse osmosis forces water through a membrane.reverse osmosis
- Check. Check the working against the definition or table.checked
- Write the answer. reverse osmosis
Check. It needs high pressure. ✓
Method. Think about the methods.
- Look. Both methods need a lot of energy, for heating or pressure.energy
- Conclude. This makes desalination costly to run.costly
- Write the answer. they need a lot of energy for heating or high pressure
Check. Energy costs are important in evaluation. ✓
Watch out
Watch out: a method is not good or bad on one factor alone. Evaluate desalination by its energy cost, its environmental impact (such as brine left behind) and its benefit (fresh water supply).
Exam tip
Exam tip. In an evaluation, give a benefit, a drawback, and a conclusion that weighs them.
Exercise 18E Desalination and the use of water
Method. Recall the definition.
- Recall. Desalination makes fresh water by removing salt.removing salt
- Write the answer. removing salt from sea water
Check. It produces potable water. ✓
Method. Recall the process.
- Recall. The vapour is condensed back into liquid water.vapour
- Write the answer. vapour
Check. The salt is left behind as the water boils. ✓
Method. Recall the purpose.
- Recall. The membrane blocks the salt ions, letting water through.salt ions
- Write the answer. salt ions
Check. Water passes through the membrane. ✓
Method. Recall the method.
- Recall. Reverse osmosis uses high pressure.reverse osmosis
- Write the answer. reverse osmosis
Check. Distillation uses heat, not pressure. ✓
Method. Recall what is left behind.
- Recall. The salt is not vaporised, so it stays in the flask.salt
- Write the answer. salt
Check. The vapour is pure water. ✓
Method. Compare the energy use.
- Compare. Distillation boils water, which needs a lot of heat.heat
- Decide. So distillation needs the most heat energy.distillation
- Write the answer. distillation
Check. Reverse osmosis needs pressure energy. ✓
Method. Think about the waste.
- Waste. The concentrated salty waste (brine) is returned to the sea.brine
- Energy. Both methods use a lot of energy, which has a carbon cost.energy
- Write the answer. Accept brine left behind, or energy use.
Check. Both can harm marine life if poorly managed. ✓
Method. Compare the energy.
- Energy. Reverse osmosis does not boil the water.does not boil
- Conclude. So it may use less energy overall, depending on the plant.less energy
- Write the answer. Accept any sentence that says it may use less energy overall.
Check. The answer depends on local energy costs. ✓
Method. Weigh the benefit against the drawback.
- Benefit. It gives a secure supply of fresh water in dry areas.secure supply
- Drawback. It uses a lot of energy and produces brine waste.energy and brine
- Conclude. It is useful where other supplies are scarce, but costly.balanced
- Write the answer. Accept any answer that gives a benefit (a secure supply of fresh water) and a drawback (energy use or brine), with a balanced conclusion.
Check. A balanced conclusion is needed for full marks. ✓
Method. Think about the demand.
- Demand. Industry uses large volumes of water.large volumes
- Conclude. So the demand must be planned for.planned
- Write the answer. Accept any sentence that says industry uses large volumes of water, which affects supply.
Check. Water use is a shared resource. ✓
Chapter summary
| Skill | Method |
|---|---|
| Atmosphere | About 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide. |
| History | Volcanic gases first; cyanobacteria released oxygen; carbon dioxide dissolved and was locked up. |
| Greenhouse effect | Greenhouse gases absorb and re-emit infrared radiation, warming the Earth. |
| Pollutants | Carbon monoxide (poison); sulfur dioxide and oxides of nitrogen (acid rain); particulates (breathing). |
| Potable water | Safe to drink, not pure. Screen, settle, filter, disinfect. |
| Desalination | Distillation (heat) or reverse osmosis (pressure); both use energy. |
Can you…?
- I can describe the composition and history of the atmosphere
- I can explain the greenhouse effect and interpret data on carbon dioxide
- I can describe atmospheric pollutants, their sources and effects, and acid rain
- I can describe the stages of water treatment in order
- I can describe and evaluate desalination methods
Key terms
- greenhouse effect
- the warming of the lower atmosphere by greenhouse gases absorbing and re-emitting infrared
- global warming
- the increase in the average temperature of the Earth
- pollutant
- a substance released into the environment that causes harm
- acid rain
- rain made acidic by sulfur dioxide and oxides of nitrogen dissolving in it
- potable water
- water that is safe to drink
- desalination
- removing salt from sea water to make fresh water
Exercise 18Z Check-up
Method. Recall the composition.
- Recall. About 78% of the atmosphere is nitrogen.78
- Write the answer. 78
Check. Oxygen is about 21%. ✓
Method. Recall the organisms.
- Recall. Cyanobacteria released oxygen by photosynthesis.cyanobacteria
- Write the answer. cyanobacteria
Check. They are ancient bacteria. ✓
Method. Recall the gas.
- Recall. Burning fossil fuels releases carbon dioxide.CO2
- Write the answer. CO2
Check. Carbon dioxide is a greenhouse gas. ✓
Method. Recall the pollutant.
- Recall. Sulfur dioxide with oxides of nitrogen causes acid rain.sulfur dioxide
- Write the answer. sulfur dioxide
Check. Both dissolve in rain. ✓
Method. Recall the first stage.
- Recall. Screening removes large objects.screening
- Write the answer. screening
Check. It is the first stage. ✓
Method. Recall the method.
- Recall. Distillation boils and condenses the water.distillation
- Write the answer. distillation
Check. It needs a lot of heat energy. ✓
Method. Recall the effect.
- Recall. More greenhouse gases trap more heat, causing global warming.global warming
- Write the answer. it increases global warming
Check. The enhanced greenhouse effect. ✓
Method. Think about the effect.
- Poison. Carbon monoxide is poisonous.poisonous
- Conclude. It binds to haemoglobin, reducing the oxygen carried.binds
- Write the answer. Accept any sentence that says it is poisonous and binds to haemoglobin in the blood.
Check. It forms in incomplete combustion. ✓
Method. Think about the standard.
- Standard. Potable water must be safe, not pure.safe
- Conclude. It may contain small amounts of dissolved substances at safe levels.safe levels
- Write the answer. Accept any sentence that says it may contain small amounts of safe dissolved substances.
Check. Pure water has no dissolved substances. ✓
Total 24 marks. Allow about 24 minutes.
Exercise 18X Exam-style questions
Method. Recall the composition.
- Nitrogen. About 78% nitrogen.78%
- Oxygen. About 21% oxygen.21%
- Write the answer. nitrogen 78%, oxygen 21%
Check. Argon is about 0.9%. ✓
Method. Describe the process.
- Absorb. Greenhouse gases absorb infrared radiation from the Earth.absorb
- Re-emit. They re-emit it in all directions, warming the lower atmosphere.re-emit
- Write the answer. Accept any answer that says greenhouse gases absorb infrared radiation from the Earth and re-emit it, warming the atmosphere.
Check. Two steps are needed for full marks. ✓
Method. Compare their effects.
- CO. Carbon monoxide is poisonous.poisonous
- CO2. Carbon dioxide is a greenhouse gas that causes global warming.greenhouse
- Write the answer. Accept any sentence that says carbon monoxide is poisonous, while carbon dioxide is a greenhouse gas.
Check. The two gases have different effects. ✓
Method. Recall the order.
- Order. Screening, sedimentation, filtration, then disinfection.four stages
- Write the answer. screening, sedimentation, filtration, disinfection
Check. Each stage is in the order used at a works. ✓
Method. Give the pollutants and the process.
- Pollutants. Sulfur dioxide and oxides of nitrogen are released by burning fuels.released
- Process. They dissolve in rain water to form acids.dissolve
- Write the answer. Accept any answer that says sulfur dioxide and oxides of nitrogen dissolve in rain to form acids.
Check. Acid rain has a low pH. ✓
Method. Describe the organisms and the process.
- Organisms. Ancient cyanobacteria carried out photosynthesis.cyanobacteria
- Result. They released oxygen over a long time, and it built up.built up
- Write the answer. Accept any answer that says cyanobacteria carried out photosynthesis, releasing oxygen into the atmosphere over a long time.
Check. The oxygen level rose slowly. ✓
Method. Compare the methods.
- Distillation. Distillation needs a lot of heat to boil the water.heat
- Reverse osmosis. Reverse osmosis needs high pressure, which also uses energy.pressure
- Write the answer. Accept any answer that compares the energy costs: distillation needs heat, and reverse osmosis needs high pressure.
Check. A balanced evaluation considers energy and waste. ✓
Method. Think about testing.
- Test. Water can be tested for dissolved salts and harmful substances.dissolved salts
- Reason. So that it meets the standard and is safe to drink.safe
- Write the answer. Accept any answer that says test for dissolved salts or harmful substances, to ensure it is safe to drink.
Check. Testing confirms the standard is met. ✓
Method. Subtract the earlier value.
- Subtract. 370 - 280 = 90 ppm.90
- Write the answer. 90
Check. The concentration has risen over time. ✓
Method. Compare the materials.
- Stone. Stone contains calcium carbonate, which acid rain reacts with.calcium carbonate
- Conclude. So stone is worn away, while steel is less affected in the same way.worn away
- Write the answer. Accept any sentence that says acid rain reacts with the calcium carbonate in stone.
Check. Acids react with carbonates. ✓