OCR GCSE Chemistry B (Twenty First Century)
OCR GCSE Chemistry B C1 Air and Water Exam Questions
Free OCR GCSE Chemistry B revision for C1 Air and Water: what you need to know, common mistakes and exam-style questions with answers. Covers the evolution of the atmosphere, air pollutants, energy changes in reactions, climate change evidence and making water potable.
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About OCR GCSE Chemistry B C1 Air and Water
Chapter C1 opens the OCR Chemistry B (Twenty First Century Science) course by asking where the air we breathe came from and how human activity is changing it. You study how volcanic gases, photosynthesis and rock formation shaped today's atmosphere, which pollutants come from burning fuels, why some reactions release energy while others take it in, the evidence for climate change, and how water is treated so it is safe to drink. The chapter links to bonding and equations in C2, crude oil in C3 and rates in C6. Examiners like data on gas levels, bond energy calculations and evaluating methods to reduce pollution.
What you need to know
- C1.1 The atmosphere The early atmosphere came mainly from volcanic gases and was mostly carbon dioxide and water vapour; water vapour condensed to form oceans. Photosynthesis added oxygen, while carbon dioxide was removed by dissolving in oceans and being locked up in sedimentary rocks and fossil fuels.
- C1.1 Pollutants Carbon monoxide and particulates come from incomplete combustion, sulfur dioxide from sulfur impurities in fuels, and nitrogen oxides from nitrogen and oxygen reacting at high temperatures in engines. Catalytic converters, low-sulfur fuels and scrubbers in power stations reduce these emissions.
- C1.1 Gas tests Oxygen relights a glowing splint, hydrogen gives a squeaky pop with a lighted splint, and carbon dioxide turns limewater milky.
- C1.2 Energy changes Exothermic reactions release energy and the temperature of the surroundings rises; endothermic reactions take in energy and the temperature falls. Reaction profiles show activation energy, and bond energies are used to calculate the overall energy change: energy to break bonds minus energy released making bonds.
- C1.2 Cells and fuel cells In separate chemistry you also learn that a chemical cell produces a voltage until a reactant is used up, and you evaluate hydrogen fuel cells against rechargeable batteries.
- C1.3 Climate change Carbon dioxide and methane absorb and re-emit infrared radiation, keeping the Earth warmer. Evidence links rising carbon dioxide levels to fossil fuel use and rising global temperature, with possible effects including extreme weather, melting ice and rising sea levels.
- C1.4 Potable water Water from ground, waste or salt water sources is treated by filtration, sedimentation and chlorination to kill microorganisms; sea water can be desalinated by distillation or reverse osmosis but these need a lot of energy. Chlorine bleaches damp blue litmus paper.
Common mistakes in C1 Air and Water
- Saying the greenhouse effect is caused by the ozone layer or by ultraviolet radiation. It is caused by greenhouse gases absorbing infrared radiation emitted by the Earth.
- Writing that sulfur dioxide comes from sulfur in the air. It forms when sulfur impurities in the fuel react with oxygen.
- Getting the bond energy subtraction the wrong way round. Energy change = bonds broken minus bonds made, so a negative answer means exothermic.
- Saying chlorine is added to water to remove solids. Filtration removes solids; chlorine kills microorganisms.
- Treating correlation between carbon dioxide and temperature as proof on its own. Explain the mechanism (infrared absorption) and mention uncertainties in the data.
Try three C1 Air and Water exam-style questions
1. Describe two ways in which the amount of carbon dioxide in the Earth's early atmosphere was reduced. (2 marks)
Show the mark scheme answer
Dissolved in the oceans; taken in by plants and algae in photosynthesis; locked up in sedimentary rocks such as limestone or in fossil fuels (any two).
2. Hydrogen reacts with chlorine: H2 + Cl2 to 2HCl. Bond energies in kJ/mol: H-H = 436, Cl-Cl = 243, H-Cl = 432. Calculate the energy change and state whether the reaction is exothermic or endothermic. (3 marks)
Show the mark scheme answer
Bonds broken = 436 + 243 = 679 kJ/mol; bonds made = 2 x 432 = 864 kJ/mol; energy change = 679 - 864 = -185 kJ/mol, so exothermic.
3. Explain why distillation is not widely used to make drinking water in the UK. (2 marks)
Show the mark scheme answer
Distillation needs large amounts of energy to boil the water so it is expensive; the UK has plenty of fresh ground and surface water that can be treated more cheaply by filtration and chlorination.
C1 Air and Water FAQs
What does OCR Chemistry B C1 Air and Water cover?
It covers how the atmosphere formed and changed, air pollutants and how to reduce them, energy changes in reactions, the evidence for climate change and how potable water is produced.
Which paper is it examined in?
OCR Chemistry B has two papers at each tier, Breadth in chemistry and Depth in chemistry, and both can assess content from every chapter, so C1 can appear on either paper.
What calculations come up in C1?
The main calculation is the overall energy change from bond energies. You may also be asked to interpret graphs of gas concentrations or temperature and to balance combustion equations.
