Key term
OCR GCSE Combined Science Chemistry J250
Chemical Reactions
Topic overviewWriting formulae of elements and compoundsBalanced equations, and deducing formulae from ionsIonic equations, and state symbolsThe mole and the Avogadro constantConservation of mass, and changes of mass in open systemsStoichiometry, and calculating reacting massesExothermic and endothermic reactions, and reaction profilesActivation energy, and bond energy calculationsOxidation and reduction, by oxygen and by electronsAcids, alkalis and neutralisationThe ionic equation for neutralisation, and reactions of acidsDilute and concentrated, weak and strongThe pH scale, and hydrogen ion concentrationpH as a factor of ten, and measuring pHProducts at the electrodes, and molten ionic compoundsElectrolysis of aqueous solutionsInert and non-inert electrodes
Elements, Compounds and Mixtures
Topic overviewPurity, and melting point dataRelative formula mass and empirical formulaeFormulations, and separating mixturesPaper and thin layer chromatographyInterpreting chromatograms and Rf valuesChoosing a purification methodMetals and non-metals in the Periodic TableElectron arrangement and position in the Periodic TableIonic, covalent and metallic bonding comparedDot and cross diagrams, and the limits of modelsReactions, electron arrangement and Mendeleev's tableCarbon's four bonds, and the variety of organic compoundsDiamond, graphite, fullerenes and grapheneBond strength, intermolecular forces and predicting stateBulk properties of ionic, molecular and giant structures
Global Challenges
Topic overviewExtracting metals by reduction and by electrolysisBiological methods of metal extractionLife cycle assessmentRecycling, and the decisions behind itFractional distillation of crude oilThe fractions, and crude oil as a feedstockDependence on hydrocarbons, and crackingThe Earth's early atmosphere, and how oxygen developedThe greenhouse effect, and evidence for climate changeEffects of carbon dioxide and methane, and air pollutantsProducing potable water
Monitoring and Controlling Chemical Reactions
Topic overviewMass of solute, concentration and dilution calculationsMeasuring the rate of a reaction, and rate graphsFactors affecting rate, and collision theorySurface area and rateCatalysts and how they workReversible reactions and dynamic equilibriumChanging the conditions of an equilibriumActivation energy, and enzymes as catalysts
Particles
Global Challenges
The Earth's early atmosphere, and how oxygen developed
- 3 things to know
- Paper 10 (Chemistry)
Overview
The composition of today's atmosphere, the volcanic gases of the early atmosphere, why ideas about it are uncertain and the evidence used, how carbon dioxide dissolved in the oceans and was locked into carbonate rocks and fossil fuels, how photosynthesis slowly produced oxygen and its equation, why oxygen mattered for life, and why nitrogen became the most abundant gas.
What you need to know
Answer each one before you reveal it.
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Key term
Which other gases are thought to have been present in the early atmosphere?
Key term
What kinds of evidence do scientists use to work out how the atmosphere has changed?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is the composition of the Earth's atmosphere today?
Model answer
About 78% nitrogen and 21% oxygen, with small amounts of argon, carbon dioxide and water vapour.
- 2
Roughly what proportion of today's atmosphere is carbon dioxide?
Model answer
About 0.04%.
- 3
What is thought to have released the gases that made up the Earth's earliest atmosphere?
Model answer
Intense volcanic activity during the first billion years of the Earth's history.
- 4
Which gas is thought to have been the main one in the Earth's early atmosphere?
Model answer
Carbon dioxide.
- 5
Which other gases are thought to have been present in the early atmosphere?
Model answer
Large amounts of water vapour and small amounts of methane and ammonia, with little or no oxygen.
- 6
Why are ideas about the early atmosphere less certain than measurements of the atmosphere today?
Model answer
No one was there to measure it and the evidence from billions of years ago is fragmentary and indirect, so the models may change as new evidence is found.
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