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
Predicting and Identifying Reactions
Tests for oxygen, hydrogen, carbon dioxide and chlorine
- 3 things to know
- Paper 10 (Chemistry)
Overview
The tests and positive results for oxygen, hydrogen, carbon dioxide and chlorine, why the litmus paper must be damp, what turns limewater milky, telling oxygen from hydrogen with a splint, what chlorine looks and smells like and why it is tested in a fume cupboard, and identifying a gas from its test results.
What you need to know
Answer each one before you reveal it.
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Key term
Describe the test for hydrogen and its positive result.
Key term
Describe the test for carbon dioxide and its positive result.
Questions from this subtopic
Each one with the model answer underneath.
- 1
Describe the test for oxygen and its positive result.
Model answer
Put a glowing splint into the gas; if it relights, the gas is oxygen.
- 2
Describe the test for hydrogen and its positive result.
Model answer
Hold a lit splint at the mouth of the tube; hydrogen burns with a squeaky pop.
- 3
Describe the test for carbon dioxide and its positive result.
Model answer
Bubble the gas through limewater; carbon dioxide turns the limewater milky white.
- 4
Describe the test for chlorine and its positive result.
Model answer
Hold damp blue litmus paper in the gas; chlorine turns it red and then bleaches it white.
- 5
Why must the litmus paper used to test for chlorine be damp?
Model answer
The chlorine must dissolve in the water on the paper before it can turn the paper red and bleach it.
- 6
What substance makes limewater turn milky when carbon dioxide is bubbled through it?
Model answer
Insoluble calcium carbonate, which forms as tiny white solid particles.
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