Key term
OCR GCSE Combined Science Chemistry J250
Chemical Reactions
Topic overviewWriting formulae of elements and compoundsBalanced equations, and deducing formulae from ionsState symbols in equationsConservation of mass, and changes of mass in open systemsExothermic and endothermic reactions, and reaction profilesActivation energy and why reactions need itOxidation and reduction in terms of oxygenAcids, alkalis and neutralisationThe ionic equation for neutralisation, and reactions of acidsThe pH scale, acidity and alkalinityMeasuring pH with indicators and pH metersProducts 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 electrolysisLife 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
Particles
Chemical Reactions
Measuring pH with indicators and pH meters
- 3 things to know
- Paper 3 (Chemistry)
Overview
What universal indicator does and its colours in acidic, neutral and alkaline solutions, why it fails in coloured solutions and gives only approximate values, litmus, using and rinsing a pH meter and its advantage over indicator, calibrating a meter to prevent a systematic error, and what the resolution of a meter means.
What you need to know
Answer each one before you reveal it.
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Key term
Why is universal indicator unsuitable for testing a strongly coloured solution?
Key term
How is a pH meter used to measure the pH of a solution?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What does universal indicator do?
Model answer
It turns a range of different colours which are matched against a chart to give an approximate pH.
- 2
What colour is universal indicator in a strongly acidic, a neutral and a strongly alkaline solution?
Model answer
Red in a strong acid, green when neutral and purple in a strong alkali.
- 3
What is the advantage of a pH meter over universal indicator?
Model answer
It gives a precise numerical value on a display instead of a colour that has to be judged by eye.
- 4
Why is universal indicator unsuitable for testing a strongly coloured solution?
Model answer
The colour of the solution masks the colour of the indicator, so the pH cannot be read.
- 5
How is a pH meter used to measure the pH of a solution?
Model answer
The probe is dipped into the solution and the pH is read directly from the meter.
- 6
Why should a pH probe be rinsed with distilled water between readings?
Model answer
To wash off the previous solution, which would otherwise contaminate the next one and give a false reading.
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