Chapter C5: Chemical analysis — OCR GCSE Combined Science

Chemical analysis in OCR GCSE Combined Science covers the identification of gases, ions, and the use of chromatography and instrumental methods. It includes flame tests, precipitation reactions, and the principles of paper chromatography, plus advantages of modern instrumental techniques.

Chemical analysis is a fundamental part of chemistry that allows scientists to identify the composition of unknown substances. In this chapter, you will learn how to test for specific gases such as hydrogen, oxygen, carbon dioxide, and chlorine, as well as how to identify metal ions using flame tests and precipitation reactions. These techniques are essential in fields like forensics, environmental monitoring, and quality control in industry.

You will also explore paper chromatography, a technique used to separate mixtures and identify substances based on their solubility in a solvent. Understanding how to calculate Rf values and interpret chromatograms is a key skill. Finally, you will compare traditional lab tests with modern instrumental methods like atomic absorption spectroscopy and gas chromatography, which are faster, more accurate, and can detect tiny amounts of substances.

This topic builds on your knowledge of atomic structure, bonding, and chemical reactions. It also links to practical skills, as you will need to carry out these tests accurately and safely. Mastering chemical analysis will not only help you in exams but also give you a deeper appreciation of how chemists solve real-world problems.

• →Flame tests: lithium (crimson), sodium (yellow), potassium (lilac), calcium (orange-red), copper (blue-green).
• →Precipitation reactions: adding sodium hydroxide to identify metal ions by the colour of the precipitate formed (e.g., copper(II) gives blue, iron(III) gives brown).
• →Gas tests: hydrogen (squeaky pop with a lighted splint), oxygen (relights a glowing splint), carbon dioxide (turns limewater milky), chlorine (bleaches damp litmus paper).
• →Paper chromatography: separates mixtures based on solubility; Rf = distance moved by substance / distance moved by solvent front.
• →Instrumental methods: faster, more sensitive, and more accurate than manual tests; examples include flame emission spectroscopy and gas chromatography.

• Distinguishing between scientific and everyday use of 'pure'
• Using melting point data to identify pure substances
• Interpreting chromatograms and calculating Rf values
• Explaining separation techniques: filtration, crystallisation, simple and fractional distillation
• Applying the law of conservation of mass
• Calculating relative formula masses
• Using the mole as a unit of amount of substance
• Calculating moles from mass and relative formula mass
• Using balanced equations to calculate reacting masses
• Calculating concentration in g/dm³ and mol/dm³
• Describing the procedure for acid-base titrations
• Explaining neutralisation in terms of H+ and OH- ions

• 💡Always check if an equation is balanced before performing calculations
• 💡Ensure units are consistent (e.g., convert all volumes to dm³ for concentration calculations)
• 💡Show all working steps in multi-step calculations to gain method marks
• 💡Use the correct number of significant figures as requested in the question
• 💡Remember that the mole is a counting unit, not a mass unit
• 💡Always state the exact colour of the flame or precipitate, and use the correct terminology (e.g., 'lilac' not 'purple' for potassium).
• 💡When describing chromatography, mention the mobile phase (solvent) and stationary phase (paper) and explain how separation occurs.
• 💡For instrumental methods, remember to compare them with manual tests: they are more accurate, sensitive, and faster, but may be more expensive.

• Confusing the scientific definition of 'pure' with the everyday meaning
• Incorrectly calculating Rf values (e.g., swapping distance moved by solute and solvent)
• Failing to balance equations before using them for stoichiometric calculations
• Errors in unit conversion (e.g., cm³ to dm³)
• Misinterpreting the limiting reactant in a reaction
• Incorrectly identifying the endpoint in a titration
• Misconception: All metal ions produce a coloured flame. Correction: Only certain metal ions give characteristic flame colours; others may not produce a visible colour or may require a different test.
• Misconception: In chromatography, the baseline can be drawn in pen. Correction: Pen ink is soluble in the solvent and would run, so a pencil must be used.
• Misconception: Rf value can be greater than 1. Correction: Rf is always between 0 and 1 because the substance cannot travel further than the solvent front.

1. 1Week 1: Learn the flame tests and gas tests. Create flashcards for each ion and gas, including the colour change or result. Test yourself daily.
2. 2Week 1: Practice precipitation reactions. Write balanced equations for the formation of precipitates and memorise the colours of common hydroxides.
3. 3Week 2: Study paper chromatography. Perform a simple experiment at home using filter paper and felt-tip pens. Calculate Rf values for different inks.
4. 4Week 2: Compare manual and instrumental methods. Make a table of advantages and disadvantages. Practice exam questions on this comparison.
5. 5Week 2: Attempt past paper questions on the whole chapter. Review your mistakes and revisit any weak areas.

• 📋Identification questions: You may be given an unknown substance and asked to carry out tests to identify the ions or gases present. Practice writing clear, step-by-step procedures.
• 📋Chromatography calculations: You may be asked to calculate Rf values or determine the distance moved by a substance. Ensure you can rearrange the formula.
• 📋Comparison questions: You may be asked to evaluate instrumental methods versus traditional tests. Use comparative language and give specific advantages/disadvantages.
• 📋Practical-based questions: You may be asked to describe how to carry out a flame test or a chromatography experiment. Include safety precautions and details of the method.