Separation and chemical analysis covers two linked jobs. First, how to split a mixture into its parts. Second, how to decide what a substance is from tests and observations.
Check the current Cambridge IGCSE Chemistry 0620 syllabus for your exam year to confirm exactly what is assessed. The lessons below teach the reasoning that these questions reward. Any real testing belongs in a supervised school laboratory, following your school’s safety rules.
What should you know first?
You will use the particle model to explain why some solids dissolve and others stay as grains. You will also use formulas and equations when a test produces a gas or a precipitate. A little arithmetic helps too, for example dividing one length by another.
An orienting example
Invented scenario: a student has a grey-white powder. It is a mixture of sand and a white salt, and the salt is thought to be a chloride.
- The powder is stirred in water. The salt dissolves and the sand does not, so filtering holds back the sand and lets the salt solution through.
- Gently heating the filtrate until crystals start to form removes water and leaves crystals of the dissolved salt. Evaporation cannot work on the sand, because the sand was never dissolved.
- A chloride test on the salt solution, using acidified silver nitrate, gives a white precipitate. That is an observation.
- The inference is that chloride ions are present. It does not yet say which cation is in the salt, because the test looked only for chloride.
Notice the pattern: choose the method from the particle size and solubility, then keep observation and conclusion in separate sentences. Most questions in this module use that pattern.
In what order should you study the lessons?
- Choose filtration versus evaporation from a mixture description. It gives the solubility reasoning that all separation questions depend on.
- Interpret a chromatogram. It adds distance measurements and the Rf calculation.
- Use a stated test result to narrow an unknown. It teaches elimination from a list of candidates.
- Distinguish observation from inferred identity. It fixes the wording that most often loses marks.
- Evaluate the limits of a single positive test. It combines the earlier skills in evaluation questions.
Then attempt the mixed practice set. The mole and equation-ratio tutor and the equation balance reasoning trainer help when a question includes an equation.
What are the common traps?
- “Filtering gets the salt back.” Filtering removes insoluble solids only. Dissolved salt passes through.
- “Measure from the bottom of the paper.” Rf distances start at the baseline, not the paper edge.
- “The test names the substance.” A test shows a property. Naming the substance is an inference.
- “A white precipitate means calcium.” Several ions give white precipitates. The next step is to add excess reagent or use a second test.
- “One positive result is enough.” A single test may fit more than one candidate.
How should you use the practice set?
Work the questions in order. Write each answer as a full sentence that separates what you saw from what you conclude. Open the answer only after you have written yours.
The Chemistry learning guide shows where this module sits among the others. If you want a teacher to check your reasoning question by question, our online one-to-one Chemistry tuition starts with a paid one-hour trial class.