Cambridge IGCSE Chemistry · Syllabus 0620 · Experimental Techniques and Chemical Analysis
Filtration
What is Filtration?
A separation method in which a mixture is poured through filter paper held in a funnel, so that an insoluble solid is held back while liquid or solution passes through. The solid retained is the residue and the liquid collected below is the filtrate. Either can be the desired product: the residue is washed with a little distilled water and dried, and the dissolved solid in the filtrate can be recovered by crystallisation.
This definition is part of the Experimental Techniques and Chemical Analysis chapter in Cambridge IGCSE Chemistry.
Common mistakes with Filtration
- 5. Residue and filtrate Fix The filtrate is the liquid or solution that has passed through the filter. The residue is what remains after a process — and that process may be filtration, evaporation or distillation, not only filtration. Never define residue as “the solid on the filter paper”; that is one example of a residue, not the meaning of the word.
- 3. “The residue is the solid on the filter paper.” Why it fails That is one example of a residue, not the definition. Defining it that way makes the salt left in an evaporating basin, and the liquid left in a distillation flask, impossible to name. Correct A residue is a substance that remains after evaporation, distillation, filtration or any similar process. Check Sea water is distilled. What is the residue? The dissolved salts left in the flask — and there is no filter anywhere in the apparatus.
- 10. “Filter the sea water to get pure water.” Why it fails The salts are dissolved, so they pass through the filter paper with the water. Filtration removes insoluble solid only. Correct Simple distillation: the water vaporises, is condensed and collected as the distillate; the salts stay behind as the residue. Check Sea water is filtered and then distilled. Which step removed the salt? Only the second.
- 27. “Use a separating funnel to separate the mixture.” Why it fails A separating funnel is not one of the five methods named in statement 12.4.1, so it is not the expected answer — and it only works at all for liquids that do not mix. Correct Choose from a suitable solvent, filtration, crystallisation, simple distillation and fractional distillation, and justify the choice from the property data. Check Ethanol and water are miscible, so no layers form; fractional distillation is the method.