Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level Chemistry.
Filtration in context
Experimental techniques and chemical analysis cover how a chemist gathers and interprets evidence: choosing the right apparatus for a measurement, running a titration to find a reacting volume, using paper chromatography to separate and identify substances, separating and purifying mixtures by filtration, crystallisation or distillation, and running the qualitative analysis tests that identify anions, cations, gases and metal ions. A titration uses a volumetric pipette to measure a fixed volume and a burette to add a variable volume until an indicator shows the end-point, giving a titre as final reading minus initial reading. Chromatography separates soluble substances by how far they travel in a solvent, measured as the retention factor, Rf.
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.
Questions students ask about Filtration
What is the difference between a residue and a filtrate?
The filtrate is the liquid or solution that has passed through the filter paper. The residue is whatever remains after a process such as filtration, evaporation or distillation — not only the solid caught on filter paper, but also, for example, the salt left in an evaporating basin or the liquid left in a distillation flask. Restricting "residue" to solid on filter paper is the most common definition error in this topic.
How should you choose a separation technique on an unfamiliar substance?
Match the technique to the property difference the question gives you: use a suitable solvent or filtration when solubility differs, crystallisation to recover a dissolved solid, simple distillation to separate a solvent from a dissolved solid or non-volatile component, and fractional distillation when two or more miscible liquids have different boiling points. A full-credit answer names the method, states the property difference that makes it work, and says what is kept.