Cambridge O Level Chemistry · Syllabus 5070 · Experimental Techniques and Chemical Analysis
Residue
What is Residue?
A substance that remains after evaporation, distillation, filtration or any similar process. Sand caught on filter paper is a residue, so is salt left in an evaporating basin, and so is the liquid left behind in a distillation flask. Restricting the word to solid on filter paper is the most common definition error in this topic.
This definition is part of the Experimental Techniques and Chemical Analysis chapter in Cambridge O Level Chemistry.
Common mistakes with Residue
- 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.
- 11. “The residue is what you collect at the end of the distillation.” Why it fails What is collected is the distillate. The residue is what stays behind in the heated flask. Correct Distillate: condensed vapour, in the receiver. Residue: not vaporised, in the flask. Check In a fractional distillation of ethanol and water, which one is the ethanol at the start? The distillate — and the water is the residue until it in turn distils.
- 22. “Statement 12.1.3 is about collecting and drying gases.” Why it fails Statement 12.1.3 is the six definitions — solvent, solute, solution, saturated solution, residue and filtrate. Gas collection and drying methods are not part of Topic 12 for this cycle. Correct Learn the six definitions for 12.1.3, in the official wording. Check If a revision resource lists upward delivery and drying agents under 12.1.3, it is describing an older specification.
Questions students ask about Residue
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.