Cambridge O Level Chemistry · Syllabus 5070 · Experimental Techniques and Chemical Analysis
Solution
What is Solution?
A mixture of one or more solutes dissolved in a solvent. Sea water is a solution of several salts in one solvent. A solution is a mixture, so its components are not chemically combined and can be separated by physical means such as distillation or crystallisation.
This definition is part of the Experimental Techniques and Chemical Analysis chapter in Cambridge O Level Chemistry.
Common mistakes with Solution
- 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.
- 7. The end-point and a colour you hoped for Fix The end-point is the first permanent colour change, reached by adding dropwise near the end and swirling. A flash of colour that swirls away is not the end-point. Which colour you are waiting for depends on the indicator you chose and on which solution is in the flask — there is no single universal end-point colour.
- 12. A precipitate and a colour change Fix A precipitate is an insoluble solid appearing in a liquid — the mixture goes cloudy or a solid settles. A colour change is a solution changing colour while staying clear. Sulfite decolourises acidified aqueous potassium manganate(VII); it does not precipitate. Sulfate gives a white precipitate; it is not a colour change.
- 4. “A saturated solution is one where no more will dissolve.” Why it fails Warm it and more will dissolve, so the statement describes something that changes as soon as the temperature does. Correct A solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature. Check Why do crystals form when a hot saturated solution cools? Only the full definition, with temperature in it, can answer that.
- 18. “Copper(II) is present.” (as an answer to “state what you would observe”) Why it fails That is an inference offered where an observation was asked for. You cannot see an ion. Correct “A light blue precipitate forms, which dissolves in excess aqueous ammonia to give a dark blue solution.” Check The reverse also fails: “a cream precipitate” is not an answer to “deduce the anion”.
- 19. “Add sodium hydroxide to the tube that already has silver nitrate in it.” Why it fails The tube no longer contains the original substance. Whatever forms may come from the first reagent rather than from the sample. Correct A fresh portion of the original substance in a clean test-tube for every test. Check Why is a solution acidified with hydrochloric acid useless for a later halide test? It now contains chloride ions of its own.
- 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.
- 24. “The end-point is when the solution turns pink.” Why it fails Which colour appears depends on the indicator chosen and on which solution is in the flask. There is no universal end-point colour. Correct The end-point is the first permanent colour change of the indicator — one that does not disappear on swirling. Name a colour only if the indicator was named. Check Adding acid to alkali containing phenolphthalein means watching pink disappear, not appear.
- 28. “Chromium(III) gives a grey-green precipitate; copper(II) in excess ammonia gives a royal blue solution.” Why it fails Both are embellishments of the official wording. The notes say green and dark blue. Correct Chromium(III): green precipitate, soluble in excess. Copper(II) with excess ammonia: soluble, giving a dark blue solution. Check Iron(II) is also green — the same word, and the excess result is what separates them.
Examiner tips on Solution
- Two words that are worth marks on their own. Precipitate — not “it goes cloudy”, not “a solid”, and never “a blue solution” when a blue solid appeared. And excess — every cation test has two stages, and the second one is what separates aluminium from calcium and copper(II) from iron(II). If your answer to a cation test has only one observation in it, it is not finished.
- On chromium(III). The official result is “green precipitate, soluble in excess”. That is what to write. The notes do not specify a colour for the solution formed in excess, so do not invent one — and do not embellish the precipitate as “grey-green” or “dirty green” in a core answer. The word required is green, and the same word is used for iron(II).
Questions students ask about Solution
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.
Why does a "blue solution" observation not earn the same mark as a "blue precipitate"?
A precipitate is an insoluble solid appearing in a liquid, making the mixture cloudy or forming a solid that settles, while a colour change is a solution changing colour while staying clear. Sulfite decolourises acidified aqueous potassium manganate(VII) as a colour change, with no precipitate, while sulfate forms a white precipitate, not a colour change. The two observations describe different chemistry, so they are not interchangeable wording.