To choose between filtration and evaporation, ask one question first: is the solid dissolved or not? An insoluble solid is removed by filtration. A dissolved solid is recovered by evaporating the solvent.
This skill opens separation and chemical analysis and appears again whenever a question gives a mixture and asks how to obtain a pure part of it.
How does the choice follow from particle size?
An insoluble solid stays as grains that are far larger than the gaps in filter paper. The grains are caught and the liquid passes through. The solid left on the paper is the residue, and the liquid that passes through is the filtrate.
A dissolved solid has broken up into particles as small as the solvent particles. These pass through filter paper with the liquid, so filtering cannot catch them. Removing the solvent by evaporation leaves the solid behind.
How do you plan a separation step by step?
- Read the description and underline which substances are soluble in the solvent and which are not.
- If the mixture is solid plus solid, choose a solvent that dissolves one of them and not the other.
- Filter to remove the insoluble solid. Wash the residue with a little solvent and let it dry.
- Evaporate the filtrate to recover the dissolved solid.
- Name the parts in your answer: residue, filtrate and what each contains.
Worked example
Invented data. A 10.0 g sample is a mixture of sand and salt. Sand is insoluble in water and salt is soluble. Describe how to obtain both and find the percentage of salt by mass if the dry sand weighs 6.4 g.
Step 1, dissolve: add the mixture to water and stir so all the salt dissolves.
Step 2, filter: the sand stays on the paper as the residue. The filtrate is salt solution.
Step 3, recover the salt: evaporate the filtrate to remove the water, leaving solid salt.
Step 4, calculate: mass of salt = 10.0 − 6.4 = 3.6 g. Percentage of salt = 3.6 ÷ 10.0 × 100 = 36%.
The answer has a method and a calculation, and both depend on the same idea: the sand never dissolved, and the salt did.
The mistake to watch for
A common slip is to filter a salt solution and expect solid salt on the paper.
Mistaken answer: “Filter the salt water to get the salt.”
The student treated salt like sand. Dissolved particles are too small to be trapped.
The correction is to check solubility before choosing the method. If the substance you want is dissolved, it is in the filtrate, and the water must be removed to get it.
Check yourself
Try these, then open each answer.
1. A student stirs fine chalk powder into water. The chalk does not dissolve. Which method separates it, and where is the chalk afterwards?
Show answer
Chalk is insoluble, so filtration separates it. The chalk is the residue on the filter paper, and the water is the filtrate.
2. A solution contains dissolved blue crystals in water. The student wants the solid back. Choose the method and give the reason.
Show answer
Use evaporation (or crystallisation). The solid is dissolved, so it passes through filter paper. Removing the water leaves the solid.
3. A 15.0 g mixture of sand and salt gives 9.0 g of dry sand after dissolving and filtering. What is the percentage of salt?
Show answer
Mass of salt = 15.0 − 9.0 = 6.0 g. Percentage = 6.0 ÷ 15.0 × 100 = 40%.
Where this leads next
Once solubility reasoning feels automatic, move on to interpreting a chromatogram, which separates substances that are all dissolved. Then test the whole module with the mixed practice set.
Some students understand each separation method alone but hesitate when a question describes a new mixture. Our teachers look for exactly that pattern in online one-to-one Chemistry tuition.