Question
You have four mixtures: (a) sand and water, (b) salt and water, (c) two miscible liquids with different boiling points, (d) different coloured dyes in ink. Which separation technique would you use for each, and why?
(CBSE Class 6 and Class 9 Science)
Solution — Step by Step
Sand is an insoluble solid in water. Pour the mixture through filter paper — sand stays on the paper (residue), water passes through (filtrate).
Why filtration? Sand particles are too large to pass through the tiny pores of filter paper.
Salt is dissolved in water — it passes through filter paper. Heat the solution so water evaporates, leaving solid salt behind.
Why evaporation? We want the dissolved solid (salt), not the liquid. If we wanted both, we would use distillation instead.
If two liquids are mixed (like acetone and water), we heat the mixture. The liquid with the lower boiling point evaporates first, gets condensed in a condenser, and is collected separately.
Why distillation? The liquids have different boiling points, so we exploit this difference. For liquids with very close boiling points, we use fractional distillation (with a fractionating column).
Ink contains several dyes dissolved together. Place a drop of ink on filter paper and dip the edge in water. Different dyes travel at different speeds, separating into distinct coloured bands.
Why chromatography? The dyes are all dissolved in the same solvent — we cannot use boiling point differences. Chromatography separates based on how strongly each dye sticks to the paper vs. how easily it moves with the solvent.
Separation Technique Selection Flowchart
flowchart TD
A["What kind of mixture?"] --> B{"Solid + Liquid?"}
B -->|"Solid is insoluble"| C["Filtration"]
B -->|"Solid is dissolved"| D{"Want the solid or liquid?"}
D -->|"Want the solid"| E["Evaporation"]
D -->|"Want both"| F["Distillation"]
A --> G{"Liquid + Liquid?"}
G -->|"Immiscible"| H["Separating funnel"]
G -->|"Miscible, different BP"| I["Distillation"]
G -->|"Miscible, close BP"| J["Fractional distillation"]
A --> K{"Multiple dissolved substances?"}
K -->|"Yes"| L["Chromatography"]Why This Works
Every separation technique exploits a physical property difference between the components: particle size (filtration), boiling point (distillation), solubility (evaporation), or affinity to a surface (chromatography).
The trick is identifying which property differs between the components of your mixture, then picking the matching technique.
Common Mistake
Students confuse evaporation and distillation. In evaporation, we only collect the solid — the liquid is lost as vapour. In distillation, we collect BOTH — the vapour is condensed back to liquid. If a question asks “how to get pure water from salt water,” the answer is distillation, NOT evaporation.