Question
How do we choose the right separation technique for a given mixture? Explain filtration, evaporation, distillation, and chromatography with examples.
(CBSE 6 + CBSE 9 Board)
Solution — Step by Step
Pass the mixture through filter paper. The solid stays on the paper (residue), the liquid passes through (filtrate).
Example: Separating sand from water. Muddy water through a filter gives clean water and sand on the paper.
Heat the solution so the liquid evaporates, leaving the solid behind.
Example: Getting salt from salt water. Heat the solution — water evaporates, salt crystals remain.
But we lose the liquid. If we want BOTH components, we need distillation.
Heat the mixture. The liquid with the lower boiling point evaporates first, passes through a condenser (cools back to liquid), and is collected separately.
Example: Separating alcohol (BP 78°C) from water (BP 100°C). Heat to 78°C — alcohol evaporates first and is collected.
The mixture is placed on a paper/column. A solvent carries the components at different speeds — the most soluble component travels fastest.
Example: Separating ink colours. Put a dot of ink on paper, dip the edge in water — different colours separate as they travel different distances.
flowchart TD
A["Given a mixture to separate"] --> B{"Solid + Liquid?"}
B -- "Insoluble solid in liquid" --> C["FILTRATION"]
B -- "Dissolved solid in liquid" --> D{"Need the liquid too?"}
D -- No --> E["EVAPORATION"]
D -- Yes --> F["DISTILLATION"]
A --> G{"Two miscible liquids?"}
G -- "Different boiling points" --> F
G -- "Very close boiling points" --> H["FRACTIONAL DISTILLATION"]
A --> I{"Need to identify components?"}
I --> J["CHROMATOGRAPHY"]
A --> K{"Immiscible liquids?"}
K --> L["SEPARATING FUNNEL"]Why This Works
Each technique exploits a specific physical property difference between the components:
- Filtration uses particle size (solid particles are too large to pass through filter pores)
- Evaporation uses boiling point difference (liquid evaporates, solid does not)
- Distillation uses boiling point difference between two liquids
- Chromatography uses solubility difference (components dissolve to different degrees in the solvent)
No chemical reactions happen — these are all physical separation methods. The components remain chemically unchanged.
Alternative Method
For CBSE 9, also know these additional techniques: centrifugation (spinning to separate by density — used for blood, cream from milk), sublimation (heating a mixture where one component sublimes — camphor, ammonium chloride), and magnetic separation (using a magnet — iron filings from sand). Match the technique to the property being exploited.
Common Mistake
Students confuse evaporation with distillation. Both involve heating, but evaporation discards the vapour (we only want the solid residue), while distillation condenses and collects the vapour (we want the liquid too). If a question says “obtain pure water from salt water,” the answer is distillation, not evaporation — evaporation gives you only the salt.