CDS Chemistry · Matter and Its States

Separation Techniques

Which method separates which mixture, from sublimation and centrifugation to chromatography, and why the parts of a compound need a chemical method instead.

All Matter and Its States notes

Why this matters

Four CDS questions, all EASY or MODERATE. Each names a mixture and asks for the method, or names a method and asks what it cannot do.

Concept 1 of 1: Choosing a separation method

Each method uses one difference between the parts of a mixture: one sublimes and the other does not, one is denser, one sticks to paper more strongly. Find the difference and the method follows. A compound is different: its parts are held by chemical bonds, so only a chemical change, such as electrolysis, can split them.

Definition

The methods:

Sublimation: one part turns straight from solid to vapour — ammonium chloride, camphor, naphthalene, iodine.
Centrifugation: very fine insoluble particles that will not settle (cream from milk, blood cells from plasma).
Chromatography: parts that stick differently to a stationary phase — dyes, natural pigments, drugs in blood. It cannot separate isotopes, which are chemically alike.
Filtration, decantation: an insoluble solid from a liquid. Evaporation: a dissolved solid from its solvent.
Distillation: a liquid from a dissolved solid, or liquids far apart in boiling point. Fractional distillation: liquids with close boiling points; gases of air.
A compound cannot be separated by any of these. Its elements are split by a chemical method, such as electrolysis (water → hydrogen + oxygen).

Mixture	Method	Difference used
Salt and ammonium chloride	Sublimation	NH₄Cl sublimes, salt does notQ
Very fine insoluble particles in a liquid	Centrifugation	DensityQ
Colours in a dye; drugs in blood	Chromatography	How strongly each part is held by the stationary phaseQ CDS 2019 (II): chromatography cannot separate radio-isotopes.
Acetone and water	Fractional distillation	Boiling point
The elements in a compound (water)	Electrolysis	Breaks the chemical bondsQ

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2020 · CDS (II) 2020 — General Knowledge · Q96Easy

Example 1 · Matter and Its States · Separation Techniques

A mixture of sodium chloride (salt) and ammonium chloride can be separated by

Chromatography cannot tell isotopes apart

Isotopes of an element behave the same chemically, so they stick to the stationary phase in the same way. Chromatography separates dyes, pigments and drugs, not isotopes.

A compound needs a chemical method

Distillation, filtration and sublimation only separate mixtures. The elements of a compound are held by bonds and need a chemical change, such as electrolysis.

Drill 3 more on choosing a separation method

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Choosing a separation method5 rows

Mixture	Method	Difference used
Salt and ammonium chloride	Sublimation	NH₄Cl sublimes, salt does notQ
Very fine insoluble particles in a liquid	Centrifugation	DensityQ
Colours in a dye; drugs in blood	Chromatography	How strongly each part is held by the stationary phaseQ CDS 2019 (II): chromatography cannot separate radio-isotopes.
Acetone and water	Fractional distillation	Boiling point
The elements in a compound (water)	Electrolysis	Breaks the chemical bondsQ

Watch out for (2)

Chromatography cannot tell isotopes apart
→ Choosing a separation method
A compound needs a chemical method
→ Choosing a separation method

Test yourself on a real paper

Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Sit a CDS past paper Drill all 4 questions