Last updated: 2 October 2026
Most substances we encounter are mixtures. Seawater contains dissolved salts, ink may contain several dyes, and a solid prepared in the laboratory may still contain unreacted chemicals or soluble impurities. Chemists therefore need ways not only to separate substances, but also to purify what they collect and decide whether the final sample is actually pure.
These methods become much easier when you stop treating them as a list of apparatus to memorise. Instead, begin with the mixture itself. Ask what is different about its components: does one dissolve while the other does not? Do the liquids form separate layers? Do they boil at different temperatures? Does one solid sublime?
Once that physical difference is clear, the appropriate separation method usually follows naturally. The same reasoning is also useful in examination questions involving unfamiliar mixtures.
Try these five questions before reading the chapter. They are designed to reveal which ideas you already understand and which ones need a little more attention.
- 5 quick questions
- Instant marking
- Explanation for every option
Separation, purification and analysis
These three words are related, but they describe different stages of laboratory work.
Separation
Separation means dividing a mixture into some or all of its components.
For example, filtering a mixture of sand and water separates the insoluble sand from the water.
Purification
Purification goes one step further. The aim is to obtain the required substance with as little contamination as possible.
Suppose sand is filtered from salt solution. The sand collected on the filter paper may still have salt solution on its surface. Washing and drying the sand are therefore part of the purification process.
Analysis
Analysis uses observations or measurements to provide information about a substance. In this topic, analysis includes using melting points, boiling points and paper chromatography to help identify substances and assess purity.
A substance can be physically separated from a mixture and still contain contaminants. Always consider whether washing, crystallising, drying or another purification step is still required.
Choosing a separation method
A strong separation answer begins with the properties of the mixture rather than the name of the apparatus.
First decide whether you have:
- two solids;
- a solid and a liquid;
- two liquids; or
- a dissolved mixture that can be analysed by chromatography.
Then identify the physical property that differs.
| Useful physical difference | Possible method |
|---|---|
| One solid dissolves in a solvent but another does not | Dissolve, filter, then crystallise or evaporate |
| One solid sublimes but another does not | Sublimation |
| An insoluble solid is mixed with a liquid | Filtration |
| A soluble solid is dissolved in a liquid | Crystallisation or evaporation |
| Two liquids are immiscible and form layers | Separating funnel |
| A solvent must be collected from a solution | Simple distillation |
| Miscible liquids have different boiling points | Fractional distillation |
| Dissolved substances travel differently with a solvent | Paper chromatography |
In an examination answer, naming the method is often not enough. Be ready to explain the property being used, such as insolubility, different boiling points or formation of separate liquid layers.
Separating mixtures of solids
If two solids are mixed together, filtration cannot normally separate them immediately because both solids would remain on the filter paper. We first need to create a difference that can be exploited.
Using a suitable solvent
A suitable solvent should dissolve one component of the mixture while leaving the other component insoluble.
Consider a mixture of sodium chloride and sand. Water is useful because sodium chloride dissolves in water while sand does not.
Worked method: separating sodium chloride and sand
- Add water to the mixture.
- Stir so that the sodium chloride dissolves.
- Filter the mixture.
- Sand remains on the filter paper as the residue.
- Sodium chloride solution passes through as the filtrate.
- Wash the sand with a small amount of water to remove remaining salt solution.
- Dry the sand.
- Recover sodium chloride from the filtrate by crystallisation or evaporation.
If you want the insoluble solid: collect, wash and dry the residue.
If you want the soluble solid: continue working with the filtrate and recover the dissolved solute.
Why is washing important?
When filtration is complete, some solution may remain between particles of the residue. If the residue is not washed, dissolved impurities may remain on its surface after drying.
Washing with a small amount of suitable solvent removes these soluble impurities.
How do you know whether a solvent is suitable?
The solvent must create a useful difference in solubility.
- If both solids dissolve, filtration will not separate them.
- If neither solid dissolves, both remain as residue.
- If only one dissolves, the dissolved and undissolved components can be separated.
Sublimation
Sublimation is the direct change of a solid into a gas without passing through the liquid state.
When the vapour is cooled, it can change directly back into a solid. The solid deposited after cooling is called the sublimate.
Sublimation can separate a mixture when one solid sublimes on heating while the other does not.
For example, ammonium chloride may be separated from sodium chloride:
- Heat the mixture gently.
- Ammonium chloride forms a vapour.
- The vapour reaches a cooler surface.
- Solid ammonium chloride is deposited on the cool surface.
- Sodium chloride remains behind.
A substance that sublimes changes directly from solid to gas. It does not become liquid first.
Filtration
Filtration separates an insoluble solid from a liquid.
The mixture is poured through filter paper held inside a filter funnel. The liquid passes through the pores of the paper, while particles of insoluble solid are trapped.
| Term | Meaning |
|---|---|
| Residue | The insoluble solid remaining on the filter paper |
| Filtrate | The liquid that passes through the filter paper |
When sand and water are filtered, the sand is the residue and the water is the filtrate.
Filtration does not separate a dissolved solute from its solvent. For example, Na⁺ and Cl⁻ ions in sodium chloride solution pass through the filter paper with the water.
If a substance has dissolved, ordinary filtration cannot recover it. A different method, such as crystallisation, evaporation or distillation, is needed.
Evaporation and crystallisation
Both methods can recover a soluble solid from a solution, but they are not identical.
Evaporation to dryness
During evaporation to dryness, the solvent is removed by heating until the dissolved solid remains.
This method is suitable when:
- the solvent is not required;
- the solute can withstand the heating involved; and
- well-formed crystals are not essential.
As the solution becomes concentrated, heating should be reduced because concentrated solutions may spit.
Crystallisation
Crystallisation is used to obtain a dissolved solid as crystals. It is particularly useful when strong heating to dryness might decompose the solute.
The basic procedure is:
- Heat the solution gently to evaporate some of the solvent.
- Stop heating when a hot concentrated or saturated solution has formed.
- Allow the solution to cool.
- Crystals form as less solute can remain dissolved at the lower temperature.
- Filter off the crystals.
- Wash them with a small amount of cold solvent.
- Dry the crystals.
What is a saturated solution?
A saturated solution contains the maximum amount of dissolved solute that can remain dissolved at a particular temperature.
For many solids, a hot solution can hold more dissolved solute than a cold solution. When a hot saturated solution cools, some of the dissolved solute therefore comes out of solution as crystals.
Why is the solution not heated to complete dryness?
If the aim is crystallisation, some solvent must remain. During cooling, the desired solid forms crystals while many soluble impurities remain in the liquid surrounding the crystals.
Heating to complete dryness may also decompose a heat-sensitive solute.
The mother liquor
The solution remaining after crystals have formed is called the mother liquor.
It still contains:
- solvent;
- some dissolved product; and
- many soluble impurities.
The crystals are separated from this liquid by filtration.
Why are crystals washed with a little cold solvent?
Mother liquor may remain on the surfaces of freshly filtered crystals. Washing removes much of this contaminated solution.
Only a small amount of cold solvent is used. Too much solvent would dissolve unnecessary product, while warm solvent would usually dissolve even more.
Evaporation compared with crystallisation
| Evaporation to dryness | Crystallisation |
|---|---|
| Most or all solvent is removed by heating. | Only part of the solvent is removed before cooling. |
| Suitable when the solute can withstand the heating. | Preferred when strong heating may decompose the solute. |
| Does not necessarily produce well-formed crystals. | Allows crystals to form during cooling. |
| Soluble impurities may remain with the solid after complete evaporation. | Many soluble impurities remain in the mother liquor. |
Separating immiscible liquids
Two liquids are immiscible when they do not mix to form one uniform liquid phase. Instead, they form separate layers.
Oil and water are familiar examples.
A separating funnel is used to separate such liquid layers.
How a separating funnel is used
- Close the tap before adding the liquid mixture.
- Add the mixture to the separating funnel.
- Allow the liquids to settle until two clear layers form.
- Place a suitable receiving container below the funnel.
- Remove the stopper.
- Open the tap and drain the lower layer.
- Close the tap when the boundary between the layers reaches the tap.
- Pour the remaining upper layer from the top into a different container.
Which liquid forms the lower layer?
The denser liquid forms the lower layer.
Do not automatically assume that water is always below the other liquid. Some liquids are denser than water and will form the lower layer instead.
Why is the stopper removed before draining?
As liquid leaves the bottom of the funnel, air must be able to enter from above. Removing the stopper allows the pressure to equalise so the liquid flows smoothly.
If density data are provided, use them. The liquid with the greater density forms the lower layer.
Simple distillation
Simple distillation can be used to collect a solvent from a solution.
Suppose pure water is required from sodium chloride solution. Water can be vaporised and then condensed, while sodium chloride remains in the distillation flask.
How simple distillation works
- Heat the solution in the distillation flask.
- The solvent boils and forms vapour.
- The vapour passes into the condenser.
- Cooling water removes thermal energy from the vapour.
- The vapour condenses back into a liquid.
- The condensed liquid is collected.
The condensed liquid collected is called the distillate.
Where should the thermometer bulb be?
The thermometer bulb should be positioned near the entrance to the condenser so that it measures the temperature of the vapour entering the condenser.
Placing it deep in the solution would instead measure the temperature of the liquid in the flask.
Which way should water flow through the condenser?
Cooling water should enter at the lower end of the condenser jacket and leave at the upper end.
Water entering from below pushes air upwards and keeps the outer jacket full, giving efficient cooling.
Cooling water in: lower end of the condenser.
Cooling water out: upper end of the condenser.
Filtrate and distillate are not the same thing
| Term | How it is obtained |
|---|---|
| Filtrate | Liquid that passes through filter paper during filtration |
| Distillate | Vapour that has been condensed and collected during distillation |
Fractional distillation
Fractional distillation is used to separate miscible liquids with different boiling points, especially when their boiling points are relatively close.
Miscible liquids mix completely to form a single liquid phase. Ethanol and water are an example.
The fractionating column
The key extra piece of apparatus is the fractionating column, positioned between the heated flask and the condenser.
It provides a large surface on which repeated condensation and evaporation can occur.
As the mixture is heated:
- Both liquids may produce vapour.
- The vapour is richer in the more volatile liquid.
- Some vapour condenses as it rises through the cooler column.
- The higher-boiling component condenses more readily and returns down the column.
- Further evaporation produces vapour richer in the lower-boiling component.
- The vapour reaching the top is therefore enriched in the lower-boiling liquid.
It does not trap one liquid while allowing another through. Separation occurs because of repeated evaporation and condensation.
Simple and fractional distillation compared
| Simple distillation | Fractional distillation |
|---|---|
| Often used to obtain a solvent from a solution. | Used to separate miscible liquids. |
| Can separate liquids when their boiling points are sufficiently far apart. | More effective when boiling points are relatively close. |
| No fractionating column is required. | A fractionating column is required. |
| Fewer vaporisation–condensation stages. | Repeated vaporisation and condensation improve separation. |
Paper chromatography
Paper chromatography is used to separate and compare small quantities of dissolved substances.
It is particularly useful for mixtures such as dyes in ink, but colourless substances can also be examined if their positions are made visible afterwards.
Setting up a chromatogram
- Draw a light starting line in pencil near the bottom of the chromatography paper.
- Place a small spot of each sample on the line.
- Allow the spots to dry.
- Place the paper in a suitable solvent.
- Make sure the starting line is above the solvent level.
- Allow the solvent to travel up the paper.
- Remove the paper before the solvent reaches the top.
- Immediately mark the position of the solvent front in pencil.
- Allow the chromatogram to dry.
Why is pencil used?
Pencil contains graphite, which does not dissolve in the chromatography solvent. The starting line therefore stays in place.
Ink contains soluble dyes. If ink were used for the starting line, these dyes might move up the paper and produce additional spots.
Why must the starting line be above the solvent?
If the sample spots are submerged, they may dissolve directly into the solvent at the bottom of the container and be washed away rather than travelling up the paper.
Why should the spots be small?
Small concentrated spots give clearer results. A large spot may spread out and overlap with another component, making interpretation difficult.
Why do different substances travel different distances?
The solvent moves through the paper and carries dissolved substances with it. How far a substance travels depends on the balance between:
- its tendency to move with the solvent; and
- its tendency to remain associated with the paper.
A substance generally travels further when it dissolves more readily in the solvent and is less strongly held by the paper.
At O-Level, solubility is an important part of the explanation, but attraction to the stationary paper also affects how far a substance moves.
Interpreting a chromatogram
| Observation | What it suggests |
|---|---|
| A sample produces one spot | Only one component has been detected under these conditions |
| A sample produces several spots | The sample contains more than one component |
| An unknown spot lines up with a known substance on the same chromatogram | The unknown may contain that substance |
Calculating the Rf value
The retention factor, Rf, compares the distance travelled by a substance with the distance travelled by the solvent front.
Both distances are measured from the original pencil starting line.
Because the numerator and denominator are both distances measured in the same unit, the units cancel. An Rf value therefore has no unit.
Worked example: Rf
A substance travels 4.2 cm from the starting line while the solvent front travels 7.0 cm.
The Rf value is 0.60.
Why must the solvent front be marked immediately?
The solvent may evaporate after the paper is removed from the container. If its final position is not marked immediately, the distance travelled by the solvent can no longer be measured accurately.
Comparing Rf values
An unknown substance may be compared with a known substance using their Rf values, but the comparison is meaningful only when the experimental conditions are the same.
In particular, the same solvent should be used. Changing the solvent changes the way the substance interacts with the system and may change its Rf value.
Useful comparison: same solvent, same paper and same experimental conditions.
Unreliable comparison: values obtained using different solvents.
Locating agents
A coloured substance forms visible spots, but many substances are colourless. Their positions cannot be seen directly after chromatography.
A locating agent is used after the chromatogram has developed to make these colourless spots visible.
Testing purity
A chemically pure substance contains only one element or one compound.
The word “pure” in Chemistry describes composition. It does not mean harmless or safe. Pure chlorine remains toxic, and pure concentrated sulfuric acid remains highly corrosive.
Melting point
A pure solid melts over a narrow temperature range close to its characteristic melting point.
An impure solid usually:
- begins melting at a lower temperature; and
- melts over a wider range of temperatures.
Impurities disturb the regular arrangement within the solid. Different parts of the sample therefore begin to melt under slightly different conditions.
Worked reasoning from melting-point data
The accepted melting point of substance P is 82 °C.
| Sample | Observed melting range | Interpretation |
|---|---|---|
| A | 81.5–82.0 °C | Narrow range close to 82 °C; likely to be relatively pure P |
| B | 75–80 °C | Lower and broader range; likely to contain impurities |
Boiling point
A pure liquid boils at a characteristic temperature when the external pressure is fixed.
An impure sample may boil over a range or at a temperature different from the reference value. A dissolved non-volatile impurity commonly raises the boiling point of a liquid.
A melting or boiling temperature is useful only when you know what value is expected for the substance under the stated conditions.
Using chromatography as evidence of purity
In an O-Level interpretation, a pure substance normally produces one chromatogram spot, while a mixture produces two or more spots.
More precisely, one spot means that only one component was detected under the conditions used. Two different substances may occasionally overlap, which is why other evidence can sometimes be useful.
Why purity matters
Purity is important wherever unwanted substances could affect safety, effectiveness or product quality.
- Medicines: the correct active ingredient and dose must be present without harmful contaminants.
- Food: harmful contaminants must be controlled and ingredients must meet quality standards.
- Laboratory chemicals: impurities can interfere with reactions and measurements.
- Industrial materials: unwanted substances may change the physical or chemical properties of a product.
Common purification and analysis mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Filtering a salt solution to collect the salt | Dissolved ions pass through the filter paper. | Use crystallisation or evaporation. |
| Stopping after filtering a salt-and-sand mixture | The salt is still dissolved in the filtrate. | Recover the salt from the filtrate afterwards. |
| Calling every liquid that passes through filter paper “pure water” | The filtrate may contain dissolved solutes. | State what is actually dissolved in the filtrate. |
| Heating a heat-sensitive solute to complete dryness | The solute may decompose. | Concentrate the solution and use crystallisation. |
| Washing crystals with a large volume of warm solvent | Too much product may dissolve. | Use a small amount of cold solvent. |
| Using fractional distillation for oil and water | Oil and water are immiscible and form separate layers. | Use a separating funnel. |
| Assuming water must be the lower layer | The lower layer is determined by density. | Use density information to identify the layers. |
| Leaving the stopper in a separating funnel while draining | Air cannot enter freely and the liquid may not flow smoothly. | Remove the stopper before opening the tap. |
| Putting the thermometer bulb deep in the distillation mixture | The temperature of the vapour entering the condenser is required. | Position the bulb near the condenser entrance. |
| Sending condenser water in from the top | The water jacket may not remain completely filled. | Send cooling water in from the lower end. |
| Saying the fractionating column filters the liquids | No particles are being trapped. | Explain repeated evaporation and condensation. |
| Drawing the chromatography baseline in ink | Ink may dissolve and produce unwanted spots. | Use pencil. |
| Putting sample spots below the solvent level | The samples may dissolve into the solvent reservoir. | Keep the baseline above the solvent. |
| Measuring Rf distances from the bottom of the paper | Both distances must begin at the starting line. | Measure from the pencil baseline. |
| Writing cm as the unit for Rf | Rf is a ratio, so the distance units cancel. | Write Rf without a unit. |
| Comparing Rf values from different solvents | The solvent affects how far each substance travels. | Compare values obtained under the same conditions. |
| Saying that chemically pure means harmless | Purity describes composition, not safety. | Define purity in terms of one element or compound. |
Purification and analysis checklist
Identify the mixture
- Which substances are present?
- Are they solids or liquids?
- Is a solid dissolved or insoluble?
- Are two liquids miscible or immiscible?
Find the physical difference
- Do the solids have different solubilities?
- Can one solid sublime?
- Do the liquids have different densities?
- Do the liquids have different boiling points?
Track every component
- Which substance is the residue?
- What remains in the filtrate?
- Which liquid forms the lower layer?
- What becomes the distillate?
Purify and analyse
- Does the product need washing?
- Does it need drying?
- Should it be crystallised?
- How can its identity or purity be checked?
You have studied the chapter. Now apply the ideas to unfamiliar mixtures, experimental procedures and analytical evidence.
- 10 application questions
- Instant marking
- Explanation for every option
A: 115–121 °C
B: 121.8–122.2 °C
C: 118–119 °C
D: 124–129 °C
Frequently asked questions
Questions students often ask about purification and analysis
These questions connect the different methods and clarify choices that often cause confusion in examination questions.
How do I know whether to collect the residue or the filtrate?
Follow the substance you want. An insoluble desired solid will be in the residue. A desired dissolved substance will remain in the filtrate and must later be recovered, for example by crystallisation.
Is filtration itself enough to purify an insoluble solid?
Not necessarily. The solid may still have solution containing soluble impurities on its surface. The residue is therefore often washed with a small amount of suitable solvent and then dried.
Why does crystallisation not recover all of the dissolved solute?
Even after cooling, some solute remains dissolved in the mother liquor. Recovering every last bit would usually require removing more solvent, but doing so may reduce the purity of the crystals.
Why is slow cooling useful during crystallisation?
Slower cooling allows crystals to grow more gradually. Very rapid crystallisation tends to produce many small crystals and may trap more mother liquor between them.
Can a separating funnel be used for ethanol and water?
No. Ethanol and water are miscible, so they form one liquid phase rather than two layers. A separating funnel requires immiscible liquids.
Why is simple distillation suitable for obtaining water from salt solution?
Water can be vaporised and condensed, whereas dissolved sodium chloride remains in the heated flask under the conditions used. The water is collected as the distillate.
Why is fractional distillation better when two boiling points are close?
Both liquids may contribute significantly to the vapour when their boiling points are close. Repeated condensation and evaporation in the fractionating column progressively enrich the vapour in the more volatile component.
Can two different substances have the same Rf value?
Yes. Different substances can occasionally have similar values in one solvent. A matching Rf supports an identification but is not always absolute proof.
Why must Rf values be compared using the same solvent?
The solvent affects how readily each substance moves through the paper. The same substance can therefore have a different Rf value in a different solvent.
Does one chromatography spot prove that a substance is perfectly pure?
At O-Level, one spot is normally treated as evidence that a sample is pure. More precisely, it means that only one component was detected under those particular conditions.
Is a liquid pure just because it boils at 100 °C?
Not automatically. The observed boiling temperature must be compared with the accepted value for the suspected substance at the stated pressure.
Why are both the melting temperature and melting range important?
The temperature helps compare the sample with the known substance, while the width of the melting range gives evidence about purity.
Does chemically pure mean safe?
No. Purity describes composition. A pure sample may still be toxic, corrosive, flammable or otherwise hazardous.
What is the best way to begin an unfamiliar separation question?
Identify the components and their physical states first. Then look for a useful difference in solubility, sublimation behaviour, miscibility, density, boiling point or chromatographic movement.
Methods of purification and analysis become much easier when every procedure is connected to the physical property that makes it work. Insolubility leads to filtration, differences in solubility can allow solids to be separated, immiscibility and density lead to a separating funnel, and differences in boiling point lead to distillation. Chromatography uses differences in how substances travel through the system, while melting and boiling data provide evidence about identity and purity. Once you learn to ask “what is different about these substances?”, unfamiliar separation questions become reasoning problems rather than memory tests.