Boiling a Solution Dry Is the Worst Way to Get a Pure Solid
Learn why crystallisation beats evaporation, how to choose between simple and fractional distillation from boiling points, how paper chromatography separates a mixture, and how to pick a technique.
Why not just boil a solution dry to get the solid back?
Because boiling dry gives you everything that was dissolved, not only what you wanted.
Three things go wrong. Some solids decompose when heated strongly — sugar chars rather than reappearing as sugar. Any impurity that was dissolved alongside your substance is left behind with it, so the solid is no purer than the solution was. And the product is a crust rather than well-formed crystals.
Crystallisation avoids all three. The solution is warmed only until it is close to saturation, then left to cool slowly. As the solubility falls, the wanted substance comes out as crystals, while the impurities — present in much smaller amounts and nowhere near saturation — stay dissolved in the liquid left behind.
The liquid left over is called the mother liquor, and it carries the impurities away with it. That is the whole advantage. This page covers the second part of the CBSE Class 9 Science chapter on mixtures and their separation.
Three things go wrong. Some solids decompose when heated strongly — sugar chars rather than reappearing as sugar. Any impurity that was dissolved alongside your substance is left behind with it, so the solid is no purer than the solution was. And the product is a crust rather than well-formed crystals.
Crystallisation avoids all three. The solution is warmed only until it is close to saturation, then left to cool slowly. As the solubility falls, the wanted substance comes out as crystals, while the impurities — present in much smaller amounts and nowhere near saturation — stay dissolved in the liquid left behind.
The liquid left over is called the mother liquor, and it carries the impurities away with it. That is the whole advantage. This page covers the second part of the CBSE Class 9 Science chapter on mixtures and their separation.
How does crystallisation give a purer solid than evaporation?
By leaving the impurities dissolved instead of drying them onto the product.
The method, step by step:
- Dissolve the impure solid in the minimum amount of hot water
- Filter the hot solution to remove any insoluble impurity
- Warm the filtrate gently to concentrate it, until it is nearly saturated
- Cool it slowly and undisturbed; crystals of the pure substance form
- Filter the crystals out and dry them; the mother liquor is discarded
Why slow cooling matters. Cooling slowly gives large, well-formed crystals of the wanted substance alone. Cooling fast gives many small crystals that trap liquid — and therefore impurities — inside them.
Worked example — how much crystallises. A saturated solution is prepared with g of water at , where the solubility of the substance is g per g of water. It is then cooled to , where the solubility is g per g.
The remaining g stays dissolved in the mother liquor, along with the impurities. This is the calculation from the previous part of this chapter, now doing a job: the difference between two solubilities is the yield of the crystallisation.
Everyday evidence. Salt from seawater in coastal salt pans is made by evaporation — the sun dries the brine and the salt is left with everything else that was dissolved in the sea, which is why such salt needs further purification. Sugar crystals grown on a thread in a saturated sugar solution, as mishri, are made by crystallisation, and they come out clear and well-shaped.
Evaporation is not useless. Where the solute is stable to heat and no purification is needed, evaporation is quicker and simpler, and it recovers all of the solute rather than only the surplus above the cold solubility. Crystallisation recovers less but purer — so the choice depends on whether purity or yield matters more.
Distillation is the opposite choice. Evaporation and crystallisation both discard the solvent. If you want the solvent back — pure water from salt water, for instance — neither will do, and the technique needed is the subject of the next section.
The method, step by step:
- Dissolve the impure solid in the minimum amount of hot water
- Filter the hot solution to remove any insoluble impurity
- Warm the filtrate gently to concentrate it, until it is nearly saturated
- Cool it slowly and undisturbed; crystals of the pure substance form
- Filter the crystals out and dry them; the mother liquor is discarded
Why slow cooling matters. Cooling slowly gives large, well-formed crystals of the wanted substance alone. Cooling fast gives many small crystals that trap liquid — and therefore impurities — inside them.
Worked example — how much crystallises. A saturated solution is prepared with g of water at , where the solubility of the substance is g per g of water. It is then cooled to , where the solubility is g per g.
The remaining g stays dissolved in the mother liquor, along with the impurities. This is the calculation from the previous part of this chapter, now doing a job: the difference between two solubilities is the yield of the crystallisation.
Everyday evidence. Salt from seawater in coastal salt pans is made by evaporation — the sun dries the brine and the salt is left with everything else that was dissolved in the sea, which is why such salt needs further purification. Sugar crystals grown on a thread in a saturated sugar solution, as mishri, are made by crystallisation, and they come out clear and well-shaped.
Evaporation is not useless. Where the solute is stable to heat and no purification is needed, evaporation is quicker and simpler, and it recovers all of the solute rather than only the surplus above the cold solubility. Crystallisation recovers less but purer — so the choice depends on whether purity or yield matters more.
Distillation is the opposite choice. Evaporation and crystallisation both discard the solvent. If you want the solvent back — pure water from salt water, for instance — neither will do, and the technique needed is the subject of the next section.
When do you use simple distillation and when fractional?
Simple distillation when the boiling points are far apart; fractional when they are close.
Distillation separates liquids using their different boiling points. The mixture is heated, the component with the lower boiling point vaporises first, the vapour passes into a condenser where it cools back to a liquid, and the liquid is collected separately.
The rule for choosing, as the syllabus states it: use simple distillation when the boiling points differ by **more than about K, and fractional distillation when they differ by less than that.
Why fractional distillation needs a column. A fractionating column — packed with glass beads or shaped into a series of surfaces — sits between the flask and the condenser. The rising vapour condenses and evaporates many times over as it climbs, and each cycle enriches it further in the more volatile component. Liquids too close in boiling point for one vaporisation to separate are separated by many.
Worked example 1 — a wide gap.** Separating acetone from water. Acetone boils at K and water at K:
The gap is greater than K, so simple distillation is enough. The acetone distils over first and the water remains.
Converting to the Celsius scale for a sense of these numbers: and .
Worked example 2 — a narrow gap. Separating the gases of liquefied air. Nitrogen boils at K and oxygen at K:
The gap is less than K, so fractional distillation is required. Nitrogen, being more volatile, comes off first.
Worked example 3 — a mixture of many components. Crude petroleum contains many liquids with boiling points spread across a wide range and often close together. It is separated by fractional distillation in a tall column, drawing off different fractions at different heights — the lightest near the top where it is coolest, the heaviest near the bottom.
Both liquids must be miscible. Distillation separates liquids that mix with each other. Two liquids that do not mix — oil and water — form separate layers and are separated far more simply, with a separating funnel, using their different densities. Choosing distillation for immiscible liquids is a needless complication, and choosing a separating funnel for miscible ones does not work at all.
Distillation recovers both components. Unlike evaporation, which discards the solvent as vapour, distillation collects it. That is why distilled water is made this way, and it is the reason to choose distillation when the liquid is the valuable part.
Distillation separates liquids using their different boiling points. The mixture is heated, the component with the lower boiling point vaporises first, the vapour passes into a condenser where it cools back to a liquid, and the liquid is collected separately.
The rule for choosing, as the syllabus states it: use simple distillation when the boiling points differ by **more than about K, and fractional distillation when they differ by less than that.
Why fractional distillation needs a column. A fractionating column — packed with glass beads or shaped into a series of surfaces — sits between the flask and the condenser. The rising vapour condenses and evaporates many times over as it climbs, and each cycle enriches it further in the more volatile component. Liquids too close in boiling point for one vaporisation to separate are separated by many.
Worked example 1 — a wide gap.** Separating acetone from water. Acetone boils at K and water at K:
The gap is greater than K, so simple distillation is enough. The acetone distils over first and the water remains.
Converting to the Celsius scale for a sense of these numbers: and .
Worked example 2 — a narrow gap. Separating the gases of liquefied air. Nitrogen boils at K and oxygen at K:
The gap is less than K, so fractional distillation is required. Nitrogen, being more volatile, comes off first.
Worked example 3 — a mixture of many components. Crude petroleum contains many liquids with boiling points spread across a wide range and often close together. It is separated by fractional distillation in a tall column, drawing off different fractions at different heights — the lightest near the top where it is coolest, the heaviest near the bottom.
Both liquids must be miscible. Distillation separates liquids that mix with each other. Two liquids that do not mix — oil and water — form separate layers and are separated far more simply, with a separating funnel, using their different densities. Choosing distillation for immiscible liquids is a needless complication, and choosing a separating funnel for miscible ones does not work at all.
Distillation recovers both components. Unlike evaporation, which discards the solvent as vapour, distillation collects it. That is why distilled water is made this way, and it is the reason to choose distillation when the liquid is the valuable part.
How does paper chromatography separate a mixture, and how do you read it?
Because different components travel at different speeds along the paper, so they end up at different heights.
A spot of the mixture is placed near the bottom of a strip of filter paper, and the bottom edge is dipped into a solvent — the level kept below the spot. The solvent rises through the paper by capillary action and carries the components with it.
Each component is pulled two ways: it is attracted to the paper, which holds it back, and it dissolves in the moving solvent, which carries it forward. A component that dissolves readily and sticks to the paper weakly travels far; one that sticks strongly and dissolves poorly travels little. Since no two components have the same balance, they separate.
The result is a chromatogram, and the rule for reading it is simple: the number of distinct spots is the number of components.
Worked example 1 — counting components. A spot of black ink is run and gives three separate coloured spots. The ink therefore contains three dyes, whatever it looked like in the bottle.
Worked example 2 — identifying a component. Run an unknown sample beside a known pure substance on the same paper. If the unknown gives a single spot at the same height as the known, the two are behaving identically and are very probably the same substance.
Worked example 3 — a measured comparison. A pigment travels cm up the paper while the solvent front travels cm.
That ratio is characteristic of the substance in that solvent, so it can be compared with a known value. Measuring the ratio rather than the raw distance matters because the raw distance depends on how long the paper was left in the solvent, and the ratio does not.
Everyday evidence. Spill water on a page written with washable ink and the ink spreads into rings of different colours — an accidental chromatogram. Marker pen on wet cloth does the same.
Where it is used. Separating dyes in ink, pigments extracted from flowers and leaves, and detecting small amounts of a substance in blood or urine. Its great advantage is that it works with very small quantities — far too little for distillation or crystallisation to handle.
Chromatography needs the components to behave differently. If two components happened to have exactly the same attraction to the paper and the same solubility in the solvent, they would travel together and appear as one spot. So one spot means one component, or components that this particular solvent cannot separate — which is why a chemist trying to confirm purity runs the same sample in more than one solvent.
Keep the solvent level below the spot. If the starting spot is submerged, the mixture dissolves into the solvent reservoir instead of climbing the paper, and the experiment gives nothing. It is the single practical detail a diagram-based question most often tests.
A spot of the mixture is placed near the bottom of a strip of filter paper, and the bottom edge is dipped into a solvent — the level kept below the spot. The solvent rises through the paper by capillary action and carries the components with it.
Each component is pulled two ways: it is attracted to the paper, which holds it back, and it dissolves in the moving solvent, which carries it forward. A component that dissolves readily and sticks to the paper weakly travels far; one that sticks strongly and dissolves poorly travels little. Since no two components have the same balance, they separate.
The result is a chromatogram, and the rule for reading it is simple: the number of distinct spots is the number of components.
Worked example 1 — counting components. A spot of black ink is run and gives three separate coloured spots. The ink therefore contains three dyes, whatever it looked like in the bottle.
Worked example 2 — identifying a component. Run an unknown sample beside a known pure substance on the same paper. If the unknown gives a single spot at the same height as the known, the two are behaving identically and are very probably the same substance.
Worked example 3 — a measured comparison. A pigment travels cm up the paper while the solvent front travels cm.
That ratio is characteristic of the substance in that solvent, so it can be compared with a known value. Measuring the ratio rather than the raw distance matters because the raw distance depends on how long the paper was left in the solvent, and the ratio does not.
Everyday evidence. Spill water on a page written with washable ink and the ink spreads into rings of different colours — an accidental chromatogram. Marker pen on wet cloth does the same.
Where it is used. Separating dyes in ink, pigments extracted from flowers and leaves, and detecting small amounts of a substance in blood or urine. Its great advantage is that it works with very small quantities — far too little for distillation or crystallisation to handle.
Chromatography needs the components to behave differently. If two components happened to have exactly the same attraction to the paper and the same solubility in the solvent, they would travel together and appear as one spot. So one spot means one component, or components that this particular solvent cannot separate — which is why a chemist trying to confirm purity runs the same sample in more than one solvent.
Keep the solvent level below the spot. If the starting spot is submerged, the mixture dissolves into the solvent reservoir instead of climbing the paper, and the experiment gives nothing. It is the single practical detail a diagram-based question most often tests.
How do you choose the right separation technique?
Ask what kind of mixture it is, and which component you want to keep.
Work down this list:
- Insoluble solid in a liquid — filtration, or sedimentation and decantation if the solid settles. For very fine particles that pass through filter paper, centrifugation.
- Soluble solid in a liquid, wanting the solid — evaporation if it is heat-stable and purity does not matter; crystallisation if it decomposes on heating or must be pure.
- Soluble solid in a liquid, wanting the liquid — distillation.
- Two miscible liquids — distillation: simple if their boiling points differ by more than about K, fractional if less.
- Two immiscible liquids — separating funnel, using the density difference.
- A volatile solid mixed with a non-volatile one — sublimation. Ammonium chloride or camphor sublimes and can be collected, leaving salt or sand behind.
- Small amounts of coloured or closely similar components — chromatography.
Worked example 1 — the same mixture, two different answers. Separating common salt and water.
- If only the salt is wanted: evaporation or crystallisation
- If the water is also wanted: distillation, which recovers both
The mixture did not change. The question did.
Worked example 2 — a three-component mixture. Separating a mixture of common salt, ammonium chloride and sand. This needs three steps in the right order:
- Sublimation first, to remove the ammonium chloride as vapour and collect it
- Dissolve the residue in water and filter, which removes the sand as the residue
- Crystallise the filtrate to recover the salt
Doing these in the wrong order does not work — dissolving first would put both solids into solution together, and nothing would then separate them.
Worked example 3 — choosing between two close options. Separating a mixture of two miscible liquids boiling at K and K.
Under K, so fractional distillation. A gap of K looks large in everyday terms and is not large enough here, which is exactly why the rule is quoted as a number rather than left to judgement.
All of these are physical methods, and that is a real limit. They separate mixtures, whose components keep their own identities. They cannot separate a compound into its elements. No amount of distillation will give you hydrogen and oxygen from water, because water is not a mixture of the two — the elements are chemically combined, and only a chemical change will part them.
So identify the mixture before the method. Almost every wrong answer in this topic comes from naming a technique before asking whether the components are soluble, miscible, volatile or merely suspended. One line describing the mixture first makes the choice of technique almost automatic.
Work down this list:
- Insoluble solid in a liquid — filtration, or sedimentation and decantation if the solid settles. For very fine particles that pass through filter paper, centrifugation.
- Soluble solid in a liquid, wanting the solid — evaporation if it is heat-stable and purity does not matter; crystallisation if it decomposes on heating or must be pure.
- Soluble solid in a liquid, wanting the liquid — distillation.
- Two miscible liquids — distillation: simple if their boiling points differ by more than about K, fractional if less.
- Two immiscible liquids — separating funnel, using the density difference.
- A volatile solid mixed with a non-volatile one — sublimation. Ammonium chloride or camphor sublimes and can be collected, leaving salt or sand behind.
- Small amounts of coloured or closely similar components — chromatography.
Worked example 1 — the same mixture, two different answers. Separating common salt and water.
- If only the salt is wanted: evaporation or crystallisation
- If the water is also wanted: distillation, which recovers both
The mixture did not change. The question did.
Worked example 2 — a three-component mixture. Separating a mixture of common salt, ammonium chloride and sand. This needs three steps in the right order:
- Sublimation first, to remove the ammonium chloride as vapour and collect it
- Dissolve the residue in water and filter, which removes the sand as the residue
- Crystallise the filtrate to recover the salt
Doing these in the wrong order does not work — dissolving first would put both solids into solution together, and nothing would then separate them.
Worked example 3 — choosing between two close options. Separating a mixture of two miscible liquids boiling at K and K.
Under K, so fractional distillation. A gap of K looks large in everyday terms and is not large enough here, which is exactly why the rule is quoted as a number rather than left to judgement.
All of these are physical methods, and that is a real limit. They separate mixtures, whose components keep their own identities. They cannot separate a compound into its elements. No amount of distillation will give you hydrogen and oxygen from water, because water is not a mixture of the two — the elements are chemically combined, and only a chemical change will part them.
So identify the mixture before the method. Almost every wrong answer in this topic comes from naming a technique before asking whether the components are soluble, miscible, volatile or merely suspended. One line describing the mixture first makes the choice of technique almost automatic.
Exam tip
Exam tip: say why the technique works, not just its name
Name the technique and the property it exploits. *Fractional distillation, because the boiling points differ by less than K* earns both marks; the name alone earns one.
Crystallisation beats evaporation for three reasons: the solid may decompose on heating, dissolved impurities stay in the mother liquor, and the crystals are well-formed. Cool slowly for large pure crystals.
The yield of a crystallisation is the difference between two solubilities: g.
Simple distillation above about K difference, fractional below it. Acetone and water differ by K (simple); nitrogen and oxygen by K (fractional).
Distillation is for miscible liquids; a separating funnel for immiscible ones. Never swap those.
Distillation recovers both components; evaporation discards the solvent.
For chromatography, keep the solvent level below the spot, and remember the number of spots is the number of components. The ratio is what you compare, not the raw distance.
For a multi-step separation, get the order right — sublimation before dissolving, not after.
Sublimation removes a volatile solid such as ammonium chloride or camphor; centrifugation handles particles too fine to filter.
And remember these are physical methods for mixtures only — they cannot split a compound into its elements.
Crystallisation beats evaporation for three reasons: the solid may decompose on heating, dissolved impurities stay in the mother liquor, and the crystals are well-formed. Cool slowly for large pure crystals.
The yield of a crystallisation is the difference between two solubilities: g.
Simple distillation above about K difference, fractional below it. Acetone and water differ by K (simple); nitrogen and oxygen by K (fractional).
Distillation is for miscible liquids; a separating funnel for immiscible ones. Never swap those.
Distillation recovers both components; evaporation discards the solvent.
For chromatography, keep the solvent level below the spot, and remember the number of spots is the number of components. The ratio is what you compare, not the raw distance.
For a multi-step separation, get the order right — sublimation before dissolving, not after.
Sublimation removes a volatile solid such as ammonium chloride or camphor; centrifugation handles particles too fine to filter.
And remember these are physical methods for mixtures only — they cannot split a compound into its elements.
Did you know
Why a refinery is a single very tall column
Petroleum arriving at a refinery is a mixture of a great many liquids, and most of them boil at temperatures uncomfortably close together. Separating them one pair at a time would take an impossible number of distillations.
A tall fractionating column does the whole job at once, and it works by turning the K rule to its advantage many times over.
The crude oil is heated and fed in near the bottom, where it is hottest. Vapour rises, and as it climbs the column it cools. At each level, some of the vapour condenses back into liquid, gives up its less volatile components, and the remainder evaporates and continues upward slightly richer in the lighter substances than before.
That single step — condense, enrich, evaporate — is what a laboratory fractionating column achieves with its glass beads. A refinery column simply stacks the step dozens of times up its height, so components separated by only a few kelvin of boiling point end up at different levels.
The outlets are then placed to match. The lightest fractions are drawn off near the top, where the column is coolest; heavier fractions such as diesel come off in the middle; and the heaviest residues, which never vaporised at all, are taken from the bottom.
The same arrangement separates the gases of the air. Air is cooled until it liquefies and then fractionally distilled, and because nitrogen boils at K and oxygen at K — a gap of only K — nitrogen comes off first while oxygen collects lower down.
So the tall column is not an engineering flourish. Its height is the number of separations it performs, which is why a mixture that no single distillation could separate yields to one piece of apparatus.
A tall fractionating column does the whole job at once, and it works by turning the K rule to its advantage many times over.
The crude oil is heated and fed in near the bottom, where it is hottest. Vapour rises, and as it climbs the column it cools. At each level, some of the vapour condenses back into liquid, gives up its less volatile components, and the remainder evaporates and continues upward slightly richer in the lighter substances than before.
That single step — condense, enrich, evaporate — is what a laboratory fractionating column achieves with its glass beads. A refinery column simply stacks the step dozens of times up its height, so components separated by only a few kelvin of boiling point end up at different levels.
The outlets are then placed to match. The lightest fractions are drawn off near the top, where the column is coolest; heavier fractions such as diesel come off in the middle; and the heaviest residues, which never vaporised at all, are taken from the bottom.
The same arrangement separates the gases of the air. Air is cooled until it liquefies and then fractionally distilled, and because nitrogen boils at K and oxygen at K — a gap of only K — nitrogen comes off first while oxygen collects lower down.
So the tall column is not an engineering flourish. Its height is the number of separations it performs, which is why a mixture that no single distillation could separate yields to one piece of apparatus.
Exam relevance
Why do JEE and NEET keep returning to purification techniques?
Because a chemist's first problem with any substance is getting it pure, and these four techniques are the ones still used to do it.
This page is the foundation for the Class 11 Chemistry chapter Organic Chemistry: Some Basic Principles and Techniques, examined in both JEE Main and NEET. That chapter has a whole section on purification of organic compounds, and it covers exactly the methods here — crystallisation, simple and fractional distillation, and chromatography — with two additions: steam distillation and distillation under reduced pressure, for compounds that decompose before they boil. The Class 9 reason for preferring crystallisation over evaporation is precisely the reason those two extra methods exist.
Chromatography is developed much further there. The ratio calculated above becomes the retention factor, written , and it is defined exactly as it was used on this page — distance moved by the substance divided by distance moved by the solvent. JEE Main questions on values are common, and the Class 9 point that the ratio is characteristic while the raw distance is not is what makes useful at all.
Fractional distillation reappears in Class 11 Hydrocarbons for the refining of petroleum and in Class 12 The p-Block Elements for the fractional distillation of liquid air.
Solubility and crystallisation connect to Class 12 Solutions, where the temperature dependence used here is treated quantitatively.
What the questions look like. Match-the-column items pairing a mixture with its separation technique are the most common form — and the pairs that catch students out are two miscible liquids with close boiling points and a volatile solid in a non-volatile one. Assertion-reason questions favour the choice between simple and fractional distillation, and the claim that crystallisation gives a purer product than evaporation. Diagram-based items ask you to identify apparatus or to spot an error in a set-up, and the submerged chromatography spot is a standard error to find.
How board and competitive emphasis differ. A board paper asks you to describe crystallisation with a diagram, or to give two advantages of crystallisation over evaporation, and awards marks for the labelled apparatus. A competitive paper gives you a mixture and asks only which technique applies, sometimes with the boiling points supplied so that the K decision is the entire question — so the decision rule matters more than the description of the apparatus.
The single trap that costs the most marks. Choosing simple distillation for liquids whose boiling points are close. A gap of K feels large and is under the threshold, so the answer is fractional — and a question that bothers to give you two boiling points is almost always asking you to subtract them and compare with K.
A second trap worth naming. Using a separating funnel for miscible liquids, or distillation for immiscible ones. Establish first whether the two liquids mix; that single word decides the method, and no property of the boiling points is relevant until it has been settled.
This page is the foundation for the Class 11 Chemistry chapter Organic Chemistry: Some Basic Principles and Techniques, examined in both JEE Main and NEET. That chapter has a whole section on purification of organic compounds, and it covers exactly the methods here — crystallisation, simple and fractional distillation, and chromatography — with two additions: steam distillation and distillation under reduced pressure, for compounds that decompose before they boil. The Class 9 reason for preferring crystallisation over evaporation is precisely the reason those two extra methods exist.
Chromatography is developed much further there. The ratio calculated above becomes the retention factor, written , and it is defined exactly as it was used on this page — distance moved by the substance divided by distance moved by the solvent. JEE Main questions on values are common, and the Class 9 point that the ratio is characteristic while the raw distance is not is what makes useful at all.
Fractional distillation reappears in Class 11 Hydrocarbons for the refining of petroleum and in Class 12 The p-Block Elements for the fractional distillation of liquid air.
Solubility and crystallisation connect to Class 12 Solutions, where the temperature dependence used here is treated quantitatively.
What the questions look like. Match-the-column items pairing a mixture with its separation technique are the most common form — and the pairs that catch students out are two miscible liquids with close boiling points and a volatile solid in a non-volatile one. Assertion-reason questions favour the choice between simple and fractional distillation, and the claim that crystallisation gives a purer product than evaporation. Diagram-based items ask you to identify apparatus or to spot an error in a set-up, and the submerged chromatography spot is a standard error to find.
How board and competitive emphasis differ. A board paper asks you to describe crystallisation with a diagram, or to give two advantages of crystallisation over evaporation, and awards marks for the labelled apparatus. A competitive paper gives you a mixture and asks only which technique applies, sometimes with the boiling points supplied so that the K decision is the entire question — so the decision rule matters more than the description of the apparatus.
The single trap that costs the most marks. Choosing simple distillation for liquids whose boiling points are close. A gap of K feels large and is under the threshold, so the answer is fractional — and a question that bothers to give you two boiling points is almost always asking you to subtract them and compare with K.
A second trap worth naming. Using a separating funnel for miscible liquids, or distillation for immiscible ones. Establish first whether the two liquids mix; that single word decides the method, and no property of the boiling points is relevant until it has been settled.
Key takeaways
Separation techniques: quick revision
- Crystallisation beats evaporation because some solids decompose on strong heating, dissolved impurities remain in the mother liquor, and the crystals are well-formed.
- Method: dissolve in minimum hot water, filter, concentrate, cool slowly, filter the crystals, discard the mother liquor. Slow cooling gives large pure crystals.
- Yield is the difference of two solubilities: from g per g at to g at gives ** g.
- Salt from seawater is made by evaporation**; mishri crystals by crystallisation. Evaporation recovers more but less pure.
- Distillation separates miscible liquids by boiling point, and recovers both components.
- Simple distillation when the boiling points differ by **more than about K; fractional** when less.
- Acetone ( K) and water ( K) differ by K, so simple. Nitrogen ( K) and oxygen ( K) differ by K, so fractional. Liquids at K and K differ by K, so fractional.
- A fractionating column repeats condensation and evaporation many times, which is why a tall refinery column separates many fractions at once.
- Immiscible liquids need a separating funnel, using density — not distillation.
- Paper chromatography separates by different attraction to the paper and different solubility in the moving solvent. The number of spots is the number of components.
- Black ink giving three spots contains three dyes. A sample matching a known substance's spot height is probably that substance.
- The ratio is characteristic; the raw distance is not, since it depends on the run time.
- Keep the solvent level below the starting spot, and note that chromatography works with very small quantities.
- Choosing a technique: filtration or sedimentation for an insoluble solid, centrifugation for very fine particles, evaporation or crystallisation for a dissolved solid you want, distillation for the liquid you want, a separating funnel for immiscible liquids, sublimation for a volatile solid, chromatography for small or closely similar components.
- Salt and water: evaporation if only the salt is wanted, distillation if the water is wanted too.
- Salt, ammonium chloride and sand need three steps in order: sublimation, then dissolve and filter, then crystallise.
- These are physical methods for mixtures only — no distillation splits water into hydrogen and oxygen.
Give yourself five mixtures, describe each in one line — soluble, miscible, volatile or suspended — and only then name the technique; the description is what makes the choice obvious.
- Method: dissolve in minimum hot water, filter, concentrate, cool slowly, filter the crystals, discard the mother liquor. Slow cooling gives large pure crystals.
- Yield is the difference of two solubilities: from g per g at to g at gives ** g.
- Salt from seawater is made by evaporation**; mishri crystals by crystallisation. Evaporation recovers more but less pure.
- Distillation separates miscible liquids by boiling point, and recovers both components.
- Simple distillation when the boiling points differ by **more than about K; fractional** when less.
- Acetone ( K) and water ( K) differ by K, so simple. Nitrogen ( K) and oxygen ( K) differ by K, so fractional. Liquids at K and K differ by K, so fractional.
- A fractionating column repeats condensation and evaporation many times, which is why a tall refinery column separates many fractions at once.
- Immiscible liquids need a separating funnel, using density — not distillation.
- Paper chromatography separates by different attraction to the paper and different solubility in the moving solvent. The number of spots is the number of components.
- Black ink giving three spots contains three dyes. A sample matching a known substance's spot height is probably that substance.
- The ratio is characteristic; the raw distance is not, since it depends on the run time.
- Keep the solvent level below the starting spot, and note that chromatography works with very small quantities.
- Choosing a technique: filtration or sedimentation for an insoluble solid, centrifugation for very fine particles, evaporation or crystallisation for a dissolved solid you want, distillation for the liquid you want, a separating funnel for immiscible liquids, sublimation for a volatile solid, chromatography for small or closely similar components.
- Salt and water: evaporation if only the salt is wanted, distillation if the water is wanted too.
- Salt, ammonium chloride and sand need three steps in order: sublimation, then dissolve and filter, then crystallise.
- These are physical methods for mixtures only — no distillation splits water into hydrogen and oxygen.
Give yourself five mixtures, describe each in one line — soluble, miscible, volatile or suspended — and only then name the technique; the description is what makes the choice obvious.