What you'll learn
- What “pure” means in chemistry, and how that differs from everyday language.
- How melting point data can show whether a substance is pure or impure.
- Which separation method to choose: filtration, crystallisation, distillation, fractional distillation or chromatography.
- How potable water is made, and why water for analysis must be very pure.
Pure substances and mixtures
In everyday language, pure often means “natural”, “clean” or “nothing added”. For example, “pure orange juice” sounds pure — but in chemistry it is not pure because it contains many substances: water, sugars, acids, colourings and vitamins.
Pure substance
In chemistry, a pure substance contains only one substance: either one element or one compound. It has a fixed composition and characteristic melting and boiling points.
Mixture
A mixture contains two or more substances that are not chemically bonded together. The substances keep their own properties and can usually be separated by physical methods.
A mixture does not have a fixed composition. Salt water could contain a little salt or a lot of salt; it is still salt water. This matters because separation methods work by using differences in physical properties, such as boiling point, solubility, particle size or how far a substance moves in a solvent.
The big idea
You separate mixtures by choosing a physical property that is different for the substances in the mixture.
Using melting point to test purity
A melting point is the temperature at which a solid changes into a liquid.
Pure substances usually have a sharp melting point: they melt at one temperature, or over a very small range in a school lab. Mixtures melt over a range of temperatures, often lower and wider than the melting point of the pure substance.
Interpreting melting point data
A known pure substance melts at 80 °C. Sample A melts from 79.5–80.5 °C. Sample B melts from 72–78 °C. Which sample is more likely to be pure?
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Compare each sample with the expected melting point. Sample A melts very close to 80 °C, while Sample B melts well below 80 °C.
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Compare the melting ranges. Sample A has a narrow range of 1 °C, while Sample B has a wider range of 6 °C.
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Decide using both pieces of evidence. Sample A is more likely to be pure; Sample B is likely to be impure or a mixture.
Useful words before the techniques
A solvent is a liquid that dissolves another substance. The substance being dissolved is the solute. The mixture formed is a solution.
A substance is soluble if it dissolves in a solvent. It is insoluble if it does not dissolve.
For example, in salt water, water is the solvent and salt is the solute. Sand is insoluble in water, so sand and water do not form a solution.
Filtration
Filtration separates an insoluble solid from a liquid.
The mixture is poured through filter paper in a funnel. The insoluble solid stays on the filter paper as the residue. The liquid that passes through is the filtrate.
Use filtration for mixtures such as sand and water, or to remove insoluble impurities from a solution.
Crystallisation
Crystallisation is used to obtain a soluble solid from a solution.
You gently heat the solution to evaporate some solvent. When the solution becomes concentrated, you leave it to cool. Crystals of the solute form because less solute can stay dissolved in the cooler solution. The crystals can then be filtered off and dried.
Crystallisation or evaporation?
If you need dry crystals, heat gently and allow crystals to form as the solution cools. Do not simply boil everything to dryness unless the question says to, because some solids may decompose when strongly heated.
Simple and fractional distillation
Distillation separates substances using differences in boiling point.
In simple distillation, a solution is heated so that the solvent boils and becomes vapour. The vapour enters a condenser, cools, and turns back into liquid. The liquid collected is the distillate. The higher boiling solute stays behind.
Simple distillation is useful for obtaining pure water from salt water, or separating a solvent from ink.
In fractional distillation, a mixture of miscible liquids is separated. Miscible liquids are liquids that mix together, such as ethanol and water. The mixture is heated and vapours pass through a fractionating column. Substances with lower boiling points move through the column and are collected first.
Using filtration for dissolved substances
Filter paper cannot remove dissolved salt or sugar from water. Dissolved particles are small enough to pass through the filter paper with the water.
Choosing a separation method
A mixture contains sand, salt and water. You want to obtain dry salt crystals. What should you do?
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Identify the insoluble substance. Sand is insoluble in water, so use filtration first to remove the sand as the residue.
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Identify what remains in the filtrate. The filtrate is salt solution, because the salt is dissolved in the water.
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Choose the method for the wanted product. Since you want dry salt crystals, use crystallisation on the salt solution: evaporate some water, cool the concentrated solution, then filter and dry the crystals.
Paper chromatography
Paper chromatography separates mixtures of soluble substances, such as dyes in ink or food colourings.
The solvent moves through the paper, carrying dissolved substances with it. Different substances move at different rates, so they separate into spots.
Mobile and stationary phases
The mobile phase is the moving solvent. The stationary phase is the paper, or the material held in the paper, which stays still while the solvent moves through it.
A substance moves further up the paper if it is more soluble in the solvent and less strongly attracted to the stationary phase. A substance moves less far if it is less soluble in the solvent or more strongly attracted to the paper.
How to carry out paper chromatography
- Draw a pencil baseline near the bottom of the chromatography paper.
- Put small spots of the ink or known substances on the baseline.
- Place the paper in a small amount of solvent, with the solvent level below the baseline.
- Let the solvent rise up the paper.
- Remove the paper before the solvent reaches the top.
- Mark the solvent front in pencil and allow the paper to dry.
Use pencil for the baseline because pencil does not dissolve in the solvent. Pen ink could dissolve and interfere with the results.
Interpreting a chromatogram
A chromatogram is the pattern of spots produced by chromatography.
A pure substance should produce one spot. An impure substance or mixture produces more than one spot.
You can identify a substance by comparing its spots with known substances. If two spots travel the same distance in the same solvent and on the same paper, they may be the same substance.
Calculating Rf values
An RfR_fRf value compares how far a substance travelled with how far the solvent travelled:
Rf=distance travelled by substancedistance travelled by solvent frontR_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}Rf=distance travelled by solvent frontdistance travelled by substanceThe distances must both be measured from the pencil baseline. RfR_fRf values have no units because the units cancel.
Calculating an Rf value and identifying a dye
A dye spot travels 4.2 cm from the baseline. The solvent front travels 7.0 cm from the baseline. A known dye has an RfR_fRf value of 0.60 under the same conditions. Is the dye likely to be the known dye?
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Use the correct distances from the baseline: dye spot = 4.2 cm and solvent front = 7.0 cm.
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Substitute into the formula:
- Compare with the known value. The calculated value is 0.60, so the dye is likely to be the known dye, as long as the same solvent and paper were used.
Rf values depend on conditions
Only compare RfR_fRf values if the chromatography was done using the same solvent and stationary phase. Changing the solvent can change how far the substances travel.
Core practical: investigating inks
For this core practical, you need to understand how simple distillation and paper chromatography can be used to investigate ink.
In simple distillation, a sample of ink can be heated. The solvent evaporates, then condenses and is collected as a distillate. The coloured dyes have much higher boiling points and remain in the flask.
In paper chromatography, spots of ink are placed on a pencil baseline. The solvent separates the dyes in the ink, producing a chromatogram. You can compare unknown inks with known inks, or calculate RfR_fRf values to help identify the dyes.
Practical detail that earns marks
In chromatography, the solvent level must start below the pencil baseline. If the ink spots are under the solvent, they may dissolve into the solvent instead of travelling up the paper.
Making water potable
Potable water is water that is safe to drink. It is not the same as chemically pure water, because potable water often contains small amounts of dissolved minerals.
Potable water
Potable water is water that has low enough levels of dissolved substances and microorganisms to be safe to drink.
Waste water and ground water can be made potable by:
- Sedimentation: large insoluble solids settle at the bottom.
- Filtration: smaller suspended insoluble solids are removed.
- Chlorination: chlorine is added to kill microorganisms.
Sea water contains dissolved salts. Filtration will not remove these salts, so sea water can be made potable by distillation. The water evaporates, leaving salts behind, then condenses to form fresh water.
Water used in chemical analysis must not contain dissolved salts. This is usually distilled or deionised water. Dissolved ions could interfere with tests and give misleading results.
Choosing a water treatment method
A muddy river water sample contains sand, clay particles and microorganisms, but its dissolved salt levels are safe. How could it be made potable?
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Deal with the largest insoluble solids first. Use sedimentation so sand and larger particles settle out.
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Remove remaining suspended solids. Use filtration to remove smaller insoluble particles such as clay.
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Make the water safe from microorganisms. Use chlorination to kill bacteria and other microorganisms. Distillation is not needed here because the dissolved salt levels are already safe.
In the exam
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Link each separation method to the property it uses: filtration uses particle size, crystallisation uses solubility, distillation uses boiling point, and chromatography uses different movement through a solvent and stationary phase.
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For melting point questions, mention both ideas: a pure substance has a sharp melting point, while a mixture melts over a range.
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For chromatography, always measure from the baseline, use the centre of each spot, and only compare RfR_fRf values when the same solvent and paper were used.
Check yourself
- Why is “pure orange juice” not a pure substance in chemistry?
- A spot travels 3.0 cm and the solvent front travels 6.0 cm. What is the RfR_fRf value?
- Which method would you use to separate sand from salt solution, and which method would you use next to obtain salt crystals?
