Purity and separating mixtures
What you'll learn
- What pure means in chemistry, and why it is different from everyday use.
- How melting point data and chromatography can show whether a substance is pure.
- How to calculate relative formula mass and empirical formulae.
- How to choose between filtration, crystallisation, distillation and chromatography.
Pure substances and mixtures
In everyday language, “pure” often means natural, clean, or not tampered with. In chemistry, it has a stricter meaning.
Pure substance
A pure substance contains only one chemical substance: either one element or one compound. It is not mixed with anything else.
A mixture contains two or more substances that are not chemically bonded together. The substances in a mixture can usually be separated by physical methods, such as filtering or distilling.
A solution is a mixture where a solute has dissolved in a solvent. For example, salt water is a mixture: salt is the solute and water is the solvent.
Dissolved does not mean pure
If salt dissolves in water, the salt has not disappeared and the solution is not pure. Salt water is still a mixture.
Purity
Purity describes how much of a sample is the substance you want, and how much is unwanted material called impurities.
Using melting point to test purity
The melting point of a substance is the temperature at which it changes from solid to liquid.
A pure substance has a sharp melting point: it melts at one temperature, or over a very small range. An impure substance usually melts over a wider range and often at a lower temperature than the pure substance.
Melting point test
Pure solids melt sharply. Impure solids usually melt over a range of temperatures.
Using a melting point range
A student tests a solid that should be pure benzoic acid. Pure benzoic acid melts at 122 °C. The sample melts from 118 °C to 121 °C.
- Compare the measured range with the data value: the sample starts melting at 118 °C, below 122 °C.
- Check whether it is sharp: melting over 3 °C is a range, not a sharp melting point.
- Conclude that the sample is impure, because impurities have lowered and broadened the melting point.
Formulae and relative formula mass
Before separating and identifying substances, you also need to be comfortable with formulae.
The relative atomic mass, ArA_rAr, compares the mass of an atom with carbon-12. The relative formula mass, MrM_rMr, is found by adding the relative atomic masses of all the atoms in a formula.
Relative formula mass
The relative formula mass, MrM_rMr, is the sum of the ArA_rAr values of all atoms in a substance’s formula. It has no units.
For example, in MgO there is one magnesium atom and one oxygen atom. If Mg has Ar=24A_r = 24Ar=24 and O has Ar=16A_r = 16Ar=16, then MgO has Mr=40M_r = 40Mr=40.
In a balanced equation, remember that the large number in front of a formula multiplies the whole formula.
Calculating formula masses in an equation
For the reaction 2Mg(s) + O₂(g) → 2MgO(s), use Mg = 24 and O = 16.
- Calculate each species separately: Mg has Mr=24M_r = 24Mr=24, O₂ has Mr=2×16=32M_r = 2 \times 16 = 32Mr=2×16=32, and MgO has Mr=24+16=40M_r = 24 + 16 = 40Mr=24+16=40.
- Apply the balancing numbers: 2Mg gives 2×24=482 \times 24 = 482×24=48, and 2MgO gives 2×40=802 \times 40 = 802×40=80.
- Compare total reactants and products: reactants give 48+32=8048 + 32 = 8048+32=80, matching the products at 80.
Empirical formulae
The empirical formula gives the simplest whole-number ratio of atoms of each element in a compound.
Empirical formula
An empirical formula is the simplest whole-number ratio of atoms of each element in a compound.
For example, glucose has formula C₆H₁₂O₆. The ratio 6:12:6 can be divided by 6, so its empirical formula is CH₂O.
Finding an empirical formula from atom numbers
A model of a compound contains 4 carbon atoms, 8 hydrogen atoms and 4 oxygen atoms.
- Write the ratio of atoms: C:H:O = 4:8:4.
- Divide all parts by the largest common factor, 4, giving 1:2:1.
- Write the empirical formula using this simplest ratio: CH₂O.
Empirical formula is not always the full formula
C₆H₁₂O₆ and CH₂O are not the same formula. CH₂O only shows the simplest ratio, not the actual number of atoms in one molecule.
Useful mixtures: formulations and alloys
Not all mixtures are “bad”. Many useful materials are deliberately made as mixtures.
Formulation
A formulation is a useful mixture designed as a product with a particular purpose. Its substances are mixed in carefully chosen amounts.
Examples include medicines, fuels, paints, cleaning products and fertilisers. The exact recipe affects properties such as colour, drying time, strength, shelf life or dose.
An alloy is a mixture of a metal with one or more other elements. For example, steel is mainly iron with carbon and sometimes other elements. Alloys are often harder or more corrosion-resistant than the pure metal.
Separating mixtures
Separation techniques work because substances in a mixture have different physical properties, such as particle size, solubility, boiling point, or attraction to a surface.
No new substances
Separating a mixture is a physical process. It does not involve a chemical reaction and does not make new substances.
Filtration
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper.
- The solid left behind is the residue.
- The liquid that passes through is the filtrate.
This works because solid particles are too large to pass through the tiny holes in the filter paper, but the liquid can pass through.
Crystallisation
Crystallisation separates a dissolved solid from a solution by making solid crystals form.
A common method is:
- Gently heat the solution to evaporate some solvent.
- Stop heating when the solution is nearly saturated.
- Leave it to cool so crystals form.
- Filter and dry the crystals.
Crystallisation is better than heating to dryness if the solid might decompose when strongly heated.
Simple and fractional distillation
Distillation separates substances using different boiling points. The liquid that evaporates and then condenses is called the distillate.
Simple distillation is used to separate a solvent from a solution, or liquids with very different boiling points. Fractional distillation is used for liquids with closer boiling points, because the fractionating column allows repeated evaporation and condensation.
Choosing a separation method
A mixture contains sand, salt and water. You want to obtain dry salt crystals.
- Use filtration first, because sand is insoluble and can be trapped as the residue while salt solution passes through as the filtrate.
- Use crystallisation on the filtrate, because the salt is dissolved in water and needs the solvent to be partly evaporated.
- Let the concentrated solution cool, then filter and dry the crystals to obtain the salt.
Filtering a solution will not remove dissolved salt
Filter paper removes insoluble particles. Dissolved salt particles pass through with the water, so filtration alone cannot separate salt from salt water.
Chromatography
Chromatography separates substances based on how they move between two phases.
Phases in chromatography
The stationary phase stays still. The mobile phase moves through or across it, carrying substances with it.
In paper chromatography, the stationary phase is the paper and the mobile phase is the solvent. In thin layer chromatography, often shortened to TLC, the stationary phase is a thin layer of solid such as silica on a plate.
A substance moves further if it is more soluble in the mobile phase or less strongly attracted to the stationary phase. Different substances move different distances, so they separate into spots.
The diagram shows the setup and how to measure distances for an RfR_fRf value.
A chromatogram is the finished pattern of spots or peaks. For paper chromatography and TLC:
- Draw the baseline in pencil, because ink would dissolve and interfere.
- Keep the sample spot above the solvent level.
- Mark the solvent front before it evaporates.
- Use a locating agent or UV light to see colourless spots.
The solvent may be aqueous if it is water-based, or non-aqueous if it uses an organic solvent. The best solvent is one that dissolves the substances and gives good separation.
Choosing the solvent
If the spots do not move, the substances may not dissolve well in that solvent. If all spots move together near the solvent front, the solvent may be carrying them too strongly.
Calculating and using RfR_fRf values
The RfR_fRf value compares how far a substance moves with how far the solvent front moves:
Rf=distance moved by substancedistance moved by solvent frontR_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}Rf=distance moved by solvent frontdistance moved by substanceMeasure from the baseline to the centre of the spot. RfR_fRf values have no units and are usually between 0 and 1.
Calculating an Rf value
On a chromatogram, a spot moves 3.2 cm from the baseline. The solvent front moves 8.0 cm from the baseline.
- Substitute into the formula: Rf=3.28.0R_f = \frac{3.2}{8.0}Rf=8.03.2.
- Calculate the ratio: Rf=0.40R_f = 0.40Rf=0.40.
- Leave off units, because both distances were measured in cm and cancel out.
Rf values depend on the conditions
Only compare RfR_fRf values if the same stationary phase, solvent and temperature were used. Changing the method can change the RfR_fRf value.
Using chromatography to check purity
A pure substance usually gives:
- one spot in paper chromatography or TLC
- one peak in gas chromatography
A mixture usually gives several spots or peaks.
Gas chromatography is used for gases or substances that can easily become gases. The mobile phase is a gas, and the stationary phase is inside a long column. Substances are identified by comparing their retention times, which are the times taken to pass through the column.
Spot count and purity
One spot suggests a pure substance, but only for substances that the method can detect. Several spots show a mixture.
Picking the best purification method
Use the information about the substances:
- Insoluble solid + liquid → filtration.
- Dissolved solid from solution → crystallisation.
- Solvent from solution → simple distillation.
- Liquids with very different boiling points → simple distillation.
- Liquids with similar boiling points → fractional distillation.
- Small amounts of soluble substances, such as dyes → paper chromatography or TLC.
- Volatile substances or gases → gas chromatography.
In the exam
- Identify the physical property being used: particle size, solubility, boiling point, or distribution between phases.
- For purity questions, look for a sharp melting point, one spot, or one peak; ranges and multiple spots suggest impurities.
- In RfR_fRf calculations, measure from the baseline to the centre of the spot and to the solvent front.
- When calculating MrM_rMr in equations, add atoms inside the formula first, then multiply by the balancing number.
Check yourself
- Why is “pure orange juice” not chemically pure?
- Which method would you use to separate ethanol and water, and why?
- A chromatogram has three spots from one sample. What does that suggest about the sample?
