● A mixture:
o Consists of 2 or more elements or compounds not chemically combined together
o Chemical properties of each substance in the mixture are unchanged ●
o Chemical properties of each substance in the mixture are unchanged ●
A pure substance = a single element or compound, not mixed with any other substance
● In everyday language, a pure substance = substance that has had nothing added to it, so it is unadulterated and in its natural state, e.g. pure milk
Diagram showing how carbon (blue) and oxygen (orange) can combine chemically to form the compound CO2 but also form a mixture of C atoms and O2 molecules
2.6 Interpret melting point data to distinguish between pure substances, which have a sharp melting point and mixtures, which melt over a range of temperatures
● Pure substances melt and boil at specific/exact temperatures, mixtures do not:
o This means melting and boiling points data can be used to distinguish pure substances from mixtures (which melt over a range of temperatures due to them consisting of 2 or more elements or compounds)
o This means melting and boiling points data can be used to distinguish pure substances from mixtures (which melt over a range of temperatures due to them consisting of 2 or more elements or compounds)
2.7 Explain the experimental techniques for separation of mixtures by: simple distillation, fractional distillation, filtration, crystallisation, and paper chromatography
● Simple distillation:
o Used to separate a pure liquid from a mixture of liquids
▪ Works when the liquids have different boiling points
▪ Commonly used to separate ethanol from water ▪ (Taking the example of ethanol…) ethanol has a lower bp than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid.
▪ Sequence of events in distillation is as follows: heating -> evaporating -> cooling -> condensing ●
Fractional distillation:
watch presentation
https://drive.google.com/drive/u/0/folders/1JGYR9pZ-FGlxEGWu9QymySJHosMl4a5O
o The oil is heated in the fractionating column and the oil evaporates and condenses at a number of different temperatures.
o The many hydrocarbons in crude oil can be separated into fractions each of which contains molecules with a similar number of carbon atoms
o The fractionating column works continuously, heated crude oil is piped in at the bottom. The vaporised oil rises up the column and the various fractions are constantly tapped off at the different levels where they condense.
o The fractions can be processed to produce fuels and feedstock for the petrochemical industry.
● Filtration:
o If you have produced e.g. a precipitate (which is an insoluble salt), you would want to separate the salt/precipitate from the salt solution.
▪ You would do this by filtering the solution, leaving behind the precipitate on the filter paper
● Crystallisation:
o If you were to have produced a soluble salt and you wanted to separate this salt from the solution that it was dissolved in
▪ You would first warm the solution in an open container, allowing the solvent to evaporate, leaving a saturated solution
▪ Allow this solution to cool
▪ The solid will come out of the solution and crystals will start to grow, these can then be collected and allowed to dry
● Paper chromatography:
o Chromatography…
▪ Used to separate mixtures and give information to help identify substances
▪ Involves a stationary phase and a mobile phase
▪ Separation depends on the distribution of substances between the phases
o Rf value = distance moved by substance / distance moved by solvent ( / represents a dividing sign)
▪ Different compounds have different Rf values in different solvents, which can be used to help identify the compounds
▪ Compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents
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Diagram showing the distillation of a mixture of salt and water
|
o Used to separate a pure liquid from a mixture of liquids
▪ Works when the liquids have different boiling points
▪ Commonly used to separate ethanol from water ▪ (Taking the example of ethanol…) ethanol has a lower bp than water so it evaporates first. The ethanol vapour is then cooled and condensed inside the condenser to form a pure liquid.
▪ Sequence of events in distillation is as follows: heating -> evaporating -> cooling -> condensing ●
Fractional distillation:
watch presentation
https://drive.google.com/drive/u/0/folders/1JGYR9pZ-FGlxEGWu9QymySJHosMl4a5O
o The oil is heated in the fractionating column and the oil evaporates and condenses at a number of different temperatures.
o The many hydrocarbons in crude oil can be separated into fractions each of which contains molecules with a similar number of carbon atoms
o The fractionating column works continuously, heated crude oil is piped in at the bottom. The vaporised oil rises up the column and the various fractions are constantly tapped off at the different levels where they condense.
o The fractions can be processed to produce fuels and feedstock for the petrochemical industry.
● Filtration:
o If you have produced e.g. a precipitate (which is an insoluble salt), you would want to separate the salt/precipitate from the salt solution.
▪ You would do this by filtering the solution, leaving behind the precipitate on the filter paper
● Crystallisation:
o If you were to have produced a soluble salt and you wanted to separate this salt from the solution that it was dissolved in
▪ You would first warm the solution in an open container, allowing the solvent to evaporate, leaving a saturated solution
▪ Allow this solution to cool
▪ The solid will come out of the solution and crystals will start to grow, these can then be collected and allowed to dry
● Paper chromatography:
o Chromatography…
▪ Used to separate mixtures and give information to help identify substances
▪ Involves a stationary phase and a mobile phase
▪ Separation depends on the distribution of substances between the phases
o Rf value = distance moved by substance / distance moved by solvent ( / represents a dividing sign)
▪ Different compounds have different Rf values in different solvents, which can be used to help identify the compounds
▪ Compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents
2.8 Describe an appropriate experiment technique to separate a mixture, knowing the properties of the components of the mixture
● you need to identify which types of substance you have in the mixture and so which technique is most appropriate (from 2.7)
2.9 Describe paper chromatography as the separation of mixtures of soluble substances by running a solvent (mobile phase) through the mixture on the paper (the paper contains the stationary phase), which causes the substances to move at different rates over the paper
2.10 Interpret a paper chromatogram:
to distinguish between pure and impure substances, to identify substances by comparison with known substances and to identify substances by calculation and use of Rf values
● pure substances: should only have one spot on a chromatogram
● impure substances/mixtures: will show up with more than one spot on a chromatogram
● to identify by comparing with known substances: carry out paper chromatography with both the known substance and substance you’re testing on the same paper. If both spots are at the same height up the paper at the end then you know the substance you’re testing is the same as the known substance.
● to identify by calculation of Rf values: you can calculate Rf values and then compare them to known values for different substances
2.11 Core practical: Investigate the composition of inks using simple distillation and paper chromatography ● see 2.7- same method
● you need to identify which types of substance you have in the mixture and so which technique is most appropriate (from 2.7)
Methods of purification
- The choice of the method of separation depends on the nature of the substances being separated.
- All methods rely on there being a difference of some sort, usually in a physical property such as b.p., between the substances being separated.
Mixtures of solids
- Differences in density, magnetic properties, sublimation and solubility can be used.
- For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities.
Mixtures of liquids
- Immiscible liquids can be separated using a separating funnel or by decanting (pouring carefully).
- Examples include when an organic product is formed in aqueous conditions.
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Separating funnel being used to separate kerosene and water
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2.9 Describe paper chromatography as the separation of mixtures of soluble substances by running a solvent (mobile phase) through the mixture on the paper (the paper contains the stationary phase), which causes the substances to move at different rates over the paper
2.10 Interpret a paper chromatogram:
to distinguish between pure and impure substances, to identify substances by comparison with known substances and to identify substances by calculation and use of Rf values
● pure substances: should only have one spot on a chromatogram
● impure substances/mixtures: will show up with more than one spot on a chromatogram
● to identify by comparing with known substances: carry out paper chromatography with both the known substance and substance you’re testing on the same paper. If both spots are at the same height up the paper at the end then you know the substance you’re testing is the same as the known substance.
● to identify by calculation of Rf values: you can calculate Rf values and then compare them to known values for different substances
2.11 Core practical: Investigate the composition of inks using simple distillation and paper chromatography ● see 2.7- same method
Aim:
- To investigate the composition of ink using simple distillation and paper chromatography
Materials:
- Full distillation apparatus
- Ink samples
- Beaker with lid
- Suitable solvents
- Pencil, ruler and chromatography paper
Simple Distillation
- Ink consists of a solvent which has different dyes dissolved in it.
- The investigation must thus include analysis of both the solvent and the dyes used.
- The solvent must first be separated which can be done by simple distillation.
- Solvents tend to have low boiling points than the dyes which tend to be more viscousso it will evaporate first.
- Add anti-bumping granules and heat gently so as not to go past the boiling point.
- Record the temperature of boiling point.
Paper Chromatography
- This will separate the different colours in the ink so the number of dyes used can be determined.
- Make sure the base line is drawn in pencil and place a lid on the apparatus if the mobile phase being used is volatile.
- Rf values can then be calculated and compared with reference values.
- Alternatively further chromatography runs can be done with known dyes and compared to the sample.
Analysis of results:
- The boiling point of the solvent can be used this to determine its composition by comparing it to reference tablesg. ethanol boils at 78ºC so if the solvent evaporates at 78ºC it is likely to be ethanol.
- Rf values can be calculated and compared with reference values.
- Alternatively further chromatography runs can be done with known dyes and compared to the sample.
Conclusion:
- Both techniques are useful for separation and identification of compounds.
Hazards, risks and precautions
It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances. Eye protection must be worn.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
| Hazard | Harm | Precaution |
|---|---|---|
| Hot apparatus | Skin burns | Allow apparatus to cool before touching it |
| Bunsen burner flame | Skin burns | Keep hair and clothes tucked in |
| Bunsen burner flame | Fire | Do not bring flammable solvents near to the flame |
| Harmful solvent | Skin irritation | Avoid skin contact, eg wear gloves |
| Harmful solvent | Breathing difficulties | Ensure adequate ventilation or use a fume cupboard |
2.12 Describe how: waste and ground water can be made potable, including the need for sedimentation, filtration and chlorination, sea water can be made potable by using distillation and water used in analysis must not contain any dissolved salts
● potable water: it is suitable for drinking so must have:
○ low levels of microbes
○ low levels of contaminating substances
○ it is not the same as pure water but is still safe
● making waste and ground water potable:
1. sedimentation: large insoluble particles will sink to the bottom of the water
2. filtration: water is filtered through beds of sand which removes small insoluble particles
3. chlorination: chlorine gas is put through water to kill microbes
● making sea water potable using distillation:
1. filter the seawater
2. boil it
3. water vapour is cooled and condensed
● water used in analysis:
● potable water: it is suitable for drinking so must have:
○ low levels of microbes
○ low levels of contaminating substances
○ it is not the same as pure water but is still safe
● making waste and ground water potable:
1. sedimentation: large insoluble particles will sink to the bottom of the water
2. filtration: water is filtered through beds of sand which removes small insoluble particles
3. chlorination: chlorine gas is put through water to kill microbes
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Filtration, sedimentation and addition of chlorine are the main steps in water treatment
|
● making sea water potable using distillation:
1. filter the seawater
2. boil it
3. water vapour is cooled and condensed
● water used in analysis:
- Most chemical investigations involve the use of water at some stage of the process.
- Normally deionised water is used, which is water that has had metallic ions such as calcium or copper removed.
- Deionisation uses specifically designed ion-exchange resins that remove ions by exchange with hydrogen and hydroxide ions in water, which then recombine to form water molecules.
○ must be pure because any dissolved salts could react with the substances you are analysing, leaving you with a false result
Glossary
Glossary
- atomThe smallest part of an element that can exist.
- boiling pointThe temperature at which a substance rapidly changes from a liquid to a gas.
- chromatogramThe results of separating mixtures by chromatography.
- chromatographyChromatography is used to separate different substances dissolved in a liquid.
- compoundA substance formed by the chemical union of two or more elements.
- concentrationThe concentration of a solution tells us how much of a substance is dissolved in water. The higher the concentration, the more particles of the substance are present.
- condensedTurned from a gas or vapour into a liquid, usually as a result of cooling.
- crude oilMixture of hydrocarbons, mainly alkanes, formed over millions of years from the remains of ancient dead marine organisms.
- crystalA solid containing particles (atoms, molecules or ions) joined together to form a regular arrangement or repeating pattern.
- crystallisationThe process of producing crystals from a solution by evaporating the solvent.
- dissolvedA substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution.
- elementA substance made of one type of atom only.
- ethanolThe alcohol which is produced as a result of fermentation of sugars by yeast.
- evaporationThe process in which a liquid changes state and turns into a gas.
- excessIn chemistry, a substance is in excess if there is more than enough of it to react with another reactant.
- filtrationMethod used to separate an insoluble solid from a liquid using a physical barrier such as paper.
- fractionIn fractional distillation, such as that of crude oil, the different parts of the original mixture are called fractions. The substances in each fraction have similar boiling points to each other.
- fractional distillationIn fractional distillation a mixture of several substances, such as crude oil, is distilled and the evaporated components are collected as they condense at different temperatures.
- freezeA change of state in which liquid becomes solid by cooling.
- groundwaterWhere water is stored in rocks beneath the ground.
- insolubleUnable to dissolve in a particular solvent. For example, sand is insoluble in water.
- ionElectrically charged particle, formed when an atom or molecule gains or loses electrons.
- meltingThe process that occurs when a solid turns into a liquid when it is heated.
- melting pointThe temperature at which a solid changes into a liquid as it is heated.
- micro organismsMicroscopic living things such as archaea, bacteria and some species of eukaryotes.
- mixtureTwo or more substances that are not joined together. The substances can be elements, compounds, or both.
- mobile phasePhase in chromatography that moves, usually a solvent or mixture of solvents.
- moleculeA collection of two or more atoms held together by chemical bonds.
- particleA general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules.
- potableWater that is safe to drink.
- pureA substance that consists of only one element or only one compound.
- reactantA substance that reacts together with another substance to form products during a chemical reaction.
- simple distillationSeparation method used to separate a solvent from a solution.
- solubleAble to dissolve in solvent. For example, sugar is soluble in water because it dissolves to form sugar solution.
- soluteThe dissolved substance in a solution.
- solutionMixture formed by a solute and a solvent.
- solventThe liquid in which the solute dissolves to form a solution.
- stationary phasePhase in chromatography that does not move, for instance, the paper in chromatography.
- vapourGas formed below the boiling point of a substance.
