Methods of Purifying and Separating Substances - Topic 2 Flashcards
1
Q
What is the difference between the meaning of pure in chemistry and the meaning of pure in every day life?
A
A pure substance consists only of one element or one compound whereas in real life it means that no other substances were added to the original substance
2
Q
What is the difference between a pure substance and a mixture?
A
A mixture consists of two or more different substances, not chemically joined together whereas a pure substance is made up of only one element
3
Q
What are all the different ways to separate substances?
A
- filtration
- distillation
- fractional distillation
- separating funnel
- re-crystallisation
- decanting
- evaporation
- chromatography
4
Q
What does filtration do and give an example of substances that it can separate:
A
- Separate an insoluble solid from a liquid or separate a liquid from an insoluble solid
- Sand and water
5
Q
Method for separating salt and sand using filtration:
A
- Add water - the salt dissolves to form a solution.
- Filter out the sand from the salt solution using a filter funnel.
- Heat the filtrate to evaporate of the water - salt crystals form.
6
Q
What does evaporation do? And give an example of substances you can separate using evaporation:
A
- Separate a soluble solid (solute) from a solution
-If only solid is required in a solution it is obtained through evaporation - Salt and water can be separated using evaporation if the solvent is not required, salt from salt solutions
7
Q
Solute:
A
Solid part of the solution
8
Q
Solvent:
A
Liquid part of solution and what the solute dissolves in
9
Q
In evaporation is the solvent retained?
A
Not retained - goes into the atmosphere
10
Q
Method for Evaporation:
A
- Heat the solution until crystals start to form at the edge.
- Allow to cool.
11
Q
What does distillation do? And give an example:
A
- To separate a soluble solid from a solution and obtain the solvent
- If the solvent Is required from a solution, it is obtained through distillation
- To collect water from a salt solutions
- Alcohol from sugar dissolved in alcohol
12
Q
In distillation what must the thermometer be at the same level of?
A
The condenser
13
Q
In distillation where does the water enter and where does it come out?
A
Enters the condenser at the bottom and comes out at the top
14
Q
How distillation works e.g. to collect pure water from a sample of salty sea water:
A
- Heat up the solution.
- The solvent evaporates - vapour comes out of the flask into the condenser where it cools - pure water drops out of the condenser.
- The solid remains in the flask.
15
Q
Describe the stages of distillation:
A
Heat —> evaporate —> cool —> condense —> pure liquid runs out
16
Q
Miscible:
A
when two liquids do mix e.g. alcohol and water
17
Q
What does fractional distillation do? Give an example of substances which can be separate by fractional distillation:
A
- liquid fron mixture of miscible liquids separated by fractional distillation
- e.g. ethanol and water. petrol and diesel from crude oil, alcohol and water
18
Q
Method for Fractional Distillation:
A
- The mixture is placed in a flask and heated.
- The mixture boils. Vapour from both liquids pass into the fractionating column, but there is a higher proportion of the substance with the lower boiling point (the more volatile one). In the fractionating column they condense and rebook many times, each time with a higher proportion of the more volatile liquid.
- As a result of this, the vapour of the liquid which has the lower boiling point emerges from the top of the column first, in a pure state, and passes into the condenser.
- When all the of this liquid has distilled over, it Is followed by the liquid having the higher boiling point.
- The thermometer measures the boiling point of the liquid distilling over at any moment.
- Keep moving the Bunsen burner in and out from under the distillation flask to make sure the temp stays in between the two boiling points, when liquid stops coming out of the condenser, both substances have been separated.
19
Q
In fractional distillation what is the difference between the vapour at the bottom of the fractionating column and the vapour at the top?
A
- The particles in the vapour at the bottom of the fractionating column has more E and will be moving faster than the vapour at the bottom of the fractionating column due to being given more heat and therefore having more kinetic E.
- The particles in the vapour at the top of the fractionating column have less E as it is gaining less heat energy and is about to become a liquid.
20
Q
Immiscible:
A
when two liquids do not mix
21
Q
What mixture does a separating funnel separate? and give an example:
A
- Mixture of immiscible liquids
- Oil and water (they are immiscible because they have different densities) are separated simply by running off the more dense liquid (water) by opening the tap.
22
Q
Separating water from a mixture of copper sulphate and water (fractional distillation) - Assessed GCSE Practical: Why must the test tube be at an angle when heated?
A
Otherwise the solution will be heated too much causing the CuSO to evaporate as well as the H2O not separating the mixture
23
Q
Separating water from a mixture of copper sulphate and water (distillation) - Assessed GCSE Practical:
Why is it important to not heat the solution too violently?
A
The CuSO will evaporate as well as the H2O not separating the two liquids in the mixture
24
Q
Separating water from a mixture of copper sulphate and water (distillation) - Assessed GCSE Practical: What temp does water distill at?
A
100°C
25
Separating water from a mixture of copper sulphate and water (distillation) - Assessed GCSE Practical: Describe the appearance of the distillate:
- colourless
- odourless
26
Separating water from a mixture of copper sulphate and water (distillation) - Assessed GCSE Practical:
What has a higher boiling point the CuSO or the H2O?
CuSO
27
Separating water from a mixture of copper sulphate and water (distillation) - Assessed GCSE Practical
What were the physical changes in the experiment?
Liquid —> gas —> liquid
28
Separating water from a mixture of copper sulphate and water (distillation) - Assessed GCSE Practical:
How do you get CuSO solution?
• Anhydrous CuSO
| add small amount of water
• Hydrated copper sulphate crystals (blue solid)
| more water
• Copper sulphate solution
29
Assessed GCSE Chromatography Practical: Chromatography:
the separation of a mixture of soluble substances (from the same solvent) 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
30
Assessed GCSE Chromatography Practical: What happens to the dyes during chromatography?
- The dyes are placed at the bottom of the chromatography paper.
- This is then placed in a solvent, such as water.
- As the water is drawn up the paper it moves the different rates depending on their solubility in water.
- Dyes which are more soluble in water move more quickly up the paper.
- Dyes which are less soluble in water move more slowly up the paper.
31
Assessed GCSE Chromatography Practical: What is the mobile phase in chromatography?
the solvent is called the mobile phase
32
Assessed GCSE Chromatography Practical: What is the stationary phase?
the paper is called the stationary phase
33
Assessed GCSE Chromatography Practical: Method for Chromatography:
1. Use pencil to draw on a start line on chromatography paper.
2. Draw a dot of ink on the pencil start line.
3. Place the chromatography paper in the beaker with the solvent in. The solvent should be touching the paper but not the start line.
4. Remove the paper from the solvent before the solvent reaches the top of the chromatography paper and dry it with a paper towel.
5. Measure the Rf value to compare the sample.
34
Assessed GCSE Chromatography Practical: How do you calculate the Rf value?
Rf = Distance travelled by dye /
Distance travelled by solvent front
35
Assessed GCSE Chromatography Practical: Why was the start line on the chromatography paper drawn in pencil?
The start line was drawn in pencil so that the line wouldn't dissolve in the solvent and only the pen would
36
Assessed GCSE Chromatography Practical: Why are the spots of pen labelled on the chromatography paper?
The spots of pen are often labelled at the start to make sure that they can be identified as the separate into pigments
37
Assessed GCSE Chromatography Practical: Why must the solvent in the beaker be lower than the start line on the chromatography paper?
The water level in the beaker must be below the spots so the pen/ink doesn't dissolve in the water at the start of the experiment
38
Assessed GCSE Chromatography Practical: What does the chromatogram tell us?
The chromatogram tells us that most of the pens were made up of two or more colours and that the more soluble the pigment is the further up the chromatography paper it will travel
39
What type of mixture is separated by decanting: and give an example:
- Separate an insoluble solid from a liquid or separates a liquid from an insoluble solid
- e.g. sand and water
40
Re-crystallisation Practical - Separation of benzoic acid from a mixture containing sand and salt: What does re-crystallisation do?
Re-crystallisation is the method of purifying a solid and removing both soluble and insoluble impurities
41
Re-crystallisation Practical - Separation of benzoic acid from a mixture containing sand and salt: Aim:
to remove the impurities from a sample of benzoic acid by carrying put re-crystallisation
42
Re-crystallisation Practical - Separation of benzoic acid from a mixture containing sand and salt: Method:
Stage 1 - removing soluble impurities - heating and hot filtration
1. Half fill a 250mm beaker with water from a kettle and place it on a tripod and wire gauze over a Bunsen Burner
2. Collect a boiling tube containing impure benzoic acid and clap. it in place so that it is resting in the hot water bath
3. Use a pipette to add hot water from the hot water bath to the boiling tube, stirring after each addition
4. Continue adding water until the salt and benzoic acid has dissolved but the sand has not
5. Collect a warm funnel
6. Place a piece of filter paper inside it and filter your mixture into a clean boiling tube - the sand should be caught by the filter paper while the solution of benzoic acid and salt passes through as filtrate
(you have now removed the insoluble impurities and only need to separate the benzoic acid and soluble impurities)
Stage 2 - removing soluble impurities - Buchner filtration
1. Take the filtrate and cool it in a. beaker containing ice
2. Use a glass rod to scratch the side of the boiling tube to help the formation of crystals (not too hard in case you break the boiling tube)
3. After a few minutes of cooling and scratching. filter the mixture using a clean filter paper
4. When filtered use a little cold distilled water to wash the precipitate
43
Re-crystallisation Practical - Separation of benzoic acid from a mixture containing sand and salt: What will happen if the solution is cooled after it's heated?
If the solution containing the salt and benzoic acid is cooled the benzoic acid will start to crystallise
44
Re-crystallisation Practical - Separation of benzoic acid from a mixture containing sand and salt: Why must the solvent be chose carefully to purify be recrystallisation?
To purify by recrystallisation the solvent used (in this case water) must be chosen carefully because the solute has to be soluble in the hot solvent and insoluble in cold solvent
45
Re-crystallisation Practical - Separation of benzoic acid from a mixture containing sand and salt: Why in stage 1 does the filtration has to be carried out using a hot funnel?
In stage 1, the filtration has to be carried out using a hot funnel to make sure that the benzoic acid doesn't crystallise
46
Re-crystallisation Practical - Separation of benzoic acid from a mixture containing sand and salt: Why in stage 2 was the benzoic acid washed with cold distilled water?
in stage 2, the benzoic acid was washed with cold distilled water to get rid of any soluble impurities and the distilled water had to be cold to avoid the benzoic acid re-dissolving
47
Why is distilled water more expensive than tap water?
Distilled water is more expensive than tap water because it has to be purified (labour = expensive)
48
Re-crystallisation - Separation of benzoic acid from a mixture containing sand and salt: how can we show that our sample of benzoic acid was pure at the end of the experiment?
To show that our sample of benzoic acid was pure we could melt it - if it melts at the benzoic acid mp (122 degrees C) then its pure, if it was impure the mp of the sample would be lower
49
What are the units of the Rf value?
It has no units as its a ratio
50
Short method for re-crystallisation:
1. Heat mixture in hot water bath to dissolve the soluble substances by adding some water to the substance
2. Filter out the insoluble substance with a hot filtration
3. Cool the filtrate so one substance re-crystallises and drain the solution with a Buchner filtration
51
Why does the thermometer bulb go next to the top of the condenser?
To measure the temp of the vapour to make sure the right liquid is being evaporated, the thermometer measures the boiling point of the liquid distilling over at any moment
52
What happens in filtration?
The liquid filtrate passes through, the undissolved solid residue stays on the paper
53
What happens in distillation?
- The solid is left behind in the flask (the residue)
- The liquid evaporates when heated and then condenses back into a liquid in the condenser
- The collected liquid is called a distillate
54
Where does the water enter and come out of the condenser in distillation?
The water enters the condenser at the bottom and comes out at the top
55
Describe the energy of the particles in the fractionating column in fractional distillation:
- The particles in the vapour at the bottom of the fractionating column has more E and will be moving faster than the vapour at the top of the fractionating column due to being given more heat and therefore having more kinetic E
- The particles in the vapour at the top of the fractionating column have less E as it is gaining less heat energy and is about to become a liquid
56
Re-crystallisation Practical - Separation of benzoic acid from a mixture containing sand and salt: What happens in re-cyrstillisation?
- The solid is dissolved in the minimum amount of hot solvent
- It is filtered to remove any insoluble impurities
- The solution is then cooled and the solid crystallises out leaving soluble impurities dissolved in the solvent
- The mixture is filtered again to obtain the solid which is then washed with a little cold solvent
57
Assessed GCSE Chromatography Practical: What happens in chromatography?
- The dyes are placed at the bottom of chromatography paper
- This is then placed in a solvent, such as water
- As the water is drawn up the paper it moves the different dyes at different rates depending on their solubility
58
Assessed GCSE Chromatography Practical: How are known dyes identified in chromatography?
Unknown dyes can be identified by the level to which they rise, as the same dye will be at the same level on a chromatogram
59
Assessed GCSE Chromatography Practical: What does the number of spots above the staring line on the chromatography paper in chromatography tell us?
The number of spots above the starting point tells us the number of dyes there were in the original sample
60
Assessed GCSE Chromatography Practical: How do we know which solid is the most soluble in chromatography?
- The water is drawn up the paper it moves the different dyes at different rates depending on their solubility
- The more soluble the pigment is the further up the chromatography paper it will travel as it has a faster rate
61
Assessed GCSE Chromatography Practical: What else can chromatography be used as?
Chromatography can also be used as an analytical technique
62
Assessed GCSE Chromatography Practical: What can be separated using chromatography?
dyes and other complex molecules can be separated using chromatography, separating a mixture of food dyes
63
Assessed GCSE Chromatography Practical: How can you interpret paper chromatograms?
1. Pure substances will produce only one spot on the chromatogram
2. If two or more substances are the same, they will produce identical chromatograms
3. If the substance is a mixture, it will separate on the paper to show all the different components as separate spots
64
Potable water:
water that is suitable for drinking
65
What must water have to be potable?
- low levels of microbes
- low levels of contaminating substances e.g. dissolved substances
- not necessarily pure water as it contains dissolved substances needed in the body
66
Where do water supplies come from?
- rain water
- this water, with low levels of dissolved substances (fresh water) collects in the ground and in lakes and rivers
- it is then treated to make it potable
67
What are the main processes needed to make waste and ground water potable?
1. Sedimentation
1. in tank leave water to sit - no kinetic E
2. large insoluble particles will sink to bottom and then are removed
2. Filtration
1. water is filtered through screens and/or beds of sand which removes small insoluble particles
3. Sterilisation
1. chlorine gas dissolved in water to kill bacteria as it is toxic - high enough concentration of chlorine added that it kills bacteria but not high enough that it harms humans - safe to drink
2. can shine UV light on water to kill bacteria - requires high amount of energy
3. ozone O₃ into water
4. Br₂ liquid kills bacteria (toxic) but accumulates in kidneys - can’t be filtered out of body
68
What does chlorine look like?
very pale green gas
69
What substances may still be present in drinking water once it has gone through all its treatments?
dissolved salts e.g. Ca²⁺ and Mg²⁺ compounds
70
What is the chemical test for chlorine?
- when damp litmus paper is put into where the chlorine gas issues suspected to be the litmus paper is bleached and turns white as chlorine is a bleaching agent
- litmus paper has to be damp so the water can react with the chloride - if it isn't damp it will set on fire
71
Why do some water companies but fluoride compounds into drinking water?
- to help teeth heath
- concentrations need to be monitored
72
How is waste water treated to make potable?
- Screening and grit removal
- large solids and grit removed
- Primary sedimentation tanks
- organic waste sinks to bottom as sludge
- remaining water (effluent) taken off top
- Aeration
- air passed through
- good bacteria kill harmful bacteria
- Final settling tanks
- good bacteria sink to bottom in sludge
- treated water ready to be returned to rivers
- Sludge treatment
- anaerobic digestion of sludge by bacteria
- Burners
- methane burned to provide heat and electricity
73
How is sea water made potable?
- Seawater is first filtered - insoluble particles are removed
- Desalination - distillation
- water is boiled and impurities stay behind
- water vapour is then cooled and condensed - contains no salt
- require large amounts of energy - so very expensive (only used when there is not enough fresh water)
- Desalination - reverse osmosis
- sea water pushed through semi-permeable membrane under pressure
- dissolved substances can’t pass through
- high energy pressure
74
Why does Spain operate desalination plants whereas the UK doesn’t?
- less rainfall + higher temperature in Spain
- boil water and collect vapour - water requires less energy to boil as natural climate is already quite hot
75
Which stage in distillation makes it so costly
- heating up and boiling water requires a lot of energy - more expensive
- using energy to force particles to flow in the opposite direction
76
What must water used in analysis be?
must be pure as any dissolved salts could react with the substances you are analysing - leaving you with a false result
77
Description of sea water + does it contain microbes + does it contain dissolved substances:
- water found in the seas and oceans
- contain microbes
- contains high amounts of dissolved substances
78
Description of ground water + does it contain microbes + does it contain dissolved substances:
- water in underground streams and rocks
- contains microbes
- contains low amount of dissolved substances
79
Description of fresh water + does it contain microbes + does it contain dissolved substances:
- water in underground streams and rocks, rivers, lakes, ice caps and glaciers
- contains microbes - very low amount of microbes from some sources
- contains low amount of dissolved substances
80
Description of waste water + does it contain microbes + does it contain dissolved substances:
- used water from homes, industry and agriculture
- contains microbes
- contains high amounts of dissolved substances
81
Description of potable water + does it contain microbes + does it contain dissolved substances:
- water that is suitable to drink
- contains no microbes (very low concentration of microbes)
- contains low amount of dissolved substances
82
Description of pure water + does it contain microbes + does it contain dissolved substances:
- water that is 100% water and contains no other substances
- contains no microbes
- contains no dissolved substances
