Separation&Purification Flashcards
1
Q
Define mixture
A
- made up 2 or more substance
- not chemically combined
2
Q
Define pure substance
A
- made up of 1 single element or compound
- not mixed with any other substance
3
Q
How do we obtain pure substances from mixtures?
A
remove impurities
4
Q
Filtration
A
- used to separate insoluble solids (suspension) from a liquid
- e.g. sand, clay
5
Q
Steps/Procedure to take in filtration (separating mixture of water and sand)
A
- Pour mixture into a filter funnel that is lined with filter paper
- Collect filtrate (water in this case) in a conical flask
- Collect residue(sand), dry it on a piece of filter paper
6
Q
How filtration works
A
- filter paper acts as sieve
- insoluble solids cannot pass thru pores, collected on filter paper (residue)
- liquid pass thru pores (filtrate)
7
Q
Evaporation to dryness
A
- used to obtain a soluble solid from a solution by heating the solution until all the water has been boiled off
- e.g obtaining salt from seawater
8
Q
Evaporation to dryness (apparatus)
A
- salt solution
- evaporating dish
- wire gauze
- tripod stand
- Caution: safety googles to be worn as very hot concentrated liquid can sputter from dish
9
Q
Evaporation to dryness (solid obtained)
A
- solid obtained may not always be pure
- when water removed, any soluble impurities left together with solid
10
Q
Crystallization
A
- used to obtain a pure solid sample from solution
- many substance decompose when heated strongly e.g. sugar decomposes to water and carbon
- most crystals e.g. copper(II)sulfate crystals give off water, become powder when heated
- in this process, water removed by heating sol
- heating stops at stage when hot saturated sol formed
- sol cool to room temp, dissolved solid turn to crystals
11
Q
Steps/Procedures to take for crystallization (obtain pure copper(II) sulfate crystals)
A
- Dissolve impure copper (II) sulfate crystals in water
- Filter to remove any insoluble impurities, collect filtrate (copper (II) sulfate solution)
- Heat copper (II) sulfate sol until its saturated
- leave it to cool and crystallize
- Filter to collect crystals, wash crystals with a little cold distilled water to remove impurities. Dry the crystals between few sheets of filter paper
12
Q
How to know whether solution is saturated? (crystallization)
A
- dip a clean glass rod into sol and removed
- small amt of sol on rod
- if small crystals form on rod as sol cools, the solution is saturated
- means that sol is at its saturation point/ crystallization point
13
Q
Solvent and solute
A
Solvent: the liquid that dissolves the solute
Solute: substance that is being dissolved
14
Q
How do we choose a suitable solvent for separating solids?
A
- diff solids dissolve in diff solvents
- most common solvent: water and ethanol
- to separate a mixture of 2 solids, use a solvent which 1 solid is soluble in and the other is not
- e.g. sodium chloride (table salt) is soluble in water but sand is not, mixture of these 2 substances separated using water as solvent
15
Q
Separating solids (sodium chloride and sand mixture)
A
- add water to dissolve sodium chloride (become sodium chloride solution and sand mixture)
- filter the mixture
- filtrate: sodium chloride sol, evaporate to dryness to obtain sodium chloride
- residue: sand + traces of sodium chloride sol, wash with distilled water, obtain sand
16
Q
Sublimation
A
- used to separate a solid that sublimes from one that does not sublime
- e.g. iodine sublimes when heated, sand does not
17
Q
Sublimation (apparatus) to separate iodine and sand
A
- funnel, wet cloth to cool, evaporating dish, wire gauze, heat source, mixture of iodine and sand
- solidified iodine collect on top of funnel where wet cloth cools down iodine (iodine vapour to solidified iodine)
18
Q
Using magnet
A
- separate non-magnetic and magnetic
- e.g can separate iron from mixture of iron and sulfur using magnet
- Cobalt Iron Nickel are magnetic
19
Q
Simple distillation
A
- used to separate a pure solvent from a solution
- distillation is the process of boiling a liquid and condensing the vapour
- e.g. pure water can be obtained from a salt solution
20
Q
Steps/Procedures to take in simple distillation
A
- In the distillation flask, solution boils. Boiling chips added to ensure smooth boiling. Water vapourizes, rises and enters condenser
- In the condenser, water vapour is cooled. Vapour condenses and changes back into pure water (distillate)
- Pure water is collected as a distillate in a receiver (conical flask)
- Salt solution remaining in flask becomes more concentrated as distillation continues. If distillation is allowed to carry on, solid residue of salt will be left in flask
21
Q
Thermometer (Procedure to note, Reason)
A
- bulb of thermometer should be placed beside side arm of distillation, should not be dipped into sol
- ensures that thermometer measures boiling of pt of substance that is distilled
22
Q
Condenser (Procedure to note, reason)
A
- should slope downwards
- 2 tubes: inner tube, outer water jacket
- cold running water allowed to enter bottom of condenser and leave by the top
- sloping ensures pure solvent runs downwards into receiver (conical flask)
- if water enters top of condenser it will exit the condenser before water jacket can be completely filled
- water from bottom to ensure entire jacket is always filled
- provides efficient cooling system
23
Q
Receiver (Procedure to note, reason)
A
- if distillate is volatile, receiver put into a large container filled with ice
- helps keep temp of distillate low so it can remain in liquid state
24
Q
Using a separating funnel
A
- can be used to separate immiscible liquids e.g. oil and water (do not dissolve into each other)
- when immiscible liquids are shaken, they appear to mix (but they actually just form an emulsion)
- emulsion eventually separates into diff layers of liquids
25
Separating funnel procedures (oil and water) (5 pts)
1. pour mixture of oil and water into separating funnel (make sure tap is closed)
2. support separating funnel using retort stand. place a clean beaker below the funnel
3. allow liquids to separate completely, may take some time, denser liquid (water) will be bottom layer
4. open tap of funnel to allow bottom layer to drain into beaker. Close the tap before top layer of liquid runs out
5. place another beaker below funnel. open tap to allow a little of top layer of liquid into beaker. now separating funnel only contains oil while beaker from step 4 contains only water
26
Fractional distillation (5 pts)
- used to separate a mixture of miscible liquids with diff boiling pts (dissolve in each other) e.g. ethanol and water
- fractionating column attached to round-bottomed flask and condenser
- many glass beads in fractioning column for larger SA from vapour to condense on
- liquid with lowest boling pt distil over first
- vapours of liquids with higher bp condense along fractionating column, fall back into round-bottom flask
27
Fractional distillation procedures (7 pts)
1. Ethanol vapour and water vapour rises up the column as the sol is heated
2. the water vapour condenses in the fractionating column and falls back into flask
3. ethanol (lower bp that water) reaches upper part of column and is distilled over
4. at this point, thermometer shows a constant temp of 78 degrees (bp of ethanol)
5. in the condenser,
- hot ethanol vapour condenses as cold water runs thru it
- liquid ethanol flows down the inner tube of the condenser and into receiver (conical flask)
6. ethanol is collected as distillate in receiver
7. when all ethanol distilled over, temp rises rapidly to 100 degrees (bp of water) at this temp, water distills over and can be collected separately
28
What are the industrial applications of fractional distillation?
- used to obtain nitrogen, argon, oxygen from air
- separate petroleum into useful fractions
- obtain ethanol produced by fermentation of glucose solution
29
Define Chromatography
the method of separating 2 or more components that dissolve in the same solvent e.g. separating dyes in food colouring
30
Paper chromatography
Aim: to separate dyes in the green food colouring using paper chromatography
1. Draw a line with a pencil at abt 1 cm from bottom strip of filter paper/chromatography paper
2. Put a drop of green food colouring on the pencil line, allow the drop to dry
3. Dip the paper into a glass tank containing the solvent (e.g. ethanol), ethanol soaked up by the paper will dissolve the dyes
4. Leave the apparatus to stand for a while. Ethanol travels up the paper, more soluble dye is in ethanol, further it will move up paper
31
How do we interpret the result of paper chromatography?
- chromatography paper with separated components is called a chromatogram
- mixture/impure: 2 or more dye spots
- pure: made of only 1 dye, 1 spot on chromatogram
32
Rf values
- ratio between distance traveled by a substance and distanced traveled by the solvent is a constant
- Rf value= distance traveled by substance/distance traveled by solvent
- Rf value does not change when chromatography is carried out under same conditions (same solvent, same temp)
- substances on a chromatogram are easily identifiable
33
Using paper chromatography to analyse a sample
- identify components present in sample
| - e.g. gov analyse samples of food colouring to ensure dyes are safe to consume
34
How can a banned substance present in food colouring be identified?
1. Paper chromatography used to separate dyes in sample
2. Each dye is identified by comparing
(a) its position in the chromatogram with that of a known dye in the same chromatogram
(b) its Rf value with the Rf value of known dye
3. Chemists can then check against database of banned dyes to see if dyes are permitted for use in food
35
Chromatography of colourless substances
- e.g. amino acids
| - spray locating agent on chomatogram, colourless substances will show up as coloured spots
36
Chromatography of colourless substances (e.g. amino aicds) procedure
1. Separate mixture of amino acids by chromatography by using suitable solvent
2. Stop the chromatography before solvent reaches top of paper. Dry paper
3. Spray locating agent onto paper
4. locating agent reacts with each of the amino acids to form coloured spots on the paper. Identify diff amino acids by checking Rf value of each coloured spot
37
Determining purity
- measurements of purity in substances impt as it helps detect harmful impurities, ensures products meet quality standards
- impurities in drugs may cause undesirable side effects
- chemicals eg preservatives, dyes added to food and beverages to last longer, taste better, look more attractive, impt to ensure our food is safe for consumption
- in production of silicon chips for the electronics industry, small amts of impurities, make component in an electronic device less efficient
38
Pure solids have fixed melting pt
- solid is pure if it has exact and constant/fixed melting point
- impurities lower melting point of solid, greater amt of impurities, lower mp
- impurities cause melting to take place over a range of temperatures
39
Pure liquids have fixed boiling points
- a liquid is pure if it has an exact and constant/ fixed boiling point
- use distillation to find out boiling pt
- impurities increase bp, more impurities, higher boiling pt
- impurities cause boiling to take place over a range of temp
