Methods of Purification and Analysis

Question 1

what is an element?

Answer 1

pure substance only made up of one type of atom and cannot be broken down into simpler substances by chemical processes

Question 2

what is an atom?

Answer 2

smallest particle of an element having the same chemical properties of that element

Question 3

what is a molecule?

Answer 3

group of two or more atoms chemically combined together

Question 4

what are 5 differences between metals and non-metals?

Answer 4

appearance:
metal - shiny
non-metal - dull

m.p. and b.p.:
metal - general high (except group I)
non-metal - generally low (except carbon and silicon)

ductility and malleability
metal - ductile and malleable
non-metal - brittle if solid

heat and electrical conductivity:
metal - good
non-metal - poor (except carbon in the form of graphite)

Question 5

what is a compound?

Answer 5

pure substance containing two or more elements chemically combined together in a fixed ratio

*fixed m.p. and b.p.
*can be covalent (molecules) or ionic (ions)
*can be broken down by chemical means

Question 6

what is an ion?

Answer 6

atom or group of atoms that have an electrical charge

*positive - cation
*negative - anion

Question 7

what is a mixture?

Answer 7

consists of two or more substances that are not chemically combined together (e.g. alloys)

Question 8

what are 4 differences between compounds and mixtures?

Answer 8

composition:
compound - elements always combined in a fixed proportion
mixture - components can be mixed in any proportion

m.p. and b.p.:
compound - fixed
mixture - over a range of temperatures

properties:
compound - physical and chemical properties different from its elements
mixture - chemical properties same as those of its components

separation:
compound - only by chemical means
mixture - easily by physical means

Question 9

what is a pure substance?

Answer 9

made up of a single element or compound and is not mixed with other substances

*fixed m.p. and b.p.

Question 10

how does impurity affect the m.p. and b.p. of substances?

Answer 10

melts/boils over a range of temperatures; lowers melting point, raises boiling point

Question 11

how do you test for purity?

Answer 11

1) measure m.p. and b.p. (easier)
pure - fixed
impure - over a range of temperatures

2) chromatography
pure - produces only one spot on chromatogram
impure - more than one spot on chromatogram

*can prove that substance is a mixture (impure = mixture)

Question 12

what are the 4 solid-liquid separation techniques?

Answer 12

1) filtration
2) evaporation to dryness
3) crystallisation
4) simple distillation

Question 13

describe filtration

Answer 13

separates insoluble solids from liquids

smaller particles can pass through pores of filter paper and be collected as filtrate; larger particles too large to pass through pores and remain as residue

Question 14

describe evaporation to dryness

Answer 14

to obtain soluble solid from liquid (only for thermally stable solids)

heat until all solvent boils off

Question 15

describe crystallisation

Answer 15

separates soluble solid from solution, forming crystals of soluble solid

1. heat solution until saturation
2. cool saturated solution to allow crystals to form
3. filter mixture to obtain crystals as residue
4. wash crystals with a little cold distilled water
5. dry crystals between pieces of filter paper

*slower rate of cooling = larger crystals formed
*crystallisation occurs as solubility of solute decreases as temp. decreases; when solution cools, extra solute unable to remain dissolve separated as pure crystals

Question 16

what is distillation?

Answer 16

process of boiling a liquid and condensing the vapour

Question 17

describe simple distillation

Answer 17

separates pure solvent from a solution (basically purifying solvent)

1. solution boils and pure solvent vapour rises and passes into condenser
2. in condenser, solvent vapour cools and condenses to form pure solvent (distillate)
3. pure solvent collected in receiver and solute remains in distillation flask

Question 18

why are boiling chips added in the distillation flask of a simple distillation set-up?

Answer 18

to smoothen boiling

Question 19

why is the bulb of the thermometer placed beside the arm of the distillation flask of a simple distillation set-up?

Answer 19

ensures that it measures b.p. of substance entering condenser

Question 20

why does cold water enter from bottom of condenser and leaves from the top of a simple/fractional distillation set-up?

Answer 20

ensures that water jacket of condenser is completely filled and that temperature at the bottom of condenser is coolest (provides more efficient cooling system)

Question 21

what are the 3 solid-solid separation techniques?

Answer 21

1) using a magnet
2) sublimation
3) using a suitable solvent

Question 22

describe using a magnet

Answer 22

separates solids with magnetic properties from solids which are non-magnetic

*iron, cobalt, nickel, steel

Question 23

describe sublimation

Answer 23

separates a mixture of solids of which one of the solids sublimes

*iodine, ammonium chloride, dry ice (solid carbon dioxide), naphthalene (in mothballs)

Question 24

describe using a suitable solvent

Answer 24

separates a mixture of solids which dissolve in different solvents

thermally stable (filtration + evaporation to dryness):
1. add a suitable solvent and stir to dissolve one of the solids
2. filter the mixture to obtain the solution as filtrate and insoluble solid as residue
3. heat solution to dryness to obtain pure solid crystals

thermally unstable (filtration + crystallisation):
1. add a suitable solvent and stir to dissolve one of the solids
2. filter the mixture to obtain the solution as filtrate and insoluble solid as residue
3. heat solution to saturation
4. cool saturated solution to allow crystals to form
5. filter the mixture to obtain crystals as residue
6. wash crystals with a little cold distilled water
7. dry the crystals between pieces of filter paper

Question 25

what are the 2 liquid-liquid separation techniques?

Answer 25

1) using a separating funnel
2) fractional distillation

Question 26

what are immiscible liquids?

Answer 26

liquids that cannot dissolve in each other and are separated into distinct layers

Question 27

describe using a separating funnel

Answer 27

separates immiscible liquids due to different densities

1. pour mixture into separating funnel with tap closed
2. allow liquids to completely separate (denser will be at the bottom)
3. open tap of funnel to allow bottom layer to drain into beaker (close tap before top layer runs out)
4. open tap to allow interface layer to drain into another beaker (close tap before top layer runs out) - separating funnel has less dense liquid; beaker (step 3) has denser liquid

Question 28

what are miscible liquids?

Answer 28

liquids completely soluble in each other to form one liquid

Question 29

describe fractional distillation

Answer 29

separates miscible liquids with different boiling points

1. on heating, lower b.p. liquid vapour and higher b.p. liquid vapour rises up column
2. higher b.p. liquid vapour condenses in fractionating column and drops back into flask
3. lower b.p. liquid vapour reaches top of condenser and passes into condenser (thermometer will show constant temperature of lower b.p. liquid’s b.p.)
4. hot lower b.p. liquid vapour condenses into liquid and collects in receiver
5. when lower b.p. liquid has distilled over, temperature rises to higher b.p. liquid’s b.p. and higher b.p. liquid vapour rises and distils over and is collected in receiver

Question 30

why are there glass beads in the fractionating column of a fractional distillation set-up?

Answer 30

they provide a large surface-area-to-volume ratio for cooling of vapours (condenses and drops back into flask); ensures that only the liquid of the lowest b.p. is distilled over

Question 31

what is chromatography?

Answer 31

method of separating two or more components that dissolve in the same solvent

*only a very small initial sample needed to generate chromatogram
*works due to difference in solubility in same solvent; more soluble = farther up
*components ran with standards to identify components as identical substances travel same distance up

Question 32

what is a chromatogram?

Answer 32

chromatography paper with separated components

Question 33

how do you conduct chromatography?

Answer 33

1. draw line with pencil about 1cm from bottom of strip of filter paper
2. dye to be separated placed on pencil line and allowed to dry
3. filter paper dipped into tube containing suitable solvent (ensure solvent is below pencil line)
4. difference in solubility of substances in same solvent - travel up paper at different speeds; more soluble = farther up than less soluble components, separating the components
5. when solvent reaches nearly top of filter paper, filter paper removed and solvent front is marked with pencil line
6. multiple spots show that dye is a mixture of different components

Question 34

why is the start line of chromatogram drawn with pencil and not ink?

Answer 34

dyes in ink are soluble in solvent and will interfere with chromatogram while pencil line is insoluble in solvent

Question 35

why is the tube of a chromatography stoppered?

Answer 35

prevents evaporation of solvent which affects development of chromatogram

Question 36

why is the solvent level in a chromatography tube below the spot on the pencil line?

Answer 36

if pencil line below solvent level, dye would dissolve into solvent and no chromatogram would be formed

Question 37

what is a retention factor value?

Answer 37

ratio of distance travelled by substance relative to that travelled by solvent

*no units
*start line to spot / start line to solvent front
*Rf value of a substance does not change given that chromatography is run under same conditions (i.e. same solvent and temp.)

Question 38

what is a solvent front?

Answer 38

position/distance reached by solvent

Question 39

what are locating agents?

Answer 39

substance that reacts with substances on chromatography paper to produce a visible coloured product

*allows measurements to be taken to determine Rf values of components