C2.1 - Purity & separating mixtures

Question 1

what is a pure substance?

Answer 1

single substance not mixed with anything else - e.g. distilled water

Question 2

what is a mixture?

Answer 2

contains 2 or more substances - e.g. seawater, milk, alloys

Question 3

what is an alloy?

Answer 3

a metal made by combining 2 or more metallic elements

Question 4

what do impurities do to the melting point of a substance?

Answer 4

lower it - disrupts regular arrangement of particles so they cant form as many bonds

Question 5

what do impurities do the the boiling point of a substance?

Answer 5

increase - impurities decrease water molecules available for vaporisation during boiling
- greater amount of heat needed to make same amount of impure solution vaporise than heat required to make pure solution vaporise

Question 6

pure + impure mpt + bpt

Answer 6

pure - fixed mpt + bpt - water bpt=100°C, mpt=0°C

impure - no fixed mpt + bpt - melt + boil at range of temps - e.g. starts boil at 70°C, completes boil at 78°C

Question 7

why is distillation used?

Answer 7

separate a pure liquid from a solute or mixture of liquids

• works when liquids have different boiling points
• sequence of events = heating –> evaporating –> cooling –> condensing

Question 8

what is a still?

Answer 8

the whole set of apparatus used in distillation

Question 9

what is a disadvantage of using simple distillation?

Answer 9

can only use it to separate things with very different boiling points

Question 10

what is different about fractional distillation?

Answer 10

you add a fractionating column on top of the flask

Question 11

why is fractional distillation useful?

Answer 11

can separate a mixture of different liquids - lowest bpt evapourates first, will reach top of column

Question 12

what is filtration used for?

Answer 12

separates insoluble solutes in a solution

Question 13

what is the filtrate?

Answer 13

the solution that passes through the filter paper

Question 14

when you mix salt and water why can you not see the salt?

Answer 14

water particles can get between the salt particles - breaks salt crystals up so you can no longer see them

Question 15

what is crystallisation?

Answer 15

process to produce crystals of the solute from the filtrate

Question 16

how are crystals produced from crystallisation?

Answer 16

when you leave a saturated solution to cool

Question 17

how is a filtrate turned into a saturated solution? (crystallisation)

Answer 17

by gently evaporating some of the solvent - as the saturated solution cools, the solute becomes less soluble in the solvent and some becomes out of solution as a solid crystal

Question 18

describe the process of filtration using a mixture of salt and crushed rock

Answer 18

• add distilled water + mix - so salt dissolves (rock is insoluble)
• set up filter paper in a funnel over a conical flask
• pour solution through paper - rock will be left in filter paper (residue) due to its bigger insoluble particles
• heat solution of saltwater in the flask - this creates a saturated solution - allow it to cool, solution now holds less solute so crystals begin to form

Question 19

what does paper chromatography separate?

Answer 19

coloured substances due to their different solubility in solvent used or how attracted substances are the the filter paper

Question 20

what is capillary action? (chromatography)

Answer 20

when liquids can flow in a narrow space, even against gravity

Question 21

describe how paper chromatography works

Answer 21

• draw a pencil line near bottom of paper (baseline)
• place ink sample(s) on pencil line
• add shallow amount of solvent (water or ethanol) to a beaker
• place paper in solvent, making sure solvent doesn’t touch pencil line
• place lid on top to stop solvent evaporating
• wait for solvent to seep up paper
• diff dyes that make up ink will dissolve in the solvent at different points up the paper
• draw pencil line where solvent stops - solvent front
• insoluble substances will stay on the baseline

Question 22

what is the mobile phase - paper chromatography

Answer 22

solvent - something molecules can move in

Question 23

what is the stationary phase - paper chromatography

Answer 23

paper - molecules don’t move

Question 24

how do you calculate the Rf value?

Answer 24

distance travelled by / distance travelled by solvent

substance

Question 25

paper chromatography - is a single spot a pure or impure substance?

Answer 25

pure

Question 26

the shorter the distance travelled by the substance....

Answer 26

....the less soluble it is - smaller Rf value

Question 27

describe thin layer chromatography

Answer 27

- draw baseline near bottom of plate with pencil - put small amount of solvent in beaker (water / ethanol) - dip bottom of plate in solvent - not baseline - put watch glass over top beaker - stops solvent evaporation - solvent moves up plate - when chems in mix dissolve in solvent, they will move up plate - diff chems in sample separate out, forms spots - remove plate when solvent close to top of plate, draw pencil line - solvent front - if spots can't be seen use UV or ninhydrin to turn them purple from colourless

Question 28

advantages of instrumental analysis

Answer 28

- quick - accurate - needs v. small samples - can prepare several samples to run one after another

Question 29

disadvantages of instrumental analysis

Answer 29

- costly - needs high skilled operators - large equipment - needs setting up

Question 30

what does mass spectrometry identify?

Answer 30

identifies components by splitting the molecule into different fragments and measure the mass of the fragments

Question 31

what does infrared spectrometry identify?

Answer 31

identifies components by subjecting the molecule to radiation which makes covalent bonds within 'wobble' at specific frequencies

Question 32

what is the mobile phase in gas chromatography?

Answer 32

unreactive / inert gas - e.g. nitrogen / helium

Question 33

what is the stationary phase in gas chromatography?

Answer 33

a viscous (thick) liquid - e.g. oil

Question 34

gas chromatography - if unknown substance isn't already a gas, then it has to be....

Answer 34

....vaporised

Question 35

describe the process of gas chromatography

Answer 35

- unknown mixture injected into long tube coated with stationary phase - mixture moves along tube with mobile phase until it comes out other end -diff substances spend diff amounts of time dissolved in mobile phase + stuck to stationary phase - a detector monitors compounds in + out if column + a recorder plots signal as a chromatogram

Question 36

what is retention time?

Answer 36

time it takes a chemical to travel through the tube - gas chromatography - diff for each chemical - what's used to identify each chemical

Question 37

what does the relative areas under the peaks show you? - chromatogram (gas chromatography)

Answer 37

relative amounts of each chemical in sample