chemistry - states of matter & mixtures: methods of separating & purifying substances (2.5 - 2.12)

2.5 pure substance vs mixture

pure substance:

• only 1 type of element/compound

• fixed composition

• can’t separate into diff. substances using physical methods

• e.g. sucrose

mixture:

• elements and/or compounds not chemically joined together

• no fixed composition

• can separate into diff. substances using physical methods

• e.g. air

2.6 pure substances vs mixtures - melting points

pure substances: sharp melting point

mixtures: melt over range of temperatures

2.7 simple distillation - use

separate solvent from solution/purify liquids

2.7 simple distillation - process

e.g. purifying water

1. mineral water evaporates - only water turns into gas (pure water vapour)

   • solid mineral ions have higher boiling points - left behind

2. vapour condensed - turns back to liquid water (pure)

2.7 fractional distillation - use

separate 2/more liquids

2.7 fractional distillation - process

liquids with lower boiling points evaporate first

first fraction collected contains liquid with lowest boiling point

1. hot vapour rises up column

2. as column heats up, temp. gradient - bottom = hottest, top = coldest

3. fraction with lowest bp. reaches top first - vapour → condenser

4. keep heating - fractions with higher bps. rise up column & collected

2.7 filtration - use

separate insoluble solid from liquid

2.7 filtration - process

1. filter funnel lined with filter paper

2. solvent & solute(s) pass through fine holes - form filtrate

3. insoluble substances can’t fit through holes - leave residue in paper

2.7 crystallisation - use

separate solutes from solution by evaporating solvent to leave solutes behind

2.7 crystallisation - process

1. filtrate placed in evaporating basin over Bunsen burner

2. solvent evaporates - leaves solutes behind

2.7 paper chromatography - use

find out which coloured compounds a mixture contains

2.7 paper chromatography - process

some compounds dissolve better in solvent than others

1. solvent moves along strip of paper

2. carries diff. substances in mixture at diff. speeds - separates them

2.9 paper chromatography - definition

separation of mixtures of soluble substances

by running solvent (mobile phase) through mixture on paper (stationary phase)

causes substances to move at diff. rates over paper

2.10 paper chromatogram - pure vs impure substances

pure substances: one spot on chromatogram

impure substances: multiple spots on chromatogram

2.10 paper chromatogram - identify substances by comparing with known substances

1. do paper chromatography with known & unknown substances on same paper

2. spots of both = same colour, same number, at same height on paper - unknown substance = known substance

2.10 paper chromatogram - identify substances by calculation

1. do paper chromatography for unknown substances

2. calculate R_f values

3. compare to known values - same R_f value = same substance

2.10 paper chromatography - R_f values

measured from starting positions of samples

2.11 core practical: composition of inks - simple distillation

1. set up apparatus like diagram

2. heat flask of ink using Bunsen burner

   • ensure ink simmers gentle & doesn’t boil over into delivery tube

3. continue heating until collected few cm³ of distillate (distilled solvent)

4. note maximum temperature obtained

2.11 core practical: composition of inks - paper chromatography

1. draw pencil line on chromatography paper about 2cm from bottom

2. add small spot of ink to pencil line

3. add water to container about 1cm deep

4. place paper into container, let water travel through paper

   • ensure paper supported so doesn’t fall into water

5. take paper out before water reaches top, immediately mark position of solvent with pencil, leave paper to dry

6. measure distance water & each coloured substance travelled from pencil line

7. calculate R_f values for each coloured substance

2.12 how are waste & ground water made potable?

1. sedimentation: large insoluble particles sink to bottom

2. filtration: beds of sand & gravel - removes small insoluble particles

3. chlorination: chlorine added - kills microorganisms

2.12 how is sea water made potable?

water separated from dissolved salts by simple distillation

1. sea water heated - water vapour leaves it

2. vapour cooled & condensed - forms water without dissolved salts

2.12 water used in analysis

chemical analysis: chemical reactions/sensitive machines identify & measure substances in sample

water used must not contain any dissolved salts - incorrect results obtained

• dissolved salts may react to form unexpected cloudy precipitates

• machine used for analysis may detect salts