separation techniques

Question 1

what is a pure substance?

Answer 1

pure substances include elements and compounds
they have a fixed melting and boiling point

Question 2

what is an impure substance?

Answer 2

impure substances include mixtures
they do not have a fixed melting and boiling point

Question 3

how can the purity of a substance be determined?

Answer 3

the purity of a substance can be determined by checking its melting and boiling point
presence of impurities will lower the melting point and increase the boiling point of substances. moreover the melting and boiling point will occur over a range of temperature

Question 4

what is filtration?

Answer 4

filtration is used to separate an insoluble solid from a liquid. separation of substances occurred due to difference in size

Question 5

how does filtration work?

Answer 5

solid particles are too large to pass through the filter paper and are collected as residue
liquid particles are small enough to pass through the filter paper and are collected as filtrate

Question 6

what is evaporation to dryness?

Answer 6

many substances are soluble and dissolve in water to form an aqueous solution. to retrieve the dissolved substance, we can perform evaporation to dryness

Question 7

how does evaporation to dryness work?

Answer 7

after evaporating all the water away, we can obtain sodium chloride crystals
evaporation to dryness is not suitable to retrieve solids that decompose upon heating(eg. sugar)

Question 8

what is crystallisation?

Answer 8

crystallisation is often used to retrieve dissolved solids from solutions as crystals. instead of heating all of the water away, we evaporate some of the water to obtain a saturated solution

Question 9

how to do crystallisation?

Answer 9

heat the solution until saturated
leave solution to cool and crystals will form
filter to collect crystals as residue
wash crystals with cold distilled water to remove soluble impurities
dry crystals between sheets of filter paper

Question 10

what is distillation is general?

Answer 10

distillation allows separation due to difference in boiling points

Question 11

how to do simple distillation?

Answer 11

when a salt solution is heated, the water in the solution will boil and turn into a vapour
as the vapour passes through the condenser, it cools and condenses back into a liquid
pure water will be collected as distillate while salt remains in the distillation flask

Question 12

in simple distillation, what is the thermometer reading?

Answer 12

it is measuring the temperature of the water vapour not the solution

Question 13

in simple distillation, why is water pumped in from the bottom and exiting at the top?

Answer 13

water is pumped in from the bottom and exits the top so as to ensure that the condenser is completely filled and there is maximum surface area for cooling. this increases the efficiency of condensation

Question 14

in simple distillation, what is the purpose of the condenser?

Answer 14

the purpose of the condenser is to provide a cool surface for the vapour to condense back into a liquid and get collected as a distillate

Question 15

what is fractional distillation?

Answer 15

fractional distillation can be used to separate two or more miscible liquids(liquids that dissolve and mix with each other)

Question 16

what the difference between simple and fractional distillation in term of set-up?

Answer 16

in fractional distillation, the set-up contains an additional fractionating column, which helps improve the separation

Question 17

how does fractional distillation work? (eg. water and ethanol)

Answer 17

ethanol b.p=78, water b.p=100
when the mixture is heated, both ethanol and water will turn into vapour
since ethanol has a lower boiling point, it will distil over first. now, thermometer will show 78dc
the water vapour, which has a higher boiling point, will condense into the fractonating column and drip back into the distillation flask
once all the ethanol has distilled over, the temperature continues to rise until 100dc and water will start to distil over

Question 18

what is chromatography?

Answer 18

chromatography is a physical separation technique that uses a solvent to separate a mixture into its components. substances are separated due to its difference in solubility

Question 19

in chromatography, if a substance only has one dot, what does that mean?

Answer 19

they are pure substances

Question 20

in chromatography, if a substance has multiple spots, what does that mean?

Answer 20

they are mixtures

Question 21

why is the starting line of a chromatogram drawn in pencil?

Answer 21

the pencil will not dissolve in the solvent

Question 22

in the chromatogram, the more soluble a substance?

Answer 22

the further it will travel from the starting line