Separation Techniques

Question 1

When is filtration used

Answer 1

Separate an insoluble solid from a solution

Question 2

What is filtrate

Answer 2

Liquid that passes through filter paper

Question 3

What is residue

Answer 3

Insoluble solid remains on filter paper

Question 4

What is evaporation to dryness used for

Answer 4

Obtain a dissolved solid from a solution

Question 5

Process of evaporation to dryness

Answer 5

Solution of dissolved solid is heated in an evaporating dish to evaporate off all the solvent

Question 6

When is crystallization used

Answer 6

Obtain a dissolved solid from a solution as pure well-formed crystals

Question 7

Process of crystalisation

Answer 7

Heat the solution to evaporate off most of the liquid to form a saturated solution
Allow the hot, saturated solution to cool, forming crystals
Filter the solution. Crystals are the residue of the solution
Dry crystals between sheets of filter paper

Question 8

Why does crystalisation occur

Answer 8

The solubility of most solutes decreases as temperature decreases.
Mass of solute that cannot dissolve at a lower temperature will come out as crystals

Question 9

Disadvantage of evaporation of dryness

Answer 9

The solid obtained by evaporation to dryness is not always pure. Any soluble impurities will be left together with the solid after heating
Upon heating and evaporation to dryness, some solids decompose, e.g. thermally unstable salts

Question 10

Advantage of crystalisation

Answer 10

Impurities remain in the solution and will not crystalise out
Pure crystals are obtained

Question 11

When do we use crystalisation over evaporation to dryness

Answer 11

To obtain a pure salt or solid
To prevent thermal decomposition of salt

Question 12

When do we use evaporation to dryness over crystalisation

Answer 12

When solubility of salt does not change significantly with change in temperature.

Question 13

When is simple distillation used

Answer 13

Obtain pure solid from solution

Question 14

Why are boiling chips added

Answer 14

To ensure smooth boiling process

Question 15

How can voliatile liquid be collected in simple distillation

Answer 15

Put receiver on ice

Question 15

Why is the thermometer there

Answer 15

It measures the temperature of the vapour, indicating boiling point of liquid distilling over

Question 16

Examples of substances that can sublimate

Answer 16

Iodine
Solid carbon dioxide (dry ice)
ammonium chloride
naphtalene

Question 16

Why does water enter from the bottom and leave the top of the condenser

Answer 16

To ensure complete condensation of all vapour passing though the condenser

Question 17

Process of sublimation

Answer 17

Mixture of solids is heated in evaporating dish with inverted funnel placed above dish
The solid that sublimates changes into a vapour directly, rises and condenses back to solid directly on the cold surface of the funnel
Pure solid is sublimate
Solid that does not sublime is left in dish as residue

Question 18

Why must chromatography paper be sufficiently long

Answer 18

Ensure complete separation of mixture into its components

Question 19

Why should start line of paper chromatography be drawn in pencil and not in ink

Answer 19

Ink might dissolves in solvent used and hence interfere in the separation of mixture

Question 20

Why should the start line be above the solvent level

Answer 20

If it is below the solvent level, original spot will dissolve in solvent before it can be separated into its components

Question 21

Why should the spot of the mixture not be too large

Answer 21

To prevent separated components from overlapping onto one another