ST

Question 1

Explain the importance of separation techniques

Answer 1

To obtain pure substances as most materials that occur naturally are mixtures and not pure substances
To remove impurities from manufactured products
To carry out identification and analysis on substances present in food and drugs

Question 2

List 8 examples of separation techniques

Answer 2

Magnetic attraction, filtration, crystallisation, evaporation to dryness, simple distillation, sublimation, use of separating funnel, paper chromatography

Question 3

What is filtration used for

Answer 3

Separate an insoluble solid from liquid

Question 4

Explain the process of filtration [4]

Answer 4

The mixture is poured through a filter which is usually made of paper
The filter paper has tiny holes which allow the particles of water or liquid to pass through but not the larger insoluble solid particle
The liquid passes through the filter paper and is collected as the filtrate
The insoluble solid remains in the filter paper and is collected as the residue

Question 5

What is evaporation to dryness used for

Answer 5

Obtain a dissolved solid from a solution

Question 6

Explain the process of evaporation to dryness

Answer 6

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

Question 7

What is crystallisation used for

Answer 7

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

Question 8

Explain the process of crystallisation

Answer 8

The solution of the dissolved solid is heated in an evaporating dish to evaporate off most of the solvent
As the solvent evaporates, the solution becomes saturated
Allow the hot, saturated solution to cool, where crystals of the dissolved solid form
Filter the mixture to obtain the crystals as the residue
Dry the crystals between pieces of filter paper

Question 9

Why does crystallisation occur

Answer 9

Solubility of most solutes decrease as the temperature decreases and the mass of solid that is not soluble at a lower temperature will crystallize out

Question 10

How to tell if solution is saturated

Answer 10

Thin film of crystals forms on surface of solution or sides of evaporating dish

Question 11

What are the disadvantages of evaporation to dryness

Answer 11

Solid obtained 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, thus evaporation to dryness should not be used

Question 12

Advantage of crystallisation

Answer 12

Impurities remain in the solution and will not crystalise out. Thus, pure crystals are obtained

Question 13

When to use crystallisation instead of evaporation to dryness

Answer 13

Obtain a pure salt or solid
Prevent thermal decomposition of the salt

Question 14

When to use evaporation to dryness instead of crystallisation

Answer 14

When crystallisation is not suitable - when solubility of the salt does not change significantly with changes in temperature

Question 15

What is distillation used for

Answer 15

Obtain pure liquid from a solution

Question 16

Explain the process of distillation

Answer 16

The solution of the liquid is heated in a flask to boil the liquid solvent
The hot vapour of the liquid rises, and then cooled in a condenser where it condenses to a liquid
This pure liquid is collected in a receiving flask as the distillate
The solute remains in the original flask as the residue

Question 17

What should we note when carrying out distillation

Answer 17

The boiling chips are added to the mixture to ensure the smooth boiling process of the liquid
The thermometer is positioned so that its bulb beside the side-arm leading to the condenser. This is to ensure that the thermometer measures the temperature of the vapour, which indicates the boiling point of the liquid distilling over
Water enters the condenser form the bottom and leaves from the top. This is to ensure the complete condensation of all the vapour passing through.
Volatile liquid can be kept in its liquid state by placing the receiver on ice

Question 18

What is magnetic attraction used for

Answer 18

separate magnetic solids from non-magnetic solids

Question 19

What is sublimation and what is it used for

Answer 19

Process whereby a solid change directly to a gas on heating, without going through liquid state. Used to separate a solid that sublimes form other solids that do not

Question 19

Explain the process of sublimation [4]

Answer 19

The mixture of solids is heated in an evaporating dish with an inverted funnel placed above the dish
The solid which sublimes changes to a vapour directly, rises and condenses back to solid directly on the cold surface of the funnel
The pure solid is collected as the sublimate
The solid which does not sublime is not affected by heat and remains in the evaporating dish as the residue

Question 20

How to separate a mixture of sand and sodium chloride obtain a dry sample of each [6]

Answer 20

1. Add deionised water to the mixture
2. Stir the mixture thoroughly. Sodium chloride would dissolve in water, while sand remains insoluble
3. Filter the mixture. Sand is obtained as the residue and sodium chloride solution is obtained as the filtrate
4. Wash the sand with deionised water to remove traces of filtrate
5. Dry the sand in between two places of filter paper
6. Heat up the sodium chloride solution to evaporate the solution to dryness. Dry sodium chloride is thus obtained

Question 21

Explain the process of using a separating funnel

Answer 21

Mixture of immiscible liquids is placed in the funnel, mixed well, and allowed to stand
Liquids form separate layers with the less dense liquid being on top of the denser one
The stopper is removed, and the tap opened, to allow the bottom layer to run off and be collected in a beaker
Top layer remains and can be collected in a separate beaker

Question 22

What is one advantage of paper chromatography

Answer 22

can be used with v. small amounts of a mixture

Question 23

Explain the process of paper chromatography [4]

Answer 23

1. A drop of the mixture to be separated is placed on the start line near the bottom of a strip of chromatography paper
2. The spot is allowed to dry, after which the paper is suspended in a suitable solvent with the start line above the solvent level (All this is done in a covered container to minimise evaporation of solvent)
3. As the solvent travels up the paper, different components in the mixture on the start line dissolve in the solvent and travel up the paper at different speeds according to their solubility in the solvent. (more soluble -> faster + further up paper) (Identical components -> travel same distance + produce same colour when same solvent is used
4. When solvent reaches top of the paper, the chromatogram is removed and allowed to dry

Question 24

What should be done if the substance is colourless

Answer 24

spray the chromatogram with locating agent

Question 25

What should be taken note of when using paper chromatography [4]

Answer 25

1. Chromatography paper should be sufficiently long to ensure complete separation of mixture into its components
2. Start line should be drawn in pencil and not ink cuz ink might dissolve in the solvent used and thus would interfere with the separation of the mixture
3. Start line should be above solvent level cuz if below solvent level, original spot mixture would dissolve into the solvent even before it can be separated into its components
4. Spot to mixture should not be too large to prevent separated components from ‘overlapping’ onto one another