Separation and Purification Techniques

Question 1

Function of magnetic attraction?

Answer 1

Can be used to separate magnetic materials from non-magnetic substances

Question 2

Type of mixture for magnetic attraction?

Answer 2

Solid-solid
E.g. Iron & sulfur

Question 3

Physical property that allow magnetic attraction to take place

Answer 3

Magnetic attraction

Question 4

How does magnetic attraction work?

Answer 4

To separate magnetic materials from non-magnetic ones with a magnet
Common magnetic materials: iron, steel

Question 5

Applications of magnetic attraction?

Answer 5

Food processing
Salvaging scrap iron and steel
Separating mixture of iron and sulfur → yellow powder

Question 6

Function of filtration?

Answer 6

Used to separate an insoluble solid from a solid-liquid mixture
Filter paper acts as a sieve:
Liquid can pass through pores in filter paper and solids cannot pass through

Question 7

Type of mixture for filtration?

Answer 7

Insoluble solid-liquid
E.g. Sand & water

Question 8

Physical property that allow filtration to take place

Answer 8

Solubility

Question 9

How does filtration work?

Answer 9

When a solid does not dissolves in a solvent, a suspension is formed. We can recover an insoluble solid from a mixture by filtration

Question 10

Applications of filtration?

Answer 10

Vacuum cleaners
Coffee makers
Water treatment → NEWater

Question 11

Function of evaporation to dryness?

Answer 11

Used to separate a soluble solid from a solid-liquid mixture

Question 12

Type of mixture for evaporation to dryness?

Answer 12

Soluble solid-liquid
E.g. Sodium chloride (common salt) & water

Question 13

Physical property that allow evaporation to dryness to take place

Answer 13

Solubility

Question 14

How does evaporation to dryness work?

Answer 14

When a solid dissolves in a solvent, a solution is formed. We can recover a soluble solid from its solution by evaporating the solution to dryness.
However, some substance like sugar will turn brown on heating (Crystallisation)

Question 15

Function of crystallisation?

Answer 15

Separating soluble solid from liquid
When substance decomposes on heating or can lose its water of crystallisation, we do not evaporate it to dryness
Instead, we crystallise it

Question 16

Type of mixture for crystallisation to take place?

Answer 16

Soluble solid-liquid
E.g. Sugar & water

Question 17

Physical property that allow crystallisation to take place

Answer 17

Solubility

Question 18

How does crystallisation work? (8 steps)

Answer 18

1. Dissolve solid in solvent (water) to obtain a solution
2. Filter the mixture, collect the filtrate
3. Heat filtrate to evaporate off most of the solvent
4. Use a clean glass rod to test whether the solution in the evaporating dish is saturated
5. Stop heating once the solution is saturated.
6. Cool the hot solution to room temperature allowing pure crystals to form
7. Filter the solution to separate the pure crystals from the remaining solution which may contain soluble impurities
8. Wash with some cold distilled water and dry crystals between pieces of filter paper

Question 19

What is a saturated solution?

Answer 19

A saturated solution contains the maximum amount of dissolved solute at a given temperature

Question 20

How to test for a saturated solution?

Answer 20

A clean glass rod can be used to test whether a solution is saturated.
It is dipped into the solution and removed. There will be a small amount of solution on the rod.
If small crystals form on the rod as the solution cools, the solution is saturated.
We say the solution is at its saturation point of crystallisation point

Question 21

Type of mixture for simple distillation to take place?

Answer 21

Soluble solid-liquid
E.g. Sand & water

Question 22

Physical property that allow simple distillation to take place

Answer 22

Solubility

Question 23

How does simple distillation work?

Answer 23

When a solid dissolves in a solvent, a solution is formed. We can recover the distillate from its solution by distillation
Distillation is the process of evaporation and condensation

Question 24

Type of mixture for fractional distillation to take place?

Answer 24

Miscible liquid-liquid
Unlike water and oil, ethanol nd water mix together completely to form a solution. They are said to be miscible.
If two liquids are miscible and have different boiling point, they must be separated by a technique called fractional distillation
E.g. liquids in crude oil alcohol and water

Question 25

Physical property that allow fractional distillation to take place

Answer 25

Solubility

Question 26

How does fractional distillation work?

Answer 26

A fractionating column, is attached to the round bottomed flask and the condenser for fractional distillation
Many glass beads/plates/spirals in the fractionating column provide a large surface area for vapour to condense on
Other than glass beads, a fractionating column may be filled with plates or a spiral.

Question 27

Describe the process of fractional distillation (4 steps)

Answer 27

1. In the round bottom flask, the mixture of ethanol and water boils and both liquids may distil together
2. Vapour of the liquid with higher boiling point condense along the fractionating column and re-enter the round-bottomed flask
3. Ethanol distils over. In the condenser, ethanol vapour condenses.
4. Ethanol is collected as the distillate

Question 28

How does the temperature change as a solution of ethanol and water undergo fractional distillation?

Answer 28

The temperature of the mixture increases as it is heated
At 78℃, ethanol distils over. The temperature remains constant until all the ethanol has distilled out of the round bottomed flask.
The temperature then increases until 100℃. At 100℃, water distils over. The temperature remains unchanged as water is being distilled.

Question 29

Industrial applications of fractional distillation

Answer 29

Fractional distillation is used in industries to obtain nitrogen, argon and oxygen from air
Fractional distillation can be used to separate mixtures of liquids such as crude oil.
Ethanol is formed when glucose solution undergoes fermentation in the presence of yeast. We can separate ethanol from glucose solution by fractional distillation.