3 - Separation and purification

Question 1

Define what is a mixture

Answer 1

Mixture is made up of 2< substances that are NOT chemically combined

Question 2

Possible states that can be separated from one another in a mixture

Answer 2

• Solid from liquid
• Solids
• Liquid from solution
• Liquids

Question 3

Describe how filtration is carried out

separating solid from liquid

Answer 3

1. Pour mixture into a filter funnel lines with filter paper
2. Collect filtrate (the liquid) in a beaker
3. Collect residue and dry it on a piece of filter paper

Question 4

What is TYPICALLY the main aim of filtration?

Answer 4

To get the residue

Question 5

What is the purpose of ‘evaporate to dryness’ ?

Answer 5

Obtain SOLUBLE SOLID from a solution until all the water is boiled off
(Not always pure as soluble impurities would be left with the solid)

Question 6

Define the purpose/ aim of crystallisation

Answer 6

Obtain pure (soluble) solid from solution

Question 7

Describe the procedure of crystallisation (5 steps)

Answer 7

1. Dissolve the solid in water
2. Filter to remove the excess insoluble substance
3. Heat the solution till its saturated (that’s when crystals start forming)
4. When solution is saturated, leave it to cool and crystallise
5. Filter to collect the crystals, wash it with a little cold distilled water to remove impurities, dry crystals with filter paper

Question 8

What is a solvent/ solute?

Answer 8

Solute - the solid

Solvent - The liquid that DISSOLVES the solute

Question 9

What are the common solvents?

Answer 9

Water and ethanol

Question 10

Purpose of sumbilation

Answer 10

To separate a solid that sublimes from one that does not

Question 11

Magnet (separation technique, purpose)

Answer 11

A magnet can be used to separate a magnetic substance from a non-magnetic substance

Question 12

Purpose of simple distillation

Answer 12

Separate a pure solvent (liquid) from a solution

Question 13

Describe the process of simple distillation

Answer 13

1. Solution boils in distillation flask
2. Boiling chips added to ensure smooth boiling
3. Water vaporise, rises and enters the condenser
4. In condenser, water vapour is cooled
5. Vapour condenses and changes back into pure water
6. Distillate is collected in the receiver (beaker/ flask)
7. Solid residue would be left in the flask

Question 14

Where the thermometer is put in simple distillation, and why it is put there

Answer 14

Procedure: Placed beside the side arm of the distillation flask
Reason: Ensures the thermometer measures the boiling point of the substance that is being distilled, and not the flame

Question 15

How should a condenser be placed in simple distillation set up

Answer 15

Procedure:

• Condenser should slop downwards
• Cold running water is allowed to enter from the bottom while the water exits from the top

Question 16

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

Answer 16

To ensure a permanent cool surface to condense the vapour effectively before it leaves the condenser

Question 17

Procedure and reason for a receiver (thing to collect distillate) in simple distillation

Answer 17

Procedure: If the distillate is volatile (easily evaporated), the receiver can be put in a tub of ice
Reason: Helps to keep the temperature of the distillate low so that it remains in the liquid state

Question 18

Purpose of a separating funnel

Answer 18

To separate 2 immiscible liquids (e.g water and oil)

Question 19

Describe the process of using a separating funnel

Answer 19

1. Pour the mixture of oil and water into the separating funnel
2. Support the separating funnel using a report stand, then put clean beaker below
3. Allow liquids to separate completely
4. Open tap of the funnel to allow the bottom liquid to drain into the beaker. Close the tap before the top layer runs out.
5. Place another clean beaker below and drain the rest of the top layer.

Question 20

Purpose of fractional distillation

Answer 20

Separate a mixture of miscible liquids with diff BOILING points.

Question 21

Purpose of tall fractionating column in fractional distillation

Answer 21

It increases TSA for condensation so liquids can be separated effectively (this is cus evaporation exists ._.)

Question 22

Describe the process of fractional distillation

e.g water and ethanol

Answer 22

1. Ethanol vapour and water vapour rise up the column as the solution is heated
2. The water vapour condenses in the fractionating column as the solution is heated
3. Ethanol, which has a lower boiling point than water, reaches the upper part of the column and is distilled over
4. The thermometer shows a constant temp. of [insert temp.] which is the boiling point of ethanol
5. In the condenser:
   - Hot ethanol vapour condenses as running water cools it;
   - Liquid ethanol glows down the inner tube of the condenser and into the receiver
6. Ethanol is collected as the distillate in the receiver

Question 23

Purpose of paper chromatography

Answer 23

Method to separate 2< components that dissolve in the same solvent (It’s a mixture i think)

Question 24

Describer the procedure of paper chromatography

Answer 24

1. Draw a line 1cm from the bottom of a strip of filter paper or chromatography
2. Put the [dye or smth] on the pencil line and allow the drops to dry
3. Dip paper into a glass tank containing the solvent. The solvent would then be soaked up by the paper to dissolve the [dye, or ink or smth]
4. Leave the apparatus to stand for a while. The solvent would travel up the paper, carrying the [dye/ ink] along. Which every is more soluble would travel up higher.

Question 25

Types of solvent for paper chromatopography

Answer 25

• water (universal solvent)

- Ethanol (organic solvent)

Question 26

How to calculate the Rf value?
AND
What is an Rf value?

Answer 26

Distance travelled by the substance/ distance travelled by the solvent
(Note: 3sf, value is equal to or less than 1)

It is the retention factor of the substance (because the distance the substance travelled is dependent on the duration of experiment)

Question 27

Why must the level of the solvent be lower than the starting line?

Answer 27

So the sample does not dissolve in the solvent before the separation

Question 28

Uses of paper chromatography

Answer 28

• Separate the components in a sample
• Identify components present in a sample
• Identify substances
• Determine whether a sample is pure

Question 29

How to determine the purity of a substance?

Answer 29

Melting point (solid) / boiling point (liquid)

Question 30

How would impurities affect m.p/ b.p

Answer 30

Lower melting point (solid) Increase boiling point (liquid)

[Note: this applies MOST of the time]