Separating mixtures (Paper chromatography)

Question 1

Define the term ‘chromatography’

Answer 1

Chromatography is the process wherby small amounts of dissolved substances are separated by running a solvent along a material such as absorbent paper.

Question 2

Give an example of separation techniques

Answer 2

Chromatography
Filtration
(Evaporation)
Crystallisation
Simple distillation
Fractional distillation

Question 3

What type of processes are all separation techniques and how?

Answer 3

All separation techniques are physical processes. This means that they do not involve chemical reactions and no new substances are made

Question 4

Give an example of a type of chromatography

Answer 4

A type of chromatography is paper chromatography

Question 5

Describe the process of paper chromatography

Answer 5

1) Draw a line near the bottom of a sheet of chromatography paper (A pencil should be used as the pencil marks are insoluble and won’t dissolve in the solvent)

2) Use a capillary tube to dab a spot of the solution on a pencil line near the bottom of the absorbent chromatography paper

3) Place the chromatography paper, so that it is standing, in a solvent at the bottom of a beaker.
Make sure the spot isn’t touching the solvent - so as to not dissolve the spot into the solvent

4) Place a lid on top of the container to stop the solvent from evaporating

5) Allow the solvent to soak up the paper, running through the spot of mixture

The relative solubility of the components making up the mixture in the solvent and the attraction the spot of mixture has to the paper will determine how far each spot of mixture will travel up the paper.

The more soluble a substance is in the solvent, and the less affinity a substance has for the chromatography paper, the further up the paper it is carried.

If any substances in the spot of mixture are insoluble in the solvent used, then those substances will stay on the baseline.

6) When the solvent has nearly reached the top of the paper, take the paper out of the beaker, and leave it to dry.

Question 6

Why is the starting line drawn in pencil

Answer 6

The starting line is drawn in pencil as pencil marks are insoluble and won’t dissolve in the solvent.

if the line was drawn in pen, the pen ink could dissolve in the solvent and move up the paper

Question 7

Why is the level of the solvent in the beaker below the sample line

Answer 7

The level of the solvent in the beaker is below the sample line so that samples don’t dissolve into the solvent in the beaker.

Question 8

What is the stationary phase in the paper chromatography experiment

Answer 8

In this experiment, the stationary phase is the chromatography paper.

Question 9

What is the mobile phase in the paper chromatography experiment and why?

Answer 9

In this experiment, the mobile phase is the solvent (e.g. water) . because the solvent moves

Question 10

During the paper chromatography experiment what happens to the mobile phase

Answer 10

During the experiment, the mobile phase absorbs up the chromatography paper (stationary phase)

Question 11

Define solvent

Answer 11

A solvent is a liquid in which another substance can be dissolved

Question 12

Define solute

Answer 12

A solute is a substance dissolved in a solvent to make a solution

Question 13

Define stationary phase

Answer 13

In Chromatography, the stationary phase is a solid where molecules are unable to move.

Question 14

Define mobile phase

Answer 14

In Chromatography, the mobile phase is a liquid where the molecules are able to move.

Question 15

Define solvent front

Answer 15

The solvent front is the distance the solvent (mobile phase) has travelled up the Chromatography paper (stationary phase).

Question 16

Why does paper chromatography work

how is separation caused

Answer 16

Paper chromatography works because some substances in the spot of mixture will dissolve better than others in the solvent (mobile phase) chosen and also will be more attracted to the stationary phase than others. This causes separation of the substances.

Question 17

State two factors that affect the rate of separation of substances in paper chromatography

Answer 17

The two factors that affect the rate of separation in paper chromatography are:
Solubility in the solvent (mobile phase)
Attraction to the chromatography paper (stationary phase)

Question 18

Explain how samples move at different speeds. (Explain separation) in terms of the mobile phase and the stationary phase

Answer 18

Different samples move at different speeds due to the solubility of the sample in the solvent and the attraction the sample has to the paper. The more soluble the sample is in the solvent and the weaker the attraction the sample has to the paper (stationary phase), the further up the paper it is carried.

Question 19

In paper chromatography why do colours smudge

Answer 19

If the colours go at the same speed, it will cause the colours to smudge

Question 20

What happens if any substances in the spot are insoluble

Answer 20

If any substances in the spot of mixture are insoluble in the solvent used, then those substances will stay on the baseline.

Question 21

What other things can be done to maximise separation

Answer 21

Different solvents can be used to maximise separation (this can also fix smudging)

Question 22

What will a pure substance produce in all solvents

Answer 22

In all substances, a pure substance will produce one pot on the chromatography paper

Question 23

for a pure substance, how many spots does it produce

Answer 23

A pure compound will produce one spot in ALL SOLVENTS

Question 24

for compounds in a MIXTURE , how many spots does it produce - does it vary?

Answer 24

Compounds in a mixture may separate into different spots depending on the solvent

Question 25

If a colour has separated into two different spots, what does it tell us about the colour

Answer 25

It tells us that this colour was a mixture of two different colours

Question 26

What is the Rf value

Answer 26

The Rf value is the distance the spot of a substance has been carried above the baseline divided by the distance of the solvent front.

Question 27

What does Rf (subscript f) stand for

Answer 27

Rf stands for retention factor.

Question 28

what is the equation for the Rf value

Answer 28

Rf = distance travelled by the spot/distance travelled by the solvent

Question 29

How do you measure the distance the spot has travelled

Answer 29

You measure the distance from the starting line to the centre of the spot.

Question 30

How do you measure the distance the solvent has travelled

Answer 30

distance travelled by solvent = solvent front - starting line