Separating mixtures (Paper chromatography) Flashcards
Define the term ‘chromatography’
Chromatography is the process wherby small amounts of dissolved substances are separated by running a solvent along a material such as absorbent paper.
Give an example of separation techniques
Chromatography
Filtration
(Evaporation)
Crystallisation
Simple distillation
Fractional distillation
What type of processes are all separation techniques and how?
All separation techniques are physical processes. This means that they do not involve chemical reactions and no new substances are made
Give an example of a type of chromatography
A type of chromatography is paper chromatography
Describe the process of paper chromatography
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.
Why is the starting line drawn in pencil
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
Why is the level of the solvent in the beaker below the sample line
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.
What is the stationary phase in the paper chromatography experiment
In this experiment, the stationary phase is the chromatography paper.
What is the mobile phase in the paper chromatography experiment and why?
In this experiment, the mobile phase is the solvent (e.g. water) . because the solvent moves
During the paper chromatography experiment what happens to the mobile phase
During the experiment, the mobile phase absorbs up the chromatography paper (stationary phase)
Define solvent
A solvent is a liquid in which another substance can be dissolved
Define solute
A solute is a substance dissolved in a solvent to make a solution
Define stationary phase
In Chromatography, the stationary phase is a solid where molecules are unable to move.
Define mobile phase
In Chromatography, the mobile phase is a liquid where the molecules are able to move.
Define solvent front
The solvent front is the distance the solvent (mobile phase) has travelled up the Chromatography paper (stationary phase).
Why does paper chromatography work
how is separation caused
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.
State two factors that affect the rate of separation of substances in paper chromatography
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)
Explain how samples move at different speeds. (Explain separation) in terms of the mobile phase and the stationary phase
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.
In paper chromatography why do colours smudge
If the colours go at the same speed, it will cause the colours to smudge
What happens if any substances in the spot are insoluble
If any substances in the spot of mixture are insoluble in the solvent used, then those substances will stay on the baseline.
What other things can be done to maximise separation
Different solvents can be used to maximise separation (this can also fix smudging)
What will a pure substance produce in all solvents
In all substances, a pure substance will produce one pot on the chromatography paper
for a pure substance, how many spots does it produce
A pure compound will produce one spot in ALL SOLVENTS
for compounds in a MIXTURE , how many spots does it produce - does it vary?
Compounds in a mixture may separate into different spots depending on the solvent
If a colour has separated into two different spots, what does it tell us about the colour
It tells us that this colour was a mixture of two different colours
What is the Rf value
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.
What does Rf (subscript f) stand for
Rf stands for retention factor.
what is the equation for the Rf value
Rf = distance travelled by the spot/distance travelled by the solvent
How do you measure the distance the spot has travelled
You measure the distance from the starting line to the centre of the spot.
How do you measure the distance the solvent has travelled
distance travelled by solvent = solvent front - starting line
