Required Practicals - Chromatography

Question 1

What is the goal of the chromatography practical?

Answer 1

To investigate how paper chromatography can be used to separate and identify chemicals in a mixture.

Question 2

What is the apparatus list for the chromatography practical?

Answer 2

-250cm3 beaker
-wooden spill
-rectangle of chromatography paper
-four known food colourings labelled.
-an unknown mixture of food colourings.
-5 glass capillary tubes.
-Paper clip
-Ruler
-Pencil

Question 3

What is the method for the chromatography practical?

Answer 3

-Use a ruler to draw a pencil line from the bottom short edge of the chromatography paper, this is the origin line.
-Mark five pencil spots at equal intervals along the origin line.
-Use a glass capillary tube to put a small spot of each colouring on four of the pencil spots. Use a different tube for each colouring. Use the 5th tube to put a small spot of the unknown mixture on the fifth spot.
-Pour water into the beaker to a depth of no more than 1cm.
-Clip the top short edge of the chromatography paper to the wooden spill.
-Carefully rest the wooden spill on the top edge of the beaker. It should dip into the water solvent.
-Wait for the water solvent to travel at least 3/4 of the paper, do not disturb the beaker during this time.
-Caredully remove the paper from the beaker. Draw another pencil line on the dry part of the paper as close to the edge as possible. This is the solvent front.
-Hang the paper to dry.
-Measure the distance in mm between the two lines.
-For each food colour, measure the distance in mm from the start line to the middle of the spot.
-Record the measurements.

Question 4

How do you calculate Rf value?

Answer 4

distance moved by solute / distance moved by solvent

Question 5

What do we have to find out in the chromatography practical?

Answer 5

An unknown sample of food colouring. We name this U.

Question 6

What do we have to compare with the unknown food colourings in the chromatography practical?

Answer 6

Four known food colourings named A-D

Question 7

What do we do first to the chromatogram in the chromatography practical?

Answer 7

Use a ruler to draw a horizonatal pencil line on the paper. This should be around 2cm from the bottom of the paper.

Question 8

What do we do after drawing the line on the chromatogram in the chromatography practical?

Answer 8

Mark five pencil spots, equidistant from each other, across the line. Leave at least 1cm clear on each side.

Question 9

What do we do after marking the pencil spots in the chromatography practical?

Answer 9

Use a capillary tube to put a small spot of each of the known food colours and the unknown colour onto the pencil spots.

Question 10

What is a capillary tube?

Answer 10

A very thin glass tube.

Question 11

Why do we keep spots small in the chromatography practical?

Answer 11

To prevent the colours from spreading into each other.

Question 12

What do we do after placing the food coloruing on the chromatogram in the chromatography practical?

Answer 12

Pour water into a beaker to a depth of 1cm

Question 13

What is the water in the chromatography practical?

Answer 13

The solvent

Question 14

What do we do after pouring water in a beaker in the chromatography practical?

Answer 14

Attach the paper to a glass rod using tape and lower the paper into the beaker.

The bottom of the paper should dip into the water.

Question 15

Where should the pencil line be, relative to the water, in the chromatography practical?

Answer 15

Above the water level

Question 16

Why mustn’t the paper touch the side walls of the beaker in the chromatography practical?

Answer 16

It will interfere with the way the water moves.

Question 17

Why do we place a lid on the beaker in the chromatography practical?

Answer 17

To reduce evaporation of the solvent

Question 18

When do we remove the paper in the chromatography practical?

What do we do when we do this?

Answer 18

When the water has travelled around 3/4 of the way up. Mark the point where the water reached with a pencil.

Question 19

Where will the spots be for the unknown food colouring in the chromatography practical?

Answer 19

At different points going up the paper.

Question 20

What does it mean if two spots, one unknown and one known, travel the same distance in the chromatography practical?

Answer 20

They are the same chemical

Question 21

How can we use rF value?

Answer 21

If we look it up in a database we can see the identity of the chemical moving on the chromatogram.

Question 22

What is the problem with rF value?

Answer 22

Several chemicals may have the same or similar rF values.
So we may need to repeat the experiment using a different solvent to narrow it down further.

Question 23

What is the problem with using rF value with new chemicals?

Answer 23

If they haven’t been analysed before, there will not be an rF value to use on the database.