Required Practicle 6 Chromatography

Question 1

What is the practicle for

Answer 1

Showing how paper Chromatography can be used to identify substances

Question 2

Substances used in experiment

Answer 2

Sample of food colouring that is a mixture of chemicals called U for unknown

Four known food colourings U could contain ( A - D)

Question 3

Setting up experiment

Answer 3

Use ruler to draw horizontal line around 2 cm up the Chromatography paper in pencil as pencil is insoluble

Draw 5 pencil spots the same distance apart from 1 another, leaving adleasst a cm on the left and right of the paper

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

Spots should be small to prevent them spreading into each other

Question 4

Method

Answer 4

Pour water ( solvent) into a beaker to a depth of 1cm

Attach paper to a glass rod using tape and Lowe the paper into the beaker. The bottom of the paper should dip into the water

Pencil lines and ink must be above water
The sides if the paper mustn’t touch the sides walls of the beaker
Put lid onto beaker to reduce evaporation of solvent

The dyes will start moving up the paper

Remove paper when water is about 3 quarters up and draw a pencil line where the water reached

Hang the paper up to dry

Compare the dyes ro see which dies U contains

Question 5

How to find the chemicals in the colours

Answer 5

Find rf value

Measure distance of pencil line to centre of each spot

Measure distance moved by water

Calculate rf value = distance moved by chemical / distance moved by solvent

Rf values don’t have units

Can look the rf value up in a database and it will identify the chemical

Several different chemicals have the same rf value so we may have to repeat experiment using different solvent to narrow chemicals down futher

If chemical hasn’t been analysed there won’t be an rf value