Scienze e Viaggi

Simple substances, compound substances and paper chromatography

In order to make the chromatography we have cut a little strip of filter paper and we have rolled up it on a pencil.

Then we have put down a drop of black ink of a fountain pen, we have supported it over the beaker and gradually we have added water until grazing the little strip.

Observations:
Even if we have only grazed the water to the paper, the water is gone up, and slowly it has made to go up also the ink. The spot has become grey - yellow, therefore the ink is not an only substance because there are two different colours and two different substances at least.

We used green ink, also. The climbing colour is more dark and has many shadings. After the inferior part becomes white and where there was the ink spot you can see in transparency.

Instead the blue ink climbs little and slowly.

We have taken another beaker, we have cut a little strip of paper with to the extremity one drop of blue ink of a biro. We have poured ethanol in the beaker until touching the strip.

The alcohol climbed fastly carrying with itself a part of the clearer spot of colour. A part of the colour has been melted in the alcohol.

We have taken a tripod and supported over a filter paper with a spot of black biro ink. With a pipette we have made to fall some drops of ethanol, it has expanded the spot and lightened the ink progressively.

The black ink isn't a simple substances, you can see two zones: one very external, and one in the centre of the paper, with two different colours.

Here you can see a blue biro ink: it seems a simple substance.

Look what happen with an ink of a felt tip!:

The paper chromatography

This type of chromatography is a surface chromatography: this means that the substances move through a flat surface that can be horizontal or vertical. First of all you put a drop of the substances to study on the paper, after to have evaporated the arranged solvent from the spot, the extremity of the strip is introduced in a little tank with an adapted solvent.

The substances move along the paper: they go up for capillarity or they move horizontally. The solvent is moved along the sheet and after to be arrived to the height of the sample it drags the substances with it in various way depending on their characteristics. After sometimes, the strip is extracted and it lets to dry in airflow (temperature ambient) so as to to be able to see the various colours.