Explain the technique to separate dissolved solids which are present in a solution in very small quantities?

Solution

Chromatography is a technique of separating two (or more) dissolved solids which are present in a solution in very small quantities. Let us perform an activity to obtain different dyes from black ink by chromatography.

The dye in the black ink is not a single colour, but it is a mixture of 2 or 3 colours. In other words, black ink is a mixture of different colour dyes (solute) in water (solvent), which can be easily identified with the help of paper chromatography.

Paper chromatography is the technique used for separation of those solutes that dissolve in same solvent. The separation by this technique is based upon the principle that though, two (or more) solutes are soluble in the same solvent, but their solubilities may be different.

Procedure : Take a thin strip of filter paper. Draw a line on it with a pencil nearly 3cm above the lower edge. Put a small drop of black ink with the help of pen at the centre of line and let it dry. Suspend the filter paper into a glass containing water so that spot of ink on the paper is just above the water level (figure). Leave it undisturbed for some time.

Observation : The water gradually rises up the filter paper by capillary action and different coloured dyes present in the ink get separated as shown in figure.    

Separation of different coloured dyes by chromatography

Discussion: As water rises up the filter paper, it takes along with it dye particles present in ink. Since, ink is a mixture of two or more coloured dyes, the dye which is more soluble in water, rises faster and produces a coloured spot on the paper at a higher position.

The less soluble dyes present dissolve a little later, rise slower and form coloured spots at lower heights. In this way all the dyes present in black ink get separated (by forming separate coloured spots).

Conclusion: Separation of different coloured dyes present in black ink occurs on a chromatographic paper due to their different solubilities in water.

Hence, from above experiment we conclude that, the dye in black ink is not a single colour, but a mixture of 2 or 3 colours.