NCERT Solutions for Class 9th Science Chapter 5 .3.3 Paper Chromatography — Activity 5.5: Let us investigate

Book page 825 Updated on2026-09-08

Q1.

Observe the paper as the water rises through the paper. What do you notice?

Answer

The water creeps steadily upwards through the paper, well above the level in the jar, and when the wet front reaches the ink spot it begins to drag the ink up with it. The single black spot stretches into a streak, and colours start to appear at the edges.

Why water climbs the paper: Chromatographic paper (and filter paper) is a mesh of fine cellulose fibres with countless narrow channels between them. Water molecules are attracted to the cellulose (adhesion) and to one another (cohesion), so they are pulled into these narrow channels and rise against gravity. This is capillary action — the same reason a cotton wick draws oil up in a lamp, and a towel soaks up water.

Tip: The line must be drawn with a pencil, not a pen. Pencil is graphite, which is insoluble and does not travel with the solvent, so it cannot spoil the result.

Q2.

As the water rises, the ink starts to separate into different colour spots. What can you infer from this?

Answer

That the “black” ink is not a single substance at all — it is a mixture of several coloured dyes, and each dye travels up the paper at its own speed.

Because separate coloured spots appear, at least that many different components are present.
The number of spots tells you how many components the ink contains.
The spot that has travelled farthest is the component least held by the paper and most attracted to the water.

Why the components separate: Every dye is caught between two competing pulls — the paper (the stationary phase) holds it back, while the rising water (the mobile phase) carries it forward. A dye that dissolves readily in water and clings only weakly to the cellulose is swept far up the strip. A dye that clings strongly to the paper and dissolves poorly is left near the start. Since each dye has its own balance of these two attractions, each ends up at a different height, and what began as one spot becomes a row of separate spots.

Try this: Repeat with green food colour, or with the green extract of spinach leaves, or with the colours of flower petals. For some of these, water will not do the job and you will need alcohol, or a mixture of solvents, as the mobile phase.

Was this helpful? Report an error