Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

Colour cheese

The ‘colour cheese’ summarizes the relationship between the wavelength of light

most strongly absorbed by a substance or solution, and its colour in white light. To

use the ‘colour cheese’ (Fig. 20.11):

1.Locate the approximate wavelength of light that is absorbed. The colour of the

compound is shown in the opposite segment. Applying this to manganate(VII)

ions, the absorbed light is yellow-green and so the solution appears violet.

Reversing the procedure, the coloured component of carrots ( -carotene) must

absorb blue-green light in order to appear orange.

2.Remember that absorptions outside the visible range (i.e. not listed in the cheese)

are irrelevant to the colour of the compound. It follows that colourless substances

(such as water) do not absorb in the visible wavelength range (400–720 nm).

3.Note that the colour cheese only works if one colour is absorbed more strongly

than the others. If very different colours are absorbed equally strongly, the

observed colour depends upon the sensitivity of our eyes to the colours involved.

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Fig. 20.10Potassium manganate(VII) solution is violet because it

absorbs yellow-green light.

Dark blue

420–450

Blue–green

450–490 nm

Green

490–530 nm

Yellow

545–580 nm

Orange

580–630 nm

Red

630–720 nm

Yellow–green

530–545 nm

Violet

400–420 nm

Predicting colours using the colour cheese

(i) Chlorophyll compounds are green coloured. What colour of light do they strongly absorb?

(ii) Copper(II) sulfate solution absorbs orange light strongly. What colour would you expect its

solution to be?

(iii)Benzene shows a strong absorption between 200 and 250 nm, but does not absorb

between 400 and 720 nm. Predict its colour.

Exercise 20H

Fig. 20.11The colour cheese.