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Substances appear coloured when visible light energy is absorbed by an atom, ion or molecule.

Transition metal ions with incomplete 3d sub-levels form coloured complexes.

For example, copper(II) ions have only nine electrons in their 3d sub-level. The horizontal line represents their energy level before binding to a ligand.

When ligands bind to the copper(II) ion, the 3d sub-level is split to form two slightly different energy levels.

Three orbitals are at a lower energy level and two orbitals are at a higher energy level.

Three orbitals are at a lower energy level and two orbitals are at a higher energy level.

An electron in the lower 3d sub-level can absorb energy from visible and ultraviolet light. This causes it to be excited to the higher 3d sub-level.

The difference in energy,  E, is proportional to the frequency of light absorbed. It determines the colour of the complex.

The difference in energy,  E, is proportional to the frequency of light absorbed. It determines the colour of the complex.

The energy difference is equal to Planck's constant, h, multiplied by the frequency of the absorbed radiation.

Hexaquacopper(II) ions absorb red light between 605 nm and 750 nm, so they produce blue solutions.

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White light contains all the frequencies of the visible spectrum.

The frequency of the light increases from the red end of the spectrum to the blue end of the spectrum.

The frequency of the light increases from the red end of the spectrum to the blue end of the spectrum.

It is common when discussing visible light to use wavelength, rather than frequency.

Blue light has a shorter wavelength than red light.

The colours of the spectrum are often shown as a colour wheel when considering absorption of light.

Colours on the opposite side of the wheel are complementary.

If a substance absorbs red light, for example, it will appear blue-green.

What happens when white light passes through a transparent, colourless solution?

No wavelengths are absorbed and white light emerges from the other side.

What happens when white light passes through a transparent, coloured solution?

Some wavelengths are absorbed. In this example, these correspond to red and yellow light.

Some wavelengths are absorbed. In this example, these correspond to red and yellow light.

The light that emerges from the other side is the complementary colour to the absorbed colours. So a solution that absorbs red light appears blue-green.

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