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Why is common salt sometimes yellow instead of pure white?

Last updated date: 25th Jul 2024
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Hint :Common salt is a mineral primarily made of the compound sodium chloride (NaCl). In its natural form, it is a crystalline mineral. The colour of a substance depends on its contents and the colour could change if there is a change in the way the electrons are placed or behave.

Complete Step By Step Answer:
Once light touches the surface, the material or substance absorbs this light at certain wavelengths and the wavelengths that are not absorbed, get reflected and our eyes receive those.
Therefore, colour in materials or compounds is caused when light reaching the eyes has some of the wavelengths removed by absorption.
The intensity of absorption and the specific wavelengths being absorbed depends on the substance or compound in question.
This is because absorption and reflection of light depends on the number of electrons, their behaviour and their placement in the constituting elements. Thus, the colour is different for different materials.
Now, the compound in the questions is common salt.
Common salt is a naturally-occurring crystalline mineral that is primarily made of the compound sodium chloride (NaCl).
Generally, it is a translucent mineral that appears white. However, sometimes it could appear yellow instead of pure white.
This change in colour is mainly due to the formation of F-centres.
For NaCl, with heat or other environmental conditions, the concentration of the metal Sodium (Na) could increase in the compound. To balance this increase – as the amount of chloride remains the same – unpaired electrons occupy the empty lattice sites, which gets the compound to a neutral charge.
These sites, where the anion chloride was supposed to be originally, are called F-centres.
These F-centres get excited with the absorption of light energy (mainly blue wavelengths) and impart another colour to the salt.
Therefore, common salt (NaCl) sometimes appears yellow instead of pure white.

Additional Information:
Such a formation of F-centres is common in other chloride compounds as well. For example, KCl, originally white, gives a violet colour and HCl, usually colourless, gives a pink colour.

Note :
You need an understanding of how electrons behave and their effects on the compound shape, colour, reactivity etc. Also, F-centres is an important concept to be learned and with this, learning a few examples of the resulting phenomenon will help you answer such questions.