Friday is here, and with it the weekly column "A Taste of Science for the Weekend" — issue no. 52.
This week: why the original color of your jeans is actually yellow, and the connection to batteries, the death penalty, and even a Nobel Prize.
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Indigo is an elusive color, somewhere between blue and violet, and humanity has been using it for thousands of years.
Natural indigo is extracted from plants through a biochemical process that continues to this day, albeit on a very limited scale.
In ancient times, its production was highly complex, and it was considered exceptionally rare and precious.
A few hundred years ago, Eastern indigo was banned from import into Europe — despite its superior quality — because it threatened local indigo production.
Illegally importing indigo into Germany was even punishable by death.
Synthetic indigo was one of the first artificial dyes ever synthesized, and it earned the chemist who discovered it a Nobel Prize.
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Indigo has one problem — it is insoluble in water.
Indigo molecules are nonpolar — they have no positive or negative pole, and therefore do not bond with water.
To make indigo react with water, sodium hydrosulfite is added, which donates 2 electrons to the indigo molecules and converts them into polar molecules.
Sodium hydroxide is also added to the water, raising the basicity (lowering the acidity) to the level required for the chemical process to take place.
The change in molecular structure also causes a change in the light frequencies the molecules absorb, which is why the solution turns yellow. Indigo in this form is called *leuco-indigo*.
At this stage, the fabric can easily be dyed using the solution.
Once dyeing is complete, the fabric is taken out to dry, and as it is exposed to air, it transfers the excess electrons to oxygen molecules and reverts to ordinary indigo in its original blue-violet color.
The fact that indigo does not react with water is actually an advantage — it keeps the dye locked within the fabric fibers for years, even after many washes.
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The process of accepting and donating electrons is a particularly common chemical process known as *oxidation-reduction* (redox).
The reducing agent donates electrons to the oxidizing agent, which accepts them.
Chemical batteries and the formation of rust are two other everyday examples of this process.
Shabbat Shalom 😊
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video credit: @AdvancedTinkering