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Background

Before the First Opium War, the Chinese official Lin Zexu destroyed confiscated opium by mixing it with salt, quicklime, and water and then throwing the result into the sea. I (not a chemist) seek to confirm my understanding of what must have happened (before I mislead my students about it). Ideally, I also want to understand the advantages of this method over "normal" boiling.

My understanding

First, to my understanding, the salt plays no chemically important role. Except perhaps raising the boiling temperature of the water, so that the opium is boiled even more hotly.

Lin must have incited the reaction

$\ce{CaO + H2O —> Ca(OH)2}$

in a receptacle also containing the opium balls.

This, as I have seen, is highly exothermic. Meaning, he now had opium in boiling water. He also got $\ce{Ca(OH)2}$, which is solid and denser than water, meaning it sinks!

What is the advantage of boiling the opium this way vs conventional boiling over fire? Would the violence and quickness of the reaction allow the water to superheat?

After reading about Portlandite, I thought that Lin‘s goal might have been to bind the opium balls inside the solid $\ce{Ca(OH)2}$, so that everything would sink, once thrown into the sea. Am I correct in now assuming that this would not happen and that the $\ce{Ca(OH)2}$, though solid, would just be a white dust, in no way aiding the sinking of the balls?

In case it offers any clues, this is a picture of the process found on Wikipedia: enter image description here

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  • $\begingroup$ Why do you care about a substance which is usually considered as a drug ? $\endgroup$
    – Maurice
    Oct 7, 2022 at 19:04
  • $\begingroup$ C'mon, quicklime is pretty tame, @Maurice... $\endgroup$ Oct 7, 2022 at 22:10
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    $\begingroup$ It is quite possible particular processes did not matter. The purpose could be just a spectacular destruction by mixing with unpleasant additives and then disposing it in the sea. Simple boiling would be far less dramatic. $\endgroup$
    – Poutnik
    Oct 8, 2022 at 2:47

1 Answer 1

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I am not familiar with the material properties of opium in 19th-century China, but I am going to assume it was like most other unrefined natural products: full of cellulose and sticky fatty acids.

There are a few reasons quicklime was likely used to destroy the opium.

  1. Quicklime increases the pH of the resulting solution (i.e., make it more basic or alkaline), which would also make the actual narcotic opiate molecules in the opium soluble in water, so when they are thrown into the sea the drugs don't just float around waiting to be picked up (at night, in boats, with nets?).
  2. Bases react with fatty acids to make soaps in a process called saponification. This would make the opium less sticky so they would be broken up more easily by the mechanical action of the boiling water, and it would increase the solubility of the plant material. This is the same way that detergent works in your dishwasher to dissolve and wash away greasy organic matter.
  3. The calcium ions in quicklime, not found in lye (i.e., caustic soda; NaOH or KOH), will make the boiling water harder and cause less foaming, which would probably make a huge mess otherwise. So not only would the addition of quicklime cause the water to heat and boil without a fire, it would keep it from boiling over.

At the end of the day, all this would make it easier and faster to get rid of more opium.

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