A precipitate is an insoluble solid that emerges from a liquid solution. The emergence of the insoluble solid from solution is called precipitation. Often the precipitate emerges as a suspension.



Precipitates form when reactions produce salts with low solubility products

applejuice.jpg Apple juice is improved by removal of undesirable substances as a precipitate. Let's hope there's no strychnine in it!

Precipitates can form when two soluble salts react in solution to form one or more insoluble products - as discussed in the famous example below. The insoluble product separates from the liquid and is called a precipitate.

Precipitates can also form when the temperature of a solution is lowered. The lower temperature reduces the solubility of a salt, resulting in its precipitation as a solid.

Example: A Common Precipitation

If you've ever made wine, beer, or fruit juices, one of the stages you'll often go through is fining, which removes dissolved substances that would otherwise worsen the taste of the drinks.

A fining agent binds to soluble substances such as proteins, polyphenols and sulfides, causing them to form a precipitate which drops to the bottom of the liquid for easy removal. Fining agents include egg whites and bentonite.

Example: A Famous, Uncommon Precipitate

A precipitate was, famously, and cleverly, the means by which Agatha Christie's first ever fictional murder was achieved by strychnine poisoning in The Mysterious Affair at Styles - her first novel.

The victim was taking medicine which contained a small amount strychnine. There was too little strychnine in each dose to cause harm. However, if all of the strychnine in the bottle were taken at once...!!

The deadly insoluble salt - strychnine bromide - was formed by the reaction of two otherwise soluble salts - strychnine sulfate and potassium bromide.

"Do you mean that the murderer introduced the strychnine into her tonic?" Captain Hastings cried.

And Hercule Poirot explained: "There was no need to introduce it. It was already there - in the mixture. The strychnine that killed Mrs. Inglethorp was the identical strychnine prescribed by Dr. Wilkins.

This solution deposits in a few hours the greater part of the strychnine salt as an insoluble bromide in transparent crystals. A lady in England lost her life by taking a similar mixture.

The precipitated strychnine collected at the bottom, and in taking the last dose she swallowed nearly all of it!

Now there was, of course, no bromide in Dr. Wilkins' prescription, but you will remember that I mentioned an empty box of bromide powders. One or two of those powders introduced into the full bottle of medicine would effectually precipitate the strychnine, as the book describes, and cause it to be taken in the last dose. You will learn later that the person who usually poured out Mrs. Inglethorp's medicine was always extremely careful not to shake the bottle, but to leave the sediment at the bottom of it undisturbed.








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