Christmas with a bang

Amanda Edward
20 December 2013

Above: Image ©istockphoto.com/mightyisland

As the Christmas season approaches, stores fill their shelves with various holiday items, including Christmas crackers that are lined up in boxes, looking like empty toilet paper rolls wrapped in shiny foil. A tradition in many homes, these crackers contain prizes such as chocolates, paper hats, jokes, or small toys. They also make a loud bang when pulled apart at their ends, which you can think of as a little holiday gift from chemistry to you.

Did you know? Silver fulminate is so volatile that it can even detonate under water!

Originally produced in 1847, Christmas crackers were named “Cosaques” by their inventor, the English confectioner Tom Smith. He was inspired by the bonbons—specifically, sugared almonds wrapped in tissue paper—he had seen on a trip to France. He thought the candies would be even more attractive to buyers if he hid love messages in the packaging. But he still felt that an extra something special was needed to really boost sales.

Smith’s eureka moment came while staring at a crackling log in the fireplace. He modified the packaging so it would make a bang when pulled apart, transforming the familiar bonbon into the iconic Christmas cracker. Even today, the cracker’s basic design remains unchanged.

The secret behind the bang is literally inside every cracker. Each one is lined with a strip of paper treated with an explosive and shock-sensitive substance known as silver fulminate, or AgCNO. This substance is particularly unstable because of how the four atoms in each of its molecules—one each of silver, carbon, nitrogen, and oxygen—are arranged. Specifically, the unusual arrangement of the molecule’s electrons causes it to react very easily. In fact, silver fulminate is so sensitive that even the small amount of friction created by two people pulling on either end of a Christmas cracker causes it to immediately and violently break apart.

Did you know? The small fireworks that make a popping noise when thrown to the ground are filled with minute amounts of silver fulminate. The explosion is so small that although it produces a dramatic noise, the package containing the silver fulminate actually stays intact!

In some crackers, the explosion is caused by friction between the treated strip and the cracker roll itself. In others, there are two strips: one is coated with silver fulminate and the other has a sandpaper-like consistency to increase friction. In either case, idea remains the same: friction causes a tiny detonation that provides a festive bang.

Rest assured that the products of the explosion are largely harmless: very small amounts of silver metal, carbon monoxide, and nitrogen gas. The silver from a Christmas cracker won’t ever make you rich and the carbon monoxide won’t activate your home’s detector because only minute amounts of silver fulminate are used.

In fact, silver fulminate is so explosive that only small amounts can be synthesized (created in a lab) at any given time. Typically, each synthesis will yield less than a few milligrams of the substance. In larger amounts, silver fulminate can be detonated by its own weight or even by the static in the surrounding air. Thankfully, the amounts needed to produce Christmas cheer present no real danger to you or your loved ones.

Silver cyanate is made up of the same atoms as silver fulminate. However, the atoms are arranged very differently. As a result, silver cyanate is much less temperamental and would never produce a satisfying bang at Christmastime. It’s like having four different coloured pieces of Lego: depending on how these pieces are arranged, the result can be very different.

In fact, it the discovery of silver fulminate helped chemists develop the concept of isomerism. Silver fulminate and silver cyanate are structural isomers of one another. That means they are both are made of exactly the same number and types of atoms, but the atoms are arranged differently.

The next time you pull on the end of a Christmas cracker, be sure to share the tale of silver fulminate. Your fellow revellers will be excited to learn that silver, carbon monoxide, and nitrogen gas are among the tiny gifts they just released from their crackers. You may also wish to tell the tale of isomerism to further impress your guests while creating a bang!

References:

General information

Scholarly publications

  • Jackson D, Dicks A. 2012. The Five Senses of Christmas Chemistry. The Journal of Chemical Education. 89:1267-1273.
  • Kurzer F. 2000. Fulminic Acid in the History of Organic Chemistry. The Journal of Chemical Education. 85:1201-1203.

Amanda Edward

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