Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
5. Things That Catch Fire or Go Bang
What do cap guns have in them that make them explode?
Several compounds can be used. A contact explosive made from phosphorus, sulfur, and potassium chlorate will explode when hit with the small hammer in a cap gun. This is similar to the mixture used in “strike anywhere” matches. An early formula (from 1926) used potassium chlorate, sulfur, and antimony. You can tell if a cap has sulfur in it by the smell it makes (like a struck match) when it explodes.
As mentioned on page 105, contact explosives are those that are easy to set off. They’re made from compounds, such as potassium chlorate or potassium perchlorate, that are very good oxidizing agents. Fuels that are very easily oxidized, such as sulfur, phosphorus, or antimony, are added to the powerful oxidizer to create an explosive sensitive enough to be set off by the force of the hammer in the cap gun.
Other exploding toys, such as the “bang snaps” novelty fireworks, use a different contact explosive, a high explosive called silver fulminate. A tiny amount of silver fulminate is added to some sand, which is then wrapped in thin paper. When thrown at something, the explosive makes a loud bang, but the sand absorbs most of the energy, so no damage is done.
Fulminates are made by reacting gold, silver, mercury, or platinum with ammonia or nitric acid. Those metals don’t react easily, and the compounds they create break apart easily, releasing energy. Most contact explosives use compounds that break apart easily and release energy. Other examples are ammonium triiodide, triacetone tri-peroxide, lead azide, and lead styphnate.