Binary Chemical Weapons are chemical weapons wherein the toxic agent is not contained within the weapon in its active state, but in the form of two chemical precursors, physically separated within the weapon. The precursors are designed to be significantly less toxic than the agent they make when mixed, and this allows the weapon to be transported and stored more safely than otherwise. The safety provided by binary chemical weapons is especially important for people who live near ammunition dumps.
The chemical reaction takes place while the weapon is in flight. Firing the munition ruptures the capsules. The munition spins rapidly in flight, which thoroughly mixes the two precursors, so they can react with one another. Finally a bursting charge aerosolizes and distributes the nerve agent.
One example of a binary chemical weapon is the United States Army M687. In the M687, methylphosphonyl difluoride and a mixture of isopropyl alcohol and isopropyl amine are held in chambers within the weapon, separated by a partition. When the weapon is fired, acceleration causes the partition to break, and the precursors are mixed by the rotation of the weapon in flight, producing sarin nerve gas.