Tri-cyclic acetone peroxide. Computer graphic of a molecule of the explosive TCAP (tri-cyclic acetone peroxide, chemical formula: C9.H18.O6). The atoms are represented by colour-coded balls (oxygen is red, carbon is grey, hydrogen is blue) joined by rods (representing the molecular bonds). This is a very unstable molecule and heat or physical shock can cause a mass of TCAP to explode in a violent reaction with oxygen in air. It is used by bombers and terrorists because it is an explosive that can be prepared from common household chemicals. It is thought that this explosive was used in the 7 July 2005 bombings in London, England.