Chemistry

Chemistry abilities are for producing poisons and explosives. Chemicals can be found scattered about inhabited areas, especially chemical storage rooms. Using them, you can poison your blades, produce grenades of various types with tin cans and blast caps, and more. -the player's manual
(5) Poison Blade: You can poison a blade with malonitrile. With 25 Chemistry skill, you produce strong poison, which is twice as deadly. With 50 Chemistry skill, you produce deadly poison, which is four times as deadly.
(10) Produce Grenades: You know how to craft grenades using a tin can, a delay fuse, and various fillers. A higher Chemistry skill lets you produce a larger variety of grenades. Your throwing distance depends on your Strength skill. 
(20) Produce Chems: You can combine a variety of chemicals to make different chemicals. A higher Chemistry skill lets you produce a larger variety of chemicals. 
(25) Produce Rockets: You know rockets. You can convert a basic rocket into any other kind of rocket if you have the right chemical. Unfortunately, the current chemical filler in the rocket will be wasted. A higher Chemistry skill lets you produce a larger variety of rockets. 
(35) Rig Glaive: You know how to combine boomprill and a throwing glaive to make a so-called boom glaive, which explodes shortly after impact. 
(50) Demo Man: You've mastered the explosive arts. You can create an explosive charge with a pipe and a canister of hexogen. A hexogen charge is the most powerful explosive in the game.
(from the player's manual)